High-speed programmable frequency divider with 50% output duty cycle

Abstract

A frequency divider includes a multiplexer having a first input terminal coupled to receive a first value M and a second input terminal for receiving a second value that is M+LSB, the multiplexer is configured to alternately output the first value M and the second value. The frequency divider includes a multi-modulus divider coupled to the multiplexer for receiving the output of the multiplexer, the multi-modulus divider operable to alternately generate an output pulse at M input clock cycles and at M+LSB clock cycles. A divide-by-two counter having an input coupled to the output of the multi-modulus divider, is operable to divide the output of the multi-modulus divider to generate a divided clock signal having a frequency of N, where N is equal to 2M+LSB. Duty cycle correction logic is coupled to the output of the divide-by-two counter and is configured to correct the duty cycle of the divided clock signal to a fifty percent duty cycle when N is odd.

Description

BACKGROUND OF THE INVENTION

Programmable frequency dividers are widely deployed in applications such as clocking and frequency synthesis. Programmable frequency dividers have been developed that have a 50% duty cycle and that operate at relatively high clock speeds. However, there is a need for programmable frequency dividers that operate at even higher clock speeds. While programmable frequency dividers are relatively easy to design that either operate at high input frequencies (multi-GHZ) or that have 50% output duty cycle, it is difficult to design a programmable frequency divider that both operates at multi-GHZ clock frequencies and has a 50% duty cycle. This is especially true when the division ratio (divisor) is odd because the frequency divider must be able to count in half-cycles to achieve a 50% duty cycle. Frequency dividers that count in half cycles are more complex, use more die area, and require more power than frequency dividers that do not count in half-cycles.

Accordingly, there is a need for a programmable frequency divider that can operate at high input frequencies, that has a 50% output duty cycle and that does not have the complexity and power requirements of conventional frequency dividers that count in half cycles.

SUMMARY OF THE INVENTION

A frequency divider is disclosed that includes a multiplexer having a first input terminal coupled to receive a first value M and a second input terminal for receiving a second value that is M plus a least significant bit (LSB), the multiplexer configured to alternately output the first value M and the second value M+LSB. The frequency divider includes a multi-modulus divider coupled to the multiplexer for receiving the output of the multiplexer and has a clock input for receiving a clock signal. The multi-modulus divider is operable to alternately generate an output pulse at M input clock cycles and at M+LSB clock cycles. A divide-by-two counter having an input coupled to the output of the multi-modulus divider is operable to divide the output of the multi-modulus divider to generate a divided clock signal having a frequency of N, where N is equal to 2M+LSB. Duty cycle correction logic that is coupled to the output of the divide-by-two counter is configured to correct the duty cycle of the divided clock signal to a fifty percent duty cycle when N is odd.

A method for generating a divided clock signal having a 50% duty cycle is disclosed that includes alternately dividing an input clock signal by a divisor M and a divisor M plus a least significant bit (LSB) to alternate periodically between generating an output pulse at M input clock cycles and M+LSB input clock cycles. The generated output pulse is divided using the divide-by-two counter to generate a divided clock signal having a frequency of N. The duty cycle of the divided clock signal is corrected when the LSB is odd to generate a corrected divided clock signal having a fifty percent duty cycle.

The method and apparatus of the present invention provide a high-speed programmable frequency divider with a 50% duty cycle having a simplified architecture and reduced power consumption as compared to conventional programmable frequency dividers that require counting in half cycles.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of a frequency divider in accordance with an embodiment of the present invention.

FIG. 2 is a block diagram showing a method for generating a divided clock signal having a 50% duty cycle in accordance with an embodiment of the present invention.

FIG. 3 is an illustration of a frequency divider having duty cycle correction logic that includes an AND gate in accordance with an embodiment of the present invention.

FIG. 4 is a waveform diagram showing signals generated by an embodiment in which N is 25 in accordance with an embodiment of the present invention.

FIG. 5 is an illustration of a frequency divider having duty cycle correction logic that includes an OR gate in accordance with an embodiment of the present invention.

DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention.

FIG. 1 shows a frequency divider 10 that divides by counting a total of N cycles of the input clock signal, where N is the divisor. Any integer can be written as N=2M+LSB. If N is an odd number, then LSB is 1 and if N is an even number the LSB is 0. N and M can be obtained in binary form without any logic operation as the LSB is the last (least significant) digit of N and M consists of the remaining digits. For example, if N=7, then N=2×3+1. In binary form N is bill, where the LSB is 1 and M is the left most two digits (3).

