Patent Application
20050285631
The present invention relates generally to electronic circuits, and more specifically to latch circuits that latch digital data.
Data latches may be used to sense small voltage differences that represent stored data. Some data latches sense the small voltage differences and then produce large voltage differences on one or more circuit nodes to “latch” the data. When performing multiple data latching operations in sequence, it may take time for the large voltage differences to change, which in turn may slow the operation of the latch.
In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the spirit and scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views.
The transistors shown in
Transistor 116 is coupled in series between upper power supply node 140 and the remainder of latch 110. The source node of transistor 116 is coupled to upper power supply node 140, and the drain node of transistor 116 is coupled to cross-coupled transistors 112 and 114. The gate node of transistor 126 is coupled to receive a signal {overscore (EN)}. When {overscore (EN)} is low, transistor 116 is on, and upper power supply node 140 is able to supply current for the operation of latch 110. When {overscore (EN)} is high, transistor 116 is off, and upper power supply node 140 is not able to supply current for the operation of latch 110. Transistor 116 operates as a switch to conditionally couple or decouple a power supply node from the latch. In some embodiments, a switching element other than a PMOSFET may be utilized to decouple upper power supply node 140 from the remainder of latch 110.
Transistor 126 is coupled in series between lower power supply node 142 and the remainder of latch 110. The source node of transistor 126 is coupled to lower power supply node 142, and the drain node of transistor 126 is coupled to cross-coupled transistors 122 and 124. The gate node of transistor 126 is coupled to receive a signal EN. When EN is high, transistor 126 is on, and lower power supply node 142 is able to supply current for the operation of latch 110. When EN is low, transistor 126 is off, and lower power supply node 142 is not able to supply current for the operation of latch 110. Transistor 126 operates as a switch to conditionally couple or decouple a power supply node from the latch. In some embodiments, a switching element other than an NMOSFET may be utilized to decouple lower power supply node 142 from the remainder of latch 110.
Complementary nodes 136 and 138 serve as both input nodes and output nodes for latch 110. As described further below, latch 110 receives a voltage differential from data source 150 at complementary nodes 136 and 138 when they are serving as input nodes. Latch 110 senses the voltage differential on complementary nodes 136 and 138, and then drives complementary nodes 136 and 138 substantially to the power supply voltages present on power supply nodes 140 and 142. Nodes 136 and 138 are referred to as “complementary” in part because they are driven to complementary voltage values. This operation is described further below.
Switch 130 is coupled between complementary nodes 136 and 138. When switch 130 is closed, the voltage between the pair of complementary nodes approaches the same value, or “equalizes.” In some embodiments, switch 130 is closed when transistors 116 and 126 are off to equalize the two complementary nodes before latch 110 senses data. Switch 130 may be any type of circuit element capable of equalizing the two complementary nodes. For example, switch 130 may be an NMOSFET, a PMOSFET, or a pass gate that includes an NMOSFET and a PMOSFET coupled in parallel. The type of circuit element used for switch 130 is not a limitation of the present invention.
Switches 132 and 134 couple complementary nodes 136 and 138 to data source 150. When switches 132 and 134 are open, voltages on complementary nodes 136 and 138 are not influenced by data source 150. Instead, the operation of the two pairs of cross-coupled transistors may drive the complementary nodes to near the power supply voltages, or the operation of switch 130 may equalize the voltages on the complementary nodes when transistors 116 and 126 are off.
Capacitors 116 and 118 hold charge and influence how fast voltages may change on complementary nodes 136 and 138. In some embodiments, capacitors 116 and 118 are not lumped elements; instead, they represent parasitic capacitances distributed among the various circuit elements coupled to the complementary nodes. For example, capacitor 116 may represent the capacitance of the gate nodes of transistors 114 and 124, and the capacitance of the drain nodes of transistors 112 and 122. Capacitor 116 may also represent any other capacitance that appears on node 136, regardless of the source of the capacitance.
