Clock divide-by-three circuit
A clock divider circuit receives an input clock signal having a first frequency (f) and generates an output signal having a frequency equal to f/N, where N is an odd integer. The clock divider circuit includes an edge counter to count a number of consecutive edges of the input clock signal having a first plurality, and to assert a control signal when a threshold number (N) of consecutive edges has been counted. The clock divider circuit also includes a frequency multiplier to generate an intermediate clock signal having a frequency equal to 2f/N by doubling the frequency of the control signal based at least in part on transitions of the input clock signal, and a frequency divider to generate an output clock signal having a frequency equal to f/N by halving the frequency of the intermediate clock signal.
1. A clock divider circuit, comprising:
an input to receive an input clock signal having a first frequency (f);
an edge counter to count a number of consecutive edges of the input clock signal having a first polarity, and to assert a control signal when a threshold number (N) of consecutive edges has been counted, wherein N is an odd integer;
a frequency multiplier to generate an intermediate clock signal having a frequency equal to 2f/N by doubling the frequency of the control signal based at least in part on transitions of the input clock signal; and
a frequency divider to generate an output clock signal having a frequency equal to f/N by halving the frequency of the intermediate clock signal.
2. The clock divider circuit of claim 1 , wherein the output clock signal has a 50% duty cycle.
3. The clock divider circuit of claim 1 , wherein the edge counter is configured to assert the control signal for a duration equal to
⌊
N
2
⌋
×
T
,
where T is a period of the input clock signal.
4. The clock divider circuit of claim 1 , wherein the frequency multiplier is further configured to:
generate a first delayed control signal by delaying the control signal by a full duty cycle;
generate a second delayed control signal by delaying the control signal by a half duty cycle; and
selectively combine the first delayed control signal with the second delayed control signal, based at least in part on an output of the frequency divider, to produce the intermediate clock signal.
5. The clock divider circuit of claim 3 , wherein the control signal is a differential signal comprising a positive component signal and a negative component signal, the frequency multiplier comprising:
a first flip-flop having an input to receive the positive component signal and an output to provide the first delayed control signal based on transitions of the input clock signal;
a second flip-flop having an input to receive the negative component signal and an output to provide the second delayed control signal based on transitions of an inverted clock signal, wherein the inverted clock signal is complementary to the input clock signal; and
a multiplexer to selectively output one of the first delayed control signal or the second delayed control signal as the intermediate clock signal based on a state of the output clock signal.
6. The clock divider circuit of claim 5 , wherein the first delayed control signal is provided by an inverted output of the first flip-flop, and wherein the second delayed control signal is provided by an inverted output of the second flip-flop.
7. The clock divider circuit of claim 1 , wherein the frequency multiplier is further configured to:
generate a delayed control signal by delaying the control signal by half its period; and
selectively combine the input clock signal with an inverted clock signal to produce the intermediate clock signal based on the control signal and the delayed control signal, wherein the inverted clock signal is complementary to the input clock signal.
8. The clock divider circuit of claim 7 , wherein the frequency multiplier comprises:
a first-in first-out (FIFO) register having an input to receive the control signal and an output to provide an intermediate control signal by delaying the control signal by a duration equal to
⌊
N
2
⌋
×
T
,
where T is a period of the input clock signal;
a flip-flop having an input to receive the intermediate control signal and an output to provide the delayed control signal based on transitions of the inverted clock signal; and
a multiplexer to selectively output one of the input clock signal or the inverted clock signal as the intermediate clock signal based on respective states of the control signal and the delayed control signal.
9. The clock divider circuit of claim 8 , wherein the multiplexer is further configured to:
output the input clock signal as the intermediate clock signal when the delayed control signal is asserted; and
output the inverted clock signal as the intermediate clock signal when the control signal is asserted.
10. The clock divider circuit of claim 1 , wherein frequency divider comprises:
a flip-flop having an input to receive an inverted output signal and an output to provide the output signal based on transitions of the intermediate clock signal, wherein the inverted output signal is complementary to the output clock signal.
11. A method, comprising:
receiving an input clock signal having a first frequency (f);
counting a number of consecutive edges of the input clock signal having a first polarity;
asserting a control signal when a threshold number (N) of consecutive edges has been counted, wherein N is an odd integer;
generating an intermediate clock signal having a frequency equal to 2f/N by doubling the frequency of the control signal based at least in part on transitions of the input clock signal; and
generating an output clock signal having a frequency equal to f/N by halving the frequency of the intermediate clock signal.
12. The method of claim 11 , wherein the output clock signal has a 50% duty cycle.
13. The method of claim 11 , wherein the control signal is asserted for a duration equal to
⌊
N
2
⌋
×
T
,
where T is a period of the input clock signal.
14. The method of claim 11 , wherein generating the intermediate clock signal comprises:
generating a first delayed control signal by delaying the control signal by a full duty cycle;
generating a second delayed control signal by delaying the control signal by a half duty cycle; and
selectively combining the first delayed control signal with the second delayed control signal, based at least in part on an output of the frequency divider, to produce the intermediate clock signal.
15. The method of claim 13 , wherein the control signal is a differential signal comprising a positive component signal and a negative component signal, and wherein generating the intermediate clock signal comprises:
generating the first delayed control signal by delaying the positive component signal based on transitions of the input clock signal;
generating the second delayed control signal by delaying the negative component signal based on transitions of an inverted clock signal, wherein the inverted clock signal is complementary to the input clock signal; and
selectively providing one of the first delayed control signal or the second delayed control signal as the intermediate clock signal based on a state of the output clock signal.
16. The method of claim 15 , wherein the first delayed control signal is complementary to the delayed positive component signal, and wherein the second delayed control signal is complementary to the delayed negative component signal.
17. The method of claim 11 , wherein generating the intermