Information
-
Patent Grant
-
6815998
-
Patent Number
6,815,998
-
Date Filed
Tuesday, October 22, 200221 years ago
-
Date Issued
Tuesday, November 9, 200419 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
- Hoffman; E. Eric
- Young; Edel M.
- Cartier; Lois D.
-
CPC
-
US Classifications
Field of Search
US
- 327 143
- 327 334
- 327 530
- 327 543
- 327 546
-
International Classifications
-
Abstract
A voltage generation circuit for generating a read-back voltage in response to a supply voltage and a reference voltage. The voltage generation circuit includes a comparator configured to receive the supply voltage and the reference voltage. The voltage generation circuit activates a select signal if the supply voltage has a predetermined relationship with respect to the reference voltage, and de-activates the select signal if the supply voltage does not exhibit the predetermined relationship with respect to the reference voltage. An adjustable voltage divider circuit is coupled to receive the supply voltage and the select signal. The adjustable voltage divider circuit is configured in response to the select signal to provide an output voltage that is a first percentage of the supply voltage if the select signal is activated, and provide an output voltage that is a second percentage of the supply voltage if the select signal is de-activated.
Description
FIELD OF THE INVENTION
The present invention relates to a circuit used to regulate the read-back voltage applied to address lines of a memory circuit during a read-back operation. More specifically, the present invention relates to a circuit that provides a read-back voltage as an adjustable percentage of a core supply voltage, with the ability to select the specific percentage depending on the actual level of the core supply voltage.
BACKGROUND OF THE INVENTION
Programmable logic devices, such as field programmable gate arrays (FPGAs), include configuration memory cells that are loaded with configuration data values. These configuration data values control the configuration of the programmable logic device. FPGAs often include a read-back mechanism that enables the previously written configuration data values to be read from the configuration memory cells.
FIG. 1
shows a conventional array of configuration memory cells (i.e., a configuration memory) such as used by Xilinx, Inc., assignee of the present invention. The configuration memory of
FIG. 1
is a 16-bit by 16-bit array, which includes 256 configuration memory cells. In general, each of the configuration memory cells is identified by a reference character M
X,Y
, where X and Y correspond with the row and column of the configuration memory cell. A typical array of configuration memory cells in a commercial device has on the order of 20,000 to one million memory cells. Thus the array of
FIG. 1
is much smaller than is typically used in a commercial embodiment, but nevertheless shows the structure of prior art configuration memories. To load the configuration memory, a bit stream of configuration data values (DATA) is shifted into data control circuit DC, under control of a clocking mechanism, until a frame of data (16 bits wide in this example) has been shifted into bit positions C
0
through C
15
of data control circuit DC. This frame of data is then routed from bit positions C
0
-C
15
to data lines D
0
-D
15
, respectively. Note that only data lines D
0
and D
15
are labeled for purposes of clarity.
Address control circuit AC, which includes address drivers
0
-
15
, drives a write enable signal onto one of the address lines A
0
-A
15
, thereby enabling the configuration data values on lines D
0
-D
15
to be written to a column of the configuration memory cells. Note that only address lines A
0
and A
15
are labeled for purposes of clarity.
Hsieh in U.S. Pat. No. 4,750,155 describes a five transistor memory cell that can be reliably read and written by applying a lower read-back voltage to a memory cell access transistor than is applied to the memory cell access transistor to write a new value. The Hsieh patent is incorporated herein by reference.
FIG. 2
is a circuit of a conventional six-transistor configuration memory cell M
0,0
that includes an n-channel access transistor T
1
, an n-channel reset transistor T
2
and two CMOS inverters I
1
and I
2
. As is well known in the CMOS design art, each of the two inverters I
1
and I
2
comprise one PMOS transistor and one NMOS transistor connected in series between the V
DD
supply voltage and ground. Inverters I
1
and I
2
are cross-coupled, thereby forming a latch. This latch is connected to data line D
0
by access transistor T
1
, which is controlled by a control voltage on address line A
0
. One or more lines Q and/or Q# extends from configuration memory cell M
0,0
to the FPGA logic structure (not shown) to control the configuration of this structure.
Configuration memory cell M
0,0
is initially reset by turning on the n-channel reset transistor T
2
. This reset mechanism enables the transistors in inverters I
1
and I
2
to be made relatively small (because configuration memory cell M
0,0
does not have to be reset via data line D
0
) While it is desirable to have relatively small transistors to reduce layout area, these small transistors undesirably result in relatively weak inverters I
1
and I
2
. Thus, the configuration memory value stored by inverters I
1
and I
2
is more susceptible to being disturbed during a read back operation where the charge on a large data line can flip the value stored by the small memory cell.
To write a configuration data value to the first column of configuration memory cells M
0,0
-M
15,0
, address driver
0
is controlled to drive a write enable voltage equal to the V
DD
supply voltage to address line A
0
. This relatively high write voltage assures that the access transistors (e.g., access transistor T
1
) are completely turned on during the write operation, such that the configuration data values are properly written to the configuration memory cells.