Frequency divider 10 includes adder 2 and a multiplexer 3 that is electrically coupled to adder 2. Adder 2 has a first input for receiving a first value M and a second input for receiving a least significant bit (LSB) and is operable to generate a second value at the output of the adder that is the sum of the first value M and the LSB. Adder 2 can be synthesized by standard logic cells that calculate MSB+LSB and store the value M+LSB as one of the inputs of multiplexer 3. In the present embodiment, after calculating M+LSB, adder 2 remains idle to reduce power consumption and only recalculates if the divisor changes.

Multiplexer 3 has a first input terminal coupled to receive the first value M (first value) and having a second input terminal electrically coupled to the output of the adder for receiving M+LSB (the second value) and is operable in response to input at a select terminal to alternately output the first value M and the second value M+LSB.

Frequency divider 10 also includes a multi-modulus divider 4 that is electrically coupled to multiplexer 3 for receiving the output of multiplexer 3. Multi-modulus divider 4 includes a clock input for receiving an input clock signal Clk_in signal and an input for receiving the complement of the input clock signal. Multi-modulus divider 4 can be a conventional high-speed programmable integer-N divider such as, for example, a simple loadable digital counter that is operable to alternately generate an output pulse at M input clock cycles and at M+LSB cycles of the reference clock signal Clk_in. Multi-modulus divider 4 does not include complex half-cycle counting circuitry or duty cycle correction circuitry, providing for fast and efficient generation of output pulses having a desired frequency.

Divide-by-two counter 5 has an input coupled to the output of the multi-modulus divider 4 and is operable to divide the output of multi-modulus divider 4 to generate a divided clock signal Clk₁ having a frequency of N, where N is equal to 2M+LSB. The output of the divide-by-2 counter is electrically coupled to the select terminal of multiplexer 3 such that multiplexer 3 receives the divided clock signal Clk₁ for controlling the operation of multiplexer 3.

The divide-by-two counter 5 serves two purposes, first, its output selects the input of multiplexer 3 to alternate the divisor for multi-modulus-divider 4 and secondly it produces the correct output frequency since 2X(M+LSB/2)=2M+LSB=N.

Duty cycle correction logic 6 is electrically coupled to the output of divide-by-two counter 5 and receives as input the input clock signal, the complement of the input clock signal and the LSB. Duty cycle correction logic 6 is configured to correct the duty cycle of the divided clock signal to a fifty percent duty cycle when N is odd to generate an output clock signal Clk_out.

FIG. 2 illustrates a method 100 for generating a divided clock signal having a 50% duty cycle that includes receiving 101 an input clock signal and alternately dividing 102 the input clock signal by a divisor M and a divisor M plus a least significant bit (LSB) to alternate periodically between generating an output pulse at M input clock cycles and M+LSB input clock cycles.

As shown by step 103 the generated output pulse is divided using a divide-by-two counter to generate a divided clock signal having a frequency of N, where N=2M+LSB. In one embodiment the divided clock signal has an output that is high for M+1 cycles and low for M cycles. The duty cycle of the divided clock signal is corrected 104-105 by duty-cycle-correction-logic 6 when the LSB is odd to generate a corrected divided clock signal having a fifty percent duty cycle. When the LSB is even, the divided clock signal is not corrected and becomes the output clock signal as shown by steps 104 and 106.

In one embodiment that is illustrated in FIG. 3 a programmable frequency divider 10a is shown in which divide-by-two counter 5 further comprises a flip-flop 22 and an inverter 21 coupled to the flip-flop 22. Inverter 21 has an input coupled to receive the divided clock signal Clk₁ and has an output coupled to the D input of flip-flop 22. Flip-flop 22 has a clock input coupled to receive the output of the multi-modulus divider 4, the flip-flop 22 operable to generate the divided clock signal Clk₁.

In this embodiment, flip-flop 22 will produce a divided clock signal having a 50% duty cycle when N is even and an output having a near-50% duty cycle when N is odd. More particularly, when N is odd the output will have a pulse width that is slightly larger than 50% because the output is high for M+1 input cycles and low for M input cycles.

In this embodiment duty cycle correction logic 6 includes AND gate 25, a first flip-flop 23 and a second flip-flop 24. The output of flip-flop 22 is electrically coupled to the D input of flip-flop 23 and the output of first flip-flop 23 is coupled to a first input of AND gate 25 and a reset terminal of flip-flop 24 is electrically coupled to receive the LSB. The complement of the output of the first flip-flop 23 is coupled to a D input of second flip-flop 24 and the complement of the output of second flip-flop 24 is electrically coupled to the second input of the AND gate 25. Flip-flop 23 is clocked by the input clock signal Clk_in and flip-flop 24 is clocked by the complement of the input clock signal such that flip-flops 23-24 are clocked at a half an input period apart from each other.

When N is even, LSB is 0 and both inputs to the multiplexer 3 will be the same such that multi-modulus divider 4 will divide by the same number M each time, producing at the output of divide-by-two counter 5 a divided clock signal having a 50% duty cycle, that will pass through flip-flop 23 after synchronization such that the output of flip-flop 23 becomes Clk_out.