Data source 150 includes current sources 152 and 156, capacitors 154 and 158, and resistors 162 and 164. Current source 152 provides a current equal to I+Δ/2, and current source 154 provides a current equal to I−Δ/2. The difference between the currents provided by current sources 152 and 156 is equal to Δ. As shown in
In some embodiments, data source 150 models a data source and as a reference circuit. For example, current source 152 and resistor 162 may model a memory cell, and current source 156 and resistor 164 may model a reference circuit to which the memory cell contents are compared. In these embodiments, the voltage on node 166 depends on the value of stored data, and node 166 is referred to as a data dependent node. Various embodiments of data storage devices are described with reference to later figures.
Switch 160 is coupled between nodes 166 and 168. When switch 160 is closed, the voltages on nodes 166 and 168 approach the same value, or “equalize.” In some embodiments, switch 160 is closed to equalize nodes 166 and 168 before latch 110 senses data. Switch 160 may be any type of circuit element capable of equalizing nodes 166 and 168. For example, switch 160 may be an NMOSFET, a PMOSFET, or a pass gate that includes an NMOSFET and a PMOSFET coupled in parallel. The type of circuit element used for switch 160 is not a limitation of the present invention.
In some embodiments, the differential current Δ, or the differential voltage on nodes 166 and 168 is very small. For example, data source 150 may model a memory storage device in which the voltage on node 166 represents data held in a memory storage device, and the voltage on node 168 represents a reference, and the magnitude of the difference may be very small. In these embodiments, latch 110 is capable of detecting minute differences. In some embodiments, switch 160 is utilized to increase the ability of latch circuit 110 to quickly discriminate small differences by equalizing nodes 166 and 168 prior to latch 110 sensing the data. By starting with both nodes 166 and 168 at the same voltage, the time between closing switches 132 and 134 and enabling latch 110 may be reduced because the time required for complementary nodes 136 and 138 to achieve a minimum required input differential may be reduced.
As previously described, data source 150 represents data to be read as well as a reference to which the data should be compared. A data read cycle starts with switches 132 and 134 closed, switches 160 and 130 open, and latch 110 disabled. Latch 110 is disabled when EN is low to decouple latch 110 from lower power supply node 142, or when {overscore (EN)} is high to decouple latch 110 from power supply node 140, or both. Once capacitors 116 and 118 have charged to a point where the voltages on nodes 136 and 138 have enough differential so that latch 110 can correctly discriminate between the two, switches 132 and 134 are opened and latch 110 is enabled by bringing EN high and {overscore (EN)} low. Latch 110 then drives complementary nodes 136 and 138 substantially to the power rails (the voltages present in power supply nodes 140 and 142).
To increase the speed of repeated reads (or the setup time for the first read), as soon as switches 132 and 134 are opened, switch 160 is closed so that the voltages on nodes 166 and 168 can equalize in preparation for the next read. After the data latched by latch 110 is read out of latch 110, latch 110 is disabled and switch 130 is closed to equalize complementary nodes 136 and 138. This may also be referred to as “pre-equalizing” the two complementary nodes. After the two complementary nodes are partially or completely equalized, switch 130 is opened and switches 132 and 134 may be closed with the two complementary nodes causing little or no differential offset to nodes 166 and 168. Further, the common mode offset introduced by closing switches 132 and 134 may be smaller when latch 110 is equalized prior to reconnecting it to data source 150. Given the reduced magnitude of the offset, latch 110 may take less time to sense the data. Latch 110 is then enabled when nodes 136 and 138 have enough voltage differential so that latch 110 can discriminate between the two.
Method 200 is shown beginning at block 210 in which a latch is decoupled from a source of power. Block 210 may correspond to turning off one or more transistors coupled in series between a power supply node and a latch. For example, latch 110 (
At block 220, two complementary nodes within the latch are equalized. This may correspond to closing switch 130 (
At 230, the two complementary nodes within the latch are coupled to two nodes having a differential voltage to be sensed. This may correspond-to closing switches 132 and 134 (
At 240, the latch is coupled to a source of power. This may correspond to transistors 116 and 126 being turned on. When the latch is coupled to a source of power, or “enabled,” the latch senses any voltage differential on the complementary nodes and drives the complementary nodes to near the power rails.