The configuration data values stored in configuration memory cells M
0,0
-M
15-15
can subsequently be read back to data control circuit DC on a column-by-column basis. For example, to read the configuration data values stored in the first column of configuration memory cells M
0,0
-M
15,0
, address control circuit AC causes address driver
0
to apply a read-back voltage to address line A
0
. This read-back voltage is typically selected to be equal to the V
DD
supply voltage minus the threshold voltage (V
T
) of access transistor T
1
. Under these conditions, the configuration data values stored in configuration memory cells M
0,0
to M
15,0
are read back to the data control circuit DC on data lines D
0
-D
15
. The read-back voltage is low enough to ensure that the read-back operation does not disturb the configuration data values stored in the configuration memory cells M
0,0
-M
15,0
. Note that the read-back voltage is referenced to the V
DD
supply voltage because the associated circuitry in data control circuit DC operates in response to the V
DD
supply voltage.
During normal operation, the V
DD
supply voltage can typically vary +/−10 percent with respect to a nominal supply voltage value. Thus, a V
DD
supply voltage having a nominal value of 1.2 Volts can vary from 1.08 to 1.32 Volts. For relatively low V
DD
supply voltages, the read-back voltage (V
DD
−V
T
) might be too low to reliably read the configuration memory cell. For example, a V
DD
supply voltage of 1.08 Volts would produce a read-back voltage of about 0.710 Volts, assuming a threshold voltage of 0.370 Volts. This read-back voltage may be inadequate to reliably read the configuration data values stored in the configuration memory cells.
It would therefore be desirable to have a method and apparatus for generating acceptable read-back voltages for a memory circuit, such as a configuration memory array of a programmable logic device, for all possible values of the V
DD
supply voltage.
SUMMARY
Accordingly, the present invention provides a read-back voltage generation circuit that provides a read-back voltage as an adjustable percentage of a supply voltage. The read-back voltage generation circuit has the ability to select the specific percentage depending on the actual level of the supply voltage. For example, if the supply voltage has a relatively high value, then the read-back voltage will be a relatively low percentage of the supply voltage. Conversely, if the supply voltage has a relatively low value, then the read-back voltage will be a relatively high percentage of the supply voltage. As a result, the read-back voltage will always be high enough to reliably read the configuration data values from the configuration memory cells within a given time margin, but not so high as to overwrite these configuration data values. The read-back voltage generation circuit is especially advantageous for use in a chip having a low core supply voltage, wherein a threshold voltage drop (V
T
) represents a large percentage of the core supply voltage.
In one embodiment, the read-back voltage generation circuit buffers the read-back voltage through a low output impedance buffer that is capable of supplying the proper voltage for the address lines on the chip. The read-back voltage generation circuit is designed to use minimal DC current, but is still able to charge the address lines quickly and efficiently to a value that properly controls the read-back function.
In accordance with one embodiment, the read-back voltage generation circuit includes a comparator configured to receive the supply voltage and a reference voltage. The voltage generation circuit activates a select signal if the supply voltage has a predetermined relationship with respect to the reference voltage, and de-activates the select signal if the supply voltage does not exhibit the predetermined relationship with respect to the reference voltage. For example, the comparator can activate the select signal if the supply voltage is less than the reference voltage, and de-activate the select signal if the supply voltage is greater than the reference voltage.
An adjustable voltage divider circuit is coupled to receive the supply voltage and the select signal. The adjustable voltage divider circuit is configured in response to the select signal to provide an output voltage that is a first percentage of the supply voltage if the select signal is activated, and provide an output voltage that is a second percentage of the supply voltage if the select signal is de-activated. For example, the adjustable voltage divider circuit can be configured to provide an output voltage that is 95-100 percent of the supply voltage if the select signal is activated, and provide an output voltage that is less than 95 percent of the supply voltage if the select signal is de-activated. A low impedance, current limited output driver drives the output voltage as the read-back voltage.
In one embodiment, the reference voltage is derived from a bandgap voltage generator, thereby providing a relatively constant reference voltage.
One variation of the present invention uses multiple comparators to compare the supply voltage to a plurality of reference voltages. Such a variation enables finer control over the read-back voltage level.
The present invention will be more fully understood in view of the following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a block diagram of a conventional array of configuration memory cells, a data control circuit, and an address control circuit.
FIG. 2
is a circuit diagram of a conventional configuration memory cell.
FIG. 3
is a circuit diagram of a read-back voltage generation circuit in accordance with one embodiment of the present invention.
FIG. 4
is a circuit diagram of a band-gap buffer used in the read-back voltage generation circuit of
FIG. 3
in accordance with one embodiment of the present invention.
FIG. 5
is a circuit diagram illustrating a comparator used in the read-back voltage generation circuit of
FIG. 3
in accordance with one embodiment of the present invention.
FIG. 6
is a circuit diagram illustrating output driver used in the read-back voltage generation circuit of
FIG. 3
in accordance with one embodiment of the present invention.
FIG. 7
is a graph illustrating a read-back voltage V
RB
and an associated address line voltage V
ADDR
in accordance with one embodiment of the present invention.
FIG. 8
is a circuit diagram of a read-back voltage generation circuit in accordance with one variation of the present invention.
FIG. 9
is a circuit diagram of a read-back voltage generation circuit in accordance with yet another variation of the present invention.
FIG. 10
is a circuit diagram of a read-back voltage generation circuit in accordance with yet another variation of the present invention.