When N is odd, LSB is 1, flip-flop 23 and flip-flop 24 that can be D-type flip-flops are operable for coupling their outputs that are half of a clock cycle apart from each other to AND gate 25 so as to generate an output clock signal having the desired 50% duty cycle.

FIG. 4 illustrates an exemplary embodiment in which N=25 and in which an input clock signal (Clk_in) 41 having a frequency of 2.5 GHz is used to generate at the output of multi-modulus divider 4 an output signal 42 alternately having an output pulse at M input clock cycles and at M+LSB clock cycles. More particularly, multi-modulus divider 4 will divide first by 13, then by 12, followed by 13, etc. Flip-flop 23 generates output Q 43 and flip-flop 24 generates a corresponding output 44 that is coupled to AND gate 25 so as to generate output signal 45 at the output of AND gate 25 having the required 50% duty cycle.

In an alternate embodiment that is shown in FIG. 5 a programmable frequency divider 10b is shown that includes an OR gate 55. More particularly, the output of first flip-flop 23 is coupled to a first input of OR gate 55 and to the D-input of second flip-flop 24. The output of second flip-flop 24 is electrically coupled to the second input of the OR gate 55. As in the previous embodiment, flip-flop 23 is clocked by the input clock signal Clk_in and flip-flop 24 is clocked by the complement of the input clock signal such that flip-flops 23-24 are clocked at a half an input period apart from each other.

Continuing with FIG. 5, when N is even, LSB is 0 and both inputs to the multiplexer 3 will be the same such that the multi-modulus divider will divide by the same number M each time, producing an output at the divide-by-two counter having a 50% duty that will pass through flip-flop 23 after synchronization such that the output of flip-flop 23 becomes Clk_out. When N is odd, LSB is 1 and flip-flop 23 and flip-flop 24 are operable to couple their outputs that are half of a clock cycle apart from each other to OR gate 55 so as to generate an output clock signal having the desired 50% duty cycle.

The method and apparatus of the present invention provides a frequency divider having a simplified architecture that does not require counting in half-cycles as do conventional programmable frequency dividers. More particularly, multi-modulus divider 4 and divide-by-two counter 5 operate on full clock cycles, and therefore have a simplified architecture as compared to corresponding structures of frequency dividers that require counting in half cycles. Accordingly, the frequency divider 10 of the present invention provides for reduced die area and lower power consumption than conventional frequency dividers that require counting in half cycles. Moreover, by not requiring counting in half cycles, the method and apparatus of the present invention allows for higher speed operation.

In the present embodiment, frequency divider 10, 10a, 10b are an Application-Specific Integrated Circuit (ASIC) devices formed on a single semiconductor die. Accordingly, adder 2, multiplexer 3, multi-modulus divider 4, divide-by-two counter 5 and duty cycle correction logic 6 are disposed on a single semiconductor die.

As is known in the art, the methods and apparatus of the present invention may be implemented in a Field Programmable Gate Array (FPGA), an Application-Specific Integrated Circuit (ASIC) or a variety of other commonly known integrated circuit devices. The implementation of the invention may include both hardware and software components.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.

Claims

1. An integrated circuit device comprising: a multiplexer having a first input terminal coupled to receive a first integer value (M) and a second input terminal for receiving a second integer value that is M plus a least significant bit (LSB), the multiplexer configured to alternately output M and the second integer value;a multi-modulus divider coupled to the multiplexer for receiving the output of the multiplexer and having a clock input for receiving a clock signal, the multi-modulus divider operable to alternately generate an output pulse at M input clock cycles and at M+LSB clock cycles;a divide-by-two counter having an input coupled to the output of the multi-modulus divider, the divide-by-two counter operable to divide the output of the multi-modulus divider to generate a divided clock signal having a frequency equal to 2M+LSB; andduty cycle correction logic coupled to the output of the divide by two counter, the duty cycle correction logic configured to correct the duty cycle of the divided clock signal to a fifty percent duty cycle when the LSB is odd.

2. The integrated circuit device of claim 1 further comprising an adder having an output coupled to the multiplexer, a first input for receiving M and a second input for receiving the LSB, the adder operable to add M and the LSB.

3. The integrated circuit device of claim 1 wherein a select terminal of the multiplexer is coupled to the output of the divide-by-two counter for receiving the divided clock signal.

4. The integrated circuit device of claim 3 wherein the divided clock signal has an output that is high for M+1 cycles and low for M cycles when the frequency is odd.

5. The integrated circuit device of claim 3 wherein the divide-by-two counter further comprises a flip-flop and an inverter coupled to the flip flop, the inverter having an input coupled to receive the divided clock signal and having an output coupled to an input of the flip-flop, the flip-flop having a clock input coupled to receive the output of the multi-modulus divider, the flip-flop operable to generate the divided clock signal.