Memory 310 may be any type of memory that provides a current IDATA that varies based on the data held in the memory. For example, memory 310 may be a static random access memory (SRAM), a dynamic random access memory (DRAM), a volatile memory, or a non-volatile memory. In some embodiments, memory 310 includes a floating gate memory such as FLASH memory. Memory 310 may include a single memory cell or multiple memory cells. In some embodiments, memory 310 includes a column of memory cells, and one memory cell is selected for reading at a time. In these embodiments, latch 340 is “shared” among multiple memory cells arranged in a column.
Reference 320 is a reference device that produces a reference current IREF. IREF is compared to IDATA when determining whether memory 310 is storing a “1” or a “0.” In some embodiments, reference 320 includes a current source. The manner in which reference 320 is implemented is not a limitation of the present invention.
Column load 330 produces a voltage on node 312 based on IDATA, and produces a voltage on node 322 based on IREF. Column load 330 may include resistive devices such as resistors 162 and 164 (
Memory device 300 may include a control circuit to control the operation of switches 160, 132, and 134, and also to control the operation of latch 340. The control circuit may be synchronous or asynchronous. For example, a control circuit may include a state machine, a processor interface, or the like. The various embodiments of the present invention are not limited by the existence or non-existence of a control circuit. Further, the various embodiments of the present invention are not limited by the type of control circuit implementation.
Sense amplifiers 440 may include any number of sense amplifiers, and the sense amplifiers may include any number of latches that may be pre-equalized, such as latch 110 (
In some embodiments, memory device 400 is part of an integrated circuit. For example, memory device 400 may be a packaged integrated circuit that includes an interface to allow memory device 400 be used as part of an electronic system. Also for example, memory device 400 may be included in a packaged integrated circuit that also includes other components, functional blocks, or subsystems. Further, in some embodiments, memory device 400 may be part of an unpackaged integrated circuit die that is sold as an unpackaged device.
Integrated circuit 510 may include a radio frequency (RF) receiver, transmitter, or transceiver coupled to antenna 530. For example, in some embodiments, an RF receiver receives a signal from antenna 530 and performs “front end” processing such as low noise amplification (LNA), filtering, frequency conversion or the like.
Integrated circuit 520 includes memory 540. In some embodiments, memory 540 may include latches that may be removed from one or more power sources and pre-equalized. For example, memory 540 may include one or more latches such as latch 110 (
In some embodiments, integrated circuit 520 may be a processor such as a microprocessor, a digital signal processor, a microcontroller, or the like. In some of these embodiments, memory 540 may be a cache memory on the same integrated circuit die. In other embodiments, memory 540 may be a cache memory co-located with a processor such as in a large package or multi-chip module. Memory 540 is shown used with an antenna in system 500. Many other uses for memory 540 exist. For example, memory 540 may be used in systems without an antenna.
Either integrated circuit 510 or 520 may be a processor or an integrated circuit other than a processor such as a radio frequency (RF) receiver, transmitter, or transceiver, an application-specific integrated circuit (ASIC), a communications device, a memory controller, or a memory such as a dynamic random access memory (DRAM). For ease of illustration, portions of integrated circuits 510 and 520 are not shown. The integrated circuits may include much more circuitry than illustrated in
Systems represented by the various foregoing figures can be of any type. Examples of represented systems include computers (e.g., desktops, laptops, handhelds, servers, tablets, web appliances, routers, etc.), wireless communications devices (e.g., cellular phones, cordless phones, pagers, personal digital assistants, wireless network interfaces, etc.), computer-related peripherals (e.g., printers, scanners, monitors, etc.), entertainment devices (e.g., televisions, radios, stereos, tape and compact disc players, video cassette recorders, camcorders, digital cameras, MP3 (Motion Picture Experts Group, Audio Layer 3) players, video games, watches, etc.), and the like.
Latches, switches, transistors, memory arrays, and other embodiments of the present invention can be implemented in many ways. In some embodiments, they are implemented in integrated circuits as part of memory devices. In some embodiments, design descriptions of the various embodiments of the present invention are included in libraries that enable designers to include them in custom or semi-custom designs. For example, any of the disclosed embodiments can be implemented in a synthesizable hardware design language, such as VHDL or Verilog, and distributed to designers for inclusion in standard cell designs, gate arrays, or the like. Likewise, any embodiment of the present invention can also be represented as a hard macro targeted to a specific manufacturing process.
Although the present invention has been described in conjunction with certain embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims.