DETAILED DESCRIPTION
FIG. 3
is a circuit diagram of a read-back voltage generation circuit
300
in accordance with one embodiment of the present invention. Read-back voltage generation circuit
300
includes band-gap reference buffer
301
, comparator
302
, output driver
303
, p-channel transistors
311
-
312
, resistors
313
-
315
, p-channel pass transistor
316
and bandgap voltage generator
320
.
In accordance with the described embodiment, read-back voltage generation circuit
300
generates a read-back voltage V
RB
, which is used to access configuration memory cells of a programmable logic device, such as a field programmable gate array, during a read-back operation. For example, the read-back voltage V
RB
generated by circuit
300
can be selectively applied to the address lines A
0
-A
15
of the configuration memory array of
FIG. 1
during a read-back operation. Alternately, read-back voltage generation circuit
300
can be used to generate a read-back voltage for a general memory circuit, or generate any sensitive referenced voltage for a voltage-level sensitive circuit.
As described in more detail below, the level of the read-back voltage V
RB
is determined in response to the actual level of the V
DD
supply voltage. Thus, if the V
DD
supply voltage is greater than a predetermined voltage, then circuit
300
will generate a read-back voltage that is a first percentage of the V
DD
supply voltage. If the V
DD
supply voltage is less than or equal to the predetermined voltage, then circuit
300
will generate a read-back voltage that is a second percentage of the V
DD
supply voltage. In accordance with one embodiment of the present invention, the first percentage is less than the second percentage. For example, of the V
DD
supply voltage is greater than the predetermined voltage, the read-back voltage V
RB
may have a voltage that is 75% of the V
DD
supply voltage. Conversely, if the V
DD
supply voltage is less than or equal to a predetermined voltage, the read-back voltage V
RB
may have a voltage that is 95% of the V
DD
supply voltage. This ensures that the read-back voltage will be high enough to reliably access the configuration memory cells within a predetermined time margin during a read-back operation, without overwriting the configuration data values stored in the configuration memory cells. Note that a read-back voltage V
RB
that is too high and rises too fast can disturb memory in the associated memory cell. Also note that the read-back voltage V
RB
is referenced to the V
DD
supply voltage because the circuitry implementing the read-back function operates in response to the V
DD
supply voltage.
Read-back voltage generation circuit
300
generates a read-back voltage V
RB
in response to a core supply voltage V
DD
, an auxiliary supply voltage V
CCI
, a first active-low enable signal ENABLE#, a second active-low enable signal ENABLE#_V
DD
, a bandgap reference voltage V
BG
and a ground supply voltage LOC_GND.
Band-gap buffer
301
is configured to receive the bandgap reference voltage V
BG
from bandgap voltage generator
320
, the core supply voltage V
CCI
, the enable signal ENABLE#, and the local ground supply voltage LOC_GND. In response to these signals, band-gap butter
301
provides an output voltage V
301
, which is relatively constant with respect to voltage and temperature variations.
FIG. 4
is a circuit diagram of band-gap buffer
301
and p-channel transistors
311
-
312
in accordance with one embodiment of the present invention. Band-gap buffer
301
includes thick-oxide p-channel transistors
401
-
405
, thick-oxide n-channel transistors
411
-
416
and thick-oxide inverter
419
. Thick-oxide elements are used in band-gap buffer
301
, because these elements operate in response to the V
CCI
supply voltage, which is higher than the V
DD
supply voltage. In the described example, the V
CCI
supply voltage has a nominal value of 2.5 Volts, while the V
DD
supply voltage has a nominal value of 1.2 Volts. The V
CCI
supply voltage is used to operate all circuitry of the FPGA that requires a higher voltage than the V
DD
supply voltage, but cannot tolerate the noise associated with the input/output supply voltage V
CCO
, while the V
DD
supply voltage is used to operate the core logic of the FPGA.
N-channel transistors
414
and
415
form a differential pair
420
. The gate of n-channel transistor
414
is coupled to receive the bandgap voltage V
BG
, which is a relatively constant voltage. In the described example, the bandgap voltage V
BG
has a value of about 1.196 Volts, with negligible variations in response to variations in temperature, process or supply voltage. The bandgap voltage V
BG
is provided by a conventional bandgap voltage generator
320
. The gate of n-channel transistor
415
is coupled to an output stage
421
, which includes n-channel transistor
416
and p-channel transistors
311
-
312
(FIG.
3
). N-channel transistor
416
is coupled between the gate of transistor
415
and the V
CCI
voltage supply terminal. P-channel transistors
311
-
312
are coupled in series between the gate of transistor
415
and the ground supply terminal LOC_GND.
The differential pair
420
is supplied by a current mirror circuit
422
formed by p-channel transistors
404
-
405
, and n-channel bias transistor
412
. P-channel transistor
404
is coupled between the drain of n-channel transistor
414
and the V
CCI
supply voltage terminal, and p-channel transistor
405
is coupled between the drain of n-channel transistor
415
and the V
CCI
voltage supply terminal. The gates of p-channel transistors
404
and
405
are commonly coupled to the drain of n-channel transistor
414
.
Current mirror circuit
422
is enabled and disabled by p-channel transistor
403
, which is coupled between the V
CCI
voltage supply terminal and the gates of p-channel transistors
404
and
405
. P-channel transistor
403
is controlled by an ENABLE signal provided by inverter
419
. When the enable signal ENABLE# is activated low, the read-back voltage generation circuit
300
is enabled, thereby causing the ENABLE signal to go high. The high ENABLE signal turns off p-channel transistor
403
, thereby enabling current mirror circuit
422
.