6. The integrated circuit device of claim 3 wherein the duty cycle correction logic further comprises an OR gate, a first flip-flop and a second flip-flop, the first flip-flop coupled to the second flip-flop, wherein the first flip-flop is clocked by the input clock signal and the second flip-flop is clocked by the complement of the input clock signal and further wherein the output of the first flip-flop is coupled to a first input of the OR gate and the output of the second flip-flop is coupled to a second input of the OR gate.

7. The integrated circuit device of claim 3 wherein the duty cycle correction logic further comprises an AND gate, a first flip-flop and a second flip-flop, the first flip-flop coupled to the second flip-flop, wherein the first flip-flop is clocked by the input clock signal and the second flip-flop is clocked by the complement of the input clock signal.

8. The integrated circuit device of claim 7 wherein the output of the first flip-flop is coupled to a first input of the AND gate and the complement of the output of the first flip-flop is coupled to an input of the second flip-flop, and wherein the complement of the output of the second flip-flop is coupled to the second input of the AND gate.

9. A method comprising: alternately dividing an input clock signal by a first divisor having a first integer value (M) M and a second divisor having a value of M plus a least significant bit (LSB) to alternate periodically between generating an output pulse at M input clock cycles and M+LSB input clock cycles;dividing the generated output pulse using a divide-by-two counter to generate a divided clock signal having a frequency equal to 2M+LSB; andcorrecting the duty cycle of the divided clock signal when the LSB is odd to generate a corrected divided clock signal having a fifty percent duty cycle.

10. The method of claim 9 wherein the divided clock signal has an output that is high for M+1 cycles and low for M cycles when the LSB is odd.

11. The method of claim 9 wherein the correcting the duty cycle further comprises correcting the duty cycle using duty cycle correction logic that includes a first flip-flop and a second flip-flop that are clocked at half of a clock cycle apart from each other, and that includes an AND gate coupled to the output of the first flip-flop and to the complement of the output of the second flip-flop.

12. A frequency divider comprising: an adder having a first input for receiving a first integer value (M) and a second input for receiving a least significant bit (LSB), the adder operable to generate a second integer value at the output of the adder that is the sum of M and the LSB;a multiplexer having a first input terminal coupled to receive M and having a second input terminal electrically coupled to the output of the adder for receiving the second integer value, the multiplexer configured to alternately output M and the second integer value;a multi-modulus divider coupled to the multiplexer for receiving the output of the multiplexer and having a clock input for receiving a clock signal, the multi-modulus divider operable to alternately generate an output pulse at M input clock cycles and at M+LSB clock cycles;a divide-by-two counter having an input coupled to the output of the multi-modulus divider, the divide-by-two counter operable to divide the output of the multi-modulus divider to generate a divided clock signal having a frequency equal to 2M+LSB; andduty cycle correction logic coupled to the output of the divide by two counter, the duty cycle correction logic configured to correct the duty cycle of the divided clock signal to a fifty percent duty cycle when the LSB is odd.

13. The frequency divider of claim 12 wherein a select terminal of the multiplexer is coupled to the output of the divide-by-two counter for controlling the operation of the multiplexer.

14. The frequency divider of claim 12 wherein the divided clock signal has an output that is high for M+1 cycles and low for M cycles when the LSB is odd.

15. The frequency divider of claim 12 wherein the divide-by-two counter further comprises a flip-flop and an inverter coupled to the flip flop, the inverter having an input coupled to receive the divided clock signal and having an output coupled to an input of the flip-flop, the flip-flop having a clock input coupled to receive the output of the multi-modulus divider, the flip-flop operable to generate the divided clock signal.

16. The frequency divider of claim 12 wherein the duty cycle correction logic further comprises an OR gate, a first flip-flop and a second flip-flop, the first flip-flop coupled to the second flip-flop, wherein the first flip-flop is clocked by the input clock signal and the second flip-flop is clocked by the complement of the input clock signal and further wherein the output of the first flip-flop is coupled to a first input of the OR gate and the output of the second flip-flop is coupled to a second input of the OR gate.

17. The frequency divider of claim 12 wherein the duty cycle correction logic further comprises an AND gate, a first flip-flop and a second flip-flop, the first flip-flop coupled to the second flip-flop, wherein the first flip-flop is clocked by the input clock signal and the second flip-flop is clocked by the complement of the input clock signal.

18. The frequency divider of claim 17 wherein the output of the first flip-flop is coupled to a first input of the AND gate and the complement of the output of the first flip-flop is coupled to an input of the second flip-flop, and wherein the complement of the output of the second flip-flop is coupled to the second input of the AND gate.