The low ENABLE# signal also activates a bias control circuit
423
, which includes p-channel transistors
401
-
402
and n-channel transistors
411
-
413
. The logic low ENABLE# signal turns on p-channel transistors
401
-
402
and turns off n-channel transistor
413
. As a result, a logic high bias voltage N_BIAS is developed on the gate of n-channel transistor
412
, thereby causing a bias current to flow through this transistor
412
, as well as differential pair
420
.
Once enabled, band-gap buffer
301
operates as follows. The enabled bias circuit
423
ensures that a constant current flows through bias transistor
412
(and thereby through differential pair
420
). The enabled current mirror circuit
422
operates to maintain equal currents through differential pair transistors
414
and
415
. The constant bandgap voltage V
BG
causes a first current to flow through transistor
414
of differential pair
420
. Current mirror
422
attempts to mirror this current to transistor
415
of differential pair
420
. The voltage on the drain of transistor
415
biases the gate of transistor
416
of output stage
421
. In response, transistor
416
biases the voltage V
301
on the gate of transistor
415
to a voltage equal to the bandgap voltage V
BG
to bring differential pair
420
into equilibrium, wherein the gates of transistors
414
and
415
have the same voltage. This happens because of the feedback loop configuration of differential amplifier
420
.
The voltage V
301
is applied to p-channel transistors
311
and
312
. P-channel transistors
311
and
312
form a voltage divider circuit, wherein the node connecting these p-channel transistors
311
-
312
provides a reference voltage V
REF
. P-channel transistors
311
and
312
are sized to have on-resistances that provide the desired voltage division ratio. In the described embodiment, p-channel transistors
311
and
312
are designed to have resistances exhibiting a ratio of about 20:100. As a result, the reference voltage V
REF
has a value equal to V
301
×100/120 or about 0.997 Volts. In an alternative embodiment, p-channel transistors
311
-
312
can be replaced with resistors that exhibit the desired-ratio.
Because the reference voltage V
REF
is derived from the constant bandgap voltage V
BG
, the reference voltage V
REF
is also a constant voltage. As will become apparent in view of the following description, the reference voltage V
REF
is selected to correspond with the V
DD
voltage level at which the read-back voltage V
RB
is adjusted. The reference voltage V
REF
is applied to the minus input terminal of comparator
302
(FIG.
3
), and the V
DD
supply voltage is applied to the plus input terminal of comparator
302
. In general, comparator
302
provides a logic high output voltage V
ADJUST
if the V
DD
supply voltage is greater than the reference voltage V
REF
. Conversely, comparator
302
provides a logic low output voltage V
ADJUST
if the V
DD
supply voltage is less than the reference voltage V
REF
.
FIG. 5
is a circuit diagram illustrating comparator
302
in accordance with one embodiment of the present invention. Comparator
302
includes p-channel transistors
501
-
509
and n-channel transistors
511
-
519
.
N-channel transistors
515
and
516
form a differential pair
520
. The gate of n-channel transistor
515
is coupled to receive the reference voltage V
REF
, and the gate of n-channel transistor
516
is coupled to receive the V
DD
supply voltage.
The differential pair
520
is supplied by current mirror circuits
521
and
522
, and n-channel bias transistor
517
. Current mirror circuit
521
, which supplies current to transistor
515
of differential pair
520
, includes p-channel transistors
504
and
505
. Current mirror circuit
522
, which supplies current to transistor
516
of differential pair
520
, includes p-channel transistors
507
and
508
. Current mirror circuits
521
and
522
are coupled to current mirror circuit
523
, which is formed by n-channel transistors
514
and
519
.
Current mirror circuit
521
is enabled and disabled by p-channel transistor
503
, which is coupled between the V
DD
voltage supply terminal and the gates of p-channel transistors
504
and
505
. Similarly, current mirror circuit
522
is enabled and disabled by p-channel transistor
506
, which is coupled between the V
DD
voltage supply terminal and the gates of p-channel transistors
507
and
508
. Current mirror circuit
523
is enabled and disabled by n-channel transistor
518
, which is coupled between the ground supply voltage terminal LOC_GND, and the gates of n-channel transistors
514
and
519
.
N-channel transistor
518
is controlled by the ENABLE#_V
DD
signal provided to comparator
302
. P-channel transistors
503
and
506
are controlled by an ENABLE_V
DD
signal provided by the inverter formed by transistors
509
and
511
. When the read-back voltage generation circuit
300
is enabled, the ENABLE#_V
DD
signal is activated low, thereby turning off n-channel transistor
518
and activating current mirror circuit
523
. The logic low ENABLE#_V
DD
signal causes the ENABLE_V
DD
signal to go high, thereby turning off p-channel transistors
503
and
506
, and activating current mirror circuits
521
and
522
.
The low ENABLE#_V
DD
signal also activates a bias control circuit
524
, which includes p-channel transistors
501
-
502
and n-channel transistors
512
-
513
and
517
. The logic low ENABLE#_V
DD
signal turns on p-channel transistors
501
-
502
and turns off n-channel transistor
513
. As a result, a logic high bias voltage N_BIAS
1
is developed on the gate of n-channel transistor
517
, thereby causing a bias current to flow through this transistor
517
, as well as differential pair
520
.
Once enabled, comparator
302
operates as follows. The enabled bias circuit
524
ensures that a constant current flows through bias transistor
517
(and thereby through differential pair
520
). The V
DD
supply voltage applied to transistor
516
will typically be greater than or less than the reference voltage V
REF
applied to transistor
515
. For example, assume that the V
DD
supply voltage is greater than the reference voltage V
REF
. The relatively high voltage applied to transistor
516
will cause the current through this transistor
516
(and through transistor
507
) to increase. In response, current mirror circuit
522
causes the current through transistor
508
to similarly increase.
The relatively low voltage applied to transistor
515
will cause the current through this transistor (and through transistor
505
) to decrease. In response, current mirror circuit
521
causes the current through transistor
504
to similarly decrease. The current through transistor
514
(being equal to the current through transistor
504
) also decreases. In response, current mirror circuit
523
causes the current through transistor
519
to decrease.
Thus, the current through p-channel transistor
508
increases, while the current through n-channel transistor
519
decreases. Under these conditions, the output voltage V
ADJUST
of comparator
302
is pulled up to the V
DD
supply voltage.
In a similar manner, if the V
DD
supply voltage is less than the reference voltage V
REP
, the current through p-channel transistor
508
decreases, while the current through n-channel transistor
519
increases. Under these conditions, the output voltage V
ADJUST
Of comparator
302
is pulled down to the ground supply voltage LOC_GND. In the foregoing manner, comparator
302
provides a full rail-to-rail comparator with fairly low operating DC current.
The output voltage V
ADJUST
of comparator
302
is applied to the gate of p-channel pass transistor
316
. As described above, if the V
DD
supply voltage is greater than the reference voltage V
REF
, the output voltage V
ADJUST
of comparator
302
will be pulled up to the V
DD
supply voltage, thereby turning off pass transistor
316
. Under these conditions, the V
DD
supply voltage is applied to a voltage divider circuit that includes resistors
313
and
315
. In the described embodiment, resistors
313
and
315
are unsalicided P+
0
polysilicon resistors having nominal resistances of 33 kOhms and 100 kOhms, respectively. Thus, the voltage V
IN
will be about 75 percent of the V
DD
supply voltage. As described in more detail below, output driver
303
drives this voltage V
IN
as the read-back voltage V
RB
. Thus, when the V
DD
supply voltage is relatively high, the read-back voltage V
RB
is selected to have a value equal to about 75 percent of the V
DD
supply voltage.
Conversely, if the V
DD
supply voltage is less than the reference voltage V
REF
, the output voltage V
ADJUST
of comparator
302
will be pulled down to the ground supply voltage LOC_GND in the manner described above, thereby turning on pass transistor
316
. Under these conditions, the V
DD
supply voltage is applied to a voltage divider circuit that includes resistors
313
,
314
and
315
. In the described embodiment, resistor
314
is an unsalicided P+ polysilicon resistor having a nominal resistances of 5 kOhms. When pass transistor
316
is turned on, a voltage divider circuit is formed, with one leg of the voltage divider circuit consisting of resistors
313
and
314
coupled in parallel (with an equivalent resistance of about 4.3 kOhms), and the other leg of the voltage divider consisting of resistor
315
. Thus, the voltage V
IN
will be about 96 percent of the V
DD
supply voltage. As described in more detail below, output driver
303
drives this voltage V
IN
as the read-back voltage V
RB
. Thus, when the V
DD
supply voltage is relatively high, the read-back voltage V
RB
is selected to have a value equal to about 96 percent of the V
DD
supply voltage.
The above-described percentages were selected in view of a simulation that indicated that memory disturb was more prevalent for higher values of V
DD
supply voltage, and memory read delay time was more of an issue for low values of the V
DD
supply voltage. Other percentages can be selected in other embodiments of the present invention.
FIG. 6
is a circuit diagram illustrating output driver
303
in accordance with one embodiment of the present invention. Output driver
303
is a low output impedance buffer that is capable of supplying the proper read-back voltage for the address lines on the chip. Output driver
303
includes thick-oxide p-channel transistors
601
-
605
, thick-oxide n-channel transistors
611
-
617
, resistor
618
and thick-oxide inverter
619
. Thick-oxide elements are used in output driver
303
, because these elements operate in response to the V
CCI
supply voltage, which is higher than the V
DD
supply voltage.
N-channel transistors
614
and
615
form a differential pair
620
. The gate of n-channel transistor
614
is coupled to receive the voltage V
IN
, which is equal to either 75% or 96% of the V
DD
supply voltage, as described above. The gate of n-channel transistor
615
is coupled to current limiting regulator output stage
621
, which includes n-channel transistors
616
and
617
. N-channel transistor
616
and resistor
618
are connected in series between the gate of transistor
615
and the V
CCI
voltage supply terminal. N-channel transistor
617
is coupled between the gate of transistor
615
and the ground supply terminal LOC_GND.
The differential pair
620
is supplied by a current mirror circuit
622
formed by p-channel transistors
604
-
605
, and n-channel bias transistor
612
. P-channel transistor
604
is coupled between the drain of n-channel transistor
614
and the V
CCI
supply voltage terminal, and p-channel transistor
605
is coupled between the drain of n-channel transistor
615
and the V
CCI
voltage supply terminal. The gates of p-channel transistors
604
and
605
are commonly coupled to the drain of n-channel transistor
614
.
Current mirror circuit
622
is enabled and disabled by p-channel transistor
603
, which is coupled between the V
CCI
voltage supply terminal and the gates of p-channel transistors
604
and
605
. P-channel transistor
603
is controlled by an ENABLE signal provided by inverter
619
. When the read-back voltage generation circuit
300
is enabled, the enable signal ENABLE# is activated low, thereby causing the ENABLE signal to go high. The high ENABLE signal turns off p-channel transistor
603
, thereby enabling current mirror circuit
622
.
The low ENABLE# signal also activates a bias control circuit
623
, which includes p-channel transistors
601
-
602
and n-channel transistors
611
-
613
. The logic low ENABLE# signal turns on p-channel transistors
601
-
602
and turns off n-channel transistor
613
. As a result, a logic high bias voltage N_BIAS
2
is developed on the gate of n-channel transistor
612
, thereby causing a bias current to flow through this transistor
612
, as well as differential pair
620
. The gate of n-channel transistor
617
of output stage
621
is coupled to receive the V
CCI
supply voltage, thereby turning on this transistor
617
.
Once enabled, output driver
303
operates in a manner similar to band-gap buffer
301
. Thus, the enabled bias circuit
623
ensures that a constant current flows through bias transistor
612
(and thereby through differential pair
620
). The enabled current mirror circuit
622
operates to maintain equal currents through differential pair transistors
614
and
615
. The input voltage V
IN
causes a first current to flow through transistor
614
of differential pair
620
. Current mirror
622
mirrors this current to transistor
615
of differential pair
620
. The voltage on the drain of transistor
615
biases the gate of transistor
616
of output stage
621
. In response, transistor
616
biases the read-back voltage V
RB
on the gate of transistor
615
to a voltage equal to the input voltage V
IN
.
Resistor
618
limits the current in output stage
621
, and reduces sharp transitions in a rising address voltage on an associated address line. Resistor
618
thereby reduces disturb conditions when reading back configuration data values from the configuration memory cells. During read-back mode, read-back voltage generation circuit
300
is enabled and the read-back voltage V
RB
is always on. However, whenever an address line is coupled to receive the read-back voltage V
RB
, current is drawn from output stage
621
, with resistor
618
limiting the amount of current that is drawn into the address line.
FIG. 7
illustrates the read-back voltage V
RB
and an address voltage V
ADDR
on an associated address line. The address line is coupled to receive the read-back voltage at time T
0
. The gradual transition of the address voltage V
ADDR
helps to reduce disturb conditions in the configuration memory cells being read.
FIG. 8
is a circuit diagram of a read-back voltage generation circuit
800
in accordance with one embodiment of the present invention. Because circuit
800
is similar to circuit
300
(FIG.
3
), similar elements in
FIGS. 3 and 8
are labeled with similar reference numbers. Thus, circuit
800
includes band-gap reference buffer
301
, comparator
302
, output driver
303
, p-channel transistors
311
-
312
, resistors
313
and
315
and p-channel pass transistor
316
. In addition, circuit
800
includes p-channel pass transistor
817
, inverter
818
and resistor
3
-
819
. Resistor
819
, which replaces resistor
314
, has a value of about 4.3 kOhms (i.e., the equivalent parallel resistance of resistors
313
and
314
). Circuit
800
operates in the same manner as circuit
300
, with the following exceptions. When the V
ADJUST
voltage has a logic high value, pass transistor
817
is turned on and pass transistor
316
is turned off, thereby creating a voltage divider circuit that includes resistors
313
and
315
. Conversely, when the V
ADJUST
voltage has a logic low value, pass transistor
817
is turned off and pass transistor
316
is turned on, thereby creating a voltage divider circuit that includes resistors
819
and
315
.
FIG. 9
is a circuit diagram of a read-back voltage generation circuit
900
in accordance with yet another variation of the present invention. Circuit
900
includes comparators
901
-
903
, output driver
905
, resistors
911
-
914
, and p-channel pass transistors
921
-
923
. Comparators
901
,
902
and
903
are configured to receive voltages V
1
-V
2
, V
3
-V
4
, and V
5
-V
6
, respectively. Comparator
901
provides a logic low voltage select signal V
SEL1
if V
1
is greater than V
2
, and a logic high voltage select signal V
SEL1
if V
1
is less than V
2
. Similarly, comparator
902
provides a logic low voltage select signal V
SEL2
if V
3
is greater than V
4
, and a logic high voltage select signal V
SEL2
if V
3
is less than V
4
. Comparator
903
provides a logic low voltage select signal V
SEL3
if V
5
is greater than V
6
, and a logic high voltage select signal V
SEL3
if V
5
is less than V
6
.
The voltage select signals V
SEL1
, V
SEL
2
and V
SEL3
are provided to the gates of p-channel pass transistors
921
,
922
and
923
, respectively. Resistors
911
,
912
and
913
are connected between the voltage supply terminals V
A
, V
B
and V
C
and pass transistors
921
,
922
and
923
, respectively. One end of resistor
914
is coupled to pass transistors
921
-
923
and the input terminal of output driver
905
, and the other end of resistor
914
is coupled to the ground voltage supply terminal. In the described embodiment, voltages V
A
, V
B
and V
C
are all equal to the V
DD
supply voltage, although this is not necessary. Voltage at terminals V
A
, V
B
and V
C
can have different voltage levels in different embodiments.
Circuit
900
provides additional control over the read-back voltage V
RB
provided by output driver
905
. For example, the V
DD
supply voltage can be provided to the plus input terminals of comparators
901
-
903
as the V
2
, V
4
and V
6
signals. Different reference voltages can then be applied to the minus input terminals of comparators
901
-
903
as the V
1
, V
3
and V
5
signals. In one example, voltage V
1
is selected to have a voltage of 1.35 Volts, such that the V
SEL1
voltage select has a logic low value if V
DD
is less than 1.35 Volts, and a logic high value otherwise. In this example, voltage V
3
is selected to have a voltage of 1.08 Volts, such that the V
SEL2
voltage select has a logic low value if V
DD
is less than 1.08 Volts, and a logic high value otherwise. Finally, voltage V
5
is selected is selected to have a voltage of 0.9 Volts, such that the V
SEL3
voltage select has a logic low value if V
DD
is less than 0.8 Volts, and a logic high value otherwise.
In this example, resistors
911
-
914
can have resistances of 33 kOhms, 33 kOhms, 6 kOhms and 100 kOhms, respectively. Thus, if the V
DD
supply voltage has a value less than 1.35 Volts, but greater than 1.08 Volts, then pass transistor
921
is turned on and pass transistors
922
-
923
are turned off. Under these conditions, the input voltage V
IN
to output driver
905
will have a value of about 75% of the V
DD
supply voltage.
If the V
DD
supply voltage has a value less than 1.08 Volts, but greater than 0.9 Volts, then pass transistors
921
and
922
are turned on and pass transistor
923
is turned off. Under these conditions, the input voltage V
IN
to output driver
905
will have a value of about 86% of the V
DD
supply voltage.
If the V
DD
supply voltage has a value less than 0.9 Volts, then pass transistors
921
-
923
are turned on. Under these conditions, the input voltage V
IN
to output driver
905
will have a value of about 96% of the V
DD
supply voltage.
In the foregoing manner, circuit
900
is able to provide more fine control over the value of the read-back voltage V
RB
. Although circuit
900
uses three comparators
901
-
903
, three pass transistors
921
-
923
, and three resistors
911
-
913
, it is understood that other numbers of comparators and pass transistors and resistors can be used in other embodiments. It is also understood that other voltages can be applied to comparators
901
-
903
in other embodiments.
FIG. 10
is a circuit diagram of a read-back voltage generation circuit
1000
in accordance with yet another variation of the present invention. Circuit
1000
includes comparator
1001
, inverter
1002
, p-channel pass transistors
1003
-
1004
and output driver
1005
. Pass transistors
1003
and
1004
are configured to receive the voltages V
B
and V
A
, respectively, wherein V
B
is greater than V
A
. Comparator
1001
is configured to receive voltages V
1
and V
2
. Comparator
1001
provides a logic low voltage select signal V
SEL
if V
1
is greater than V
2
, and a logic high voltage select signal V
SEL
if V
1
is less than V
2
.
If the V
SEL
signal has a logic low state, then p-channel transistor
1003
is turned on and transistor
1004
is turned off. As a result, the voltage V
B
is routed through pass transistor
1003
as the input voltage V
IN
to output driver
1005
. Conversely, if the V
SEL1
signal has a logic high state, then p-channel transistor
1004
is turned on and p-channel transistor
1003
is turned off. As a result, the voltage V
A
is routed through pass transistor
1004
as the input voltage V
IN
to output driver
1005
.
In one embodiment, the voltage V
2
is equal to the V
DD
supply voltage, and the voltage V
1
is equal to the reference voltage V
REF
. In this embodiment, the voltage V
A
is less than the reference voltage V
REF
. Alternatively, in this embodiment, the voltage V
A
can be equal to the reference voltage V
REF
, and the voltage V
B
can be equal to the V
DD
supply voltage. As a result, when the V
DD
supply voltage is greater than the reference voltage V
REF
, then the reference voltage V
REF
is routed as the input voltage V
IN
. Conversely, when the V
DD
supply voltage is less than the reference voltage V
REF
, then the V
DD
supply voltage is routed as the input voltage V
IN
. Note that the configuration of
FIG. 10
can be substituted in the circuits of
FIGS. 3
,
8
and
9
in accordance with other embodiments of the present invention.
Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to one of ordinary skill in the art. Thus, the read-back voltage generation circuit of the present invention can be used in a variety of integrated circuit devices, including, but not limited to, field programmable gate arrays. Thus, the invention is limited only by the following claims.
Claims
- 1. A voltage generation circuit comprising:a comparator configured to receive and a supply voltage and a reference voltage and to perform a comparison therebetweeen, and in response, activate a select signal if the supply voltage has a predetermined relationship with respect to the reference voltage, and de-activate the select signal if the supply voltage does not exhibit the predetermined relationship with respect to the reference voltage; and an adjustable voltage divider circuit coupled to receive the supply voltage and the select signal, wherein the adjustable voltage divider circuit is configured in response to the select signal to provide an output voltage that is a first percentage of the supply voltage if the select signal is activated, and provide an output voltage that is a second percentage of the supply voltage if the select signal is de-activated.
- 2. The voltage generation circuit of claim 1, further comprising an output driver configured to receive the output voltage.
- 3. The voltage generation circuit of claim 2, further comprising an address line coupled to an output terminal of the output driver, wherein the output driver provides the output voltage as a read-back voltage on the address line.
- 4. The voltage generation circuit of claim 1, wherein the output driver further comprises a current limiting output stage configured to soften a rising edge of the output voltage.
- 5. The voltage generation circuit of claim 4, wherein the current limiting output stage comprises a resistor coupled between an output terminal of the output driver and a terminal that provides the supply voltage.
- 6. The voltage generation circuit of claim 1, further comprising a bandgap voltage generator configured to generate a bandgap voltage, wherein the reference voltage is derived from the bandgap voltage.
- 7. The voltage generation circuit of claim 6, further comprising a voltage divider circuit coupled to the bandgap voltage generator, wherein the voltage divider circuit provides the reference voltage in response to the bandgap voltage.
- 8. The voltage generation circuit of claim 1, wherein the predetermined relationship is defined by the supply voltage being less than the reference voltage.
- 9. The voltage generation circuit of claim 8, wherein the first percentage is greater than the second percentage.
- 10. The voltage generation circuit of claim 9, wherein the first percentage is about 95 to 100 percent, and the second percentage is less than 95 percent.
- 11. The voltage generation circuit of claim 1, wherein the adjustable voltage divider circuit comprises:a first resistor and a pass transistor coupled in series between a supply voltage terminal coupled to receive the supply voltage and an output terminal configured to provide the output voltage, the pass transistor having a gate coupled to receive the select signal; and a second resistor coupled between the output terminal and a second voltage supply terminal coupled to receive a second supply voltage, wherein the output voltage is provided on the output terminal.
- 12. The voltage generation circuit of claim 11, wherein the adjustable voltage divider circuit further comprises a third resistor coupled between the supply voltage terminal and the output terminal.
- 13. The voltage generation circuit of claim 11, wherein the adjustable voltage divider circuit further comprises a third resistor and a second pass transistor coupled in series between the supply voltage terminal and the output terminal, the second pass transistor having a gate coupled to receive the inverse of the select signal.
- 14. The voltage generation circuit of claim 1, further comprising:a second comparator configured to receive the supply voltage and a second reference voltage, and in response, activate a second select signal if the supply voltage has a predetermined relationship with respect to the second reference voltage, and de-activate the second select signal if the supply voltage does not exhibit the predetermined relationship with respect to the second reference voltage; wherein the adjustable voltage divider circuit is configured in response to the first and second select signals to provide an output voltage that is a first percentage of the supply voltage if the select signal is activated and the second select signal is de-activated, and provide an output voltage that is a second percentage of the supply voltage if the select signal and the second select signal are both activated.
- 15. A method of generating an output voltage in response to a supply voltage and a reference voltage, the method comprising:comparing the supply voltage with the reference voltage; activating a select signal if the supply voltage is less than the reference voltage; de-activating the select signal if the supply voltage is greater than the reference voltage; providing an output voltage that is a first percentage of the supply voltage if the select signal is activated; and providing an output voltage that is a second percentage of the supply voltage if the select signal is de-activated, wherein the first percentage is greater than the second percentage.
- 16. The method of claim 15, further comprising driving the output voltage as a read-back voltage onto an address line.
- 17. The method of claim 15, further comprising deriving the reference voltage from a bandgap voltage generator.
- 18. The method of claim 15, wherein the first percentage is about 95 to 100 percent, and the second percentage is less than 95 percent.
- 19. The method of claim 15, further comprising enabling a first leg of a voltage divider circuit when the select signal is activated.
- 20. The method of claim 19, further comprising enabling a second leg of a voltage divider circuit when the select signal is de-activated.
- 21. The method of claim 15, further comprising comparing the supply voltage with a second reference voltage, wherein the first reference voltage is greater than the second reference voltage;activating a second select signal if the supply voltage is less than the second reference voltage; de-activating the second select signal if the supply voltage is greater than the second reference voltage; providing an output voltage that is the first percentage of the supply voltage if the select signal is activated and the second select signal is de-activated; providing an output voltage that is the second percentage of the supply voltage if the select signal and the second select signal are de-activated; and providing an output voltage that is a third percentage of the supply voltage if the select signal and the second select signal are activated, wherein the third percentage is greater than the first percentage.
- 22. A voltage generation circuit for generating an output voltage, the voltage generation circuit comprising:a first comparator configured to receive a first supply voltage and a first reference voltage, and in response, activate a first select signal if the first supply voltage has a first predetermined relationship with respect to the first reference voltage, and de-activate the first select signal if the first supply voltage does not exhibit the first predetermined relationship with respect to the first reference voltage; a second comparator configured to receive a second supply voltage and a second reference voltage and in response, activate a second select signal if the second supply voltage has a second predetermined relationship with respect to the second reference voltage, and de-activate the second select signal if the second supply voltage does not exhibit the second predetermined relationship with respect to the second reference voltage; and an adjustable voltage divider circuit coupled to receive the first select signal and the second select signal, wherein the adjustable voltage divider circuit is configured to provide an output voltage that is a first percentage of a fifth voltage if the first select signal is activated, and provide an output voltage that is a second percentage of a sixth voltage if the second select signal is activated.
US Referenced Citations (9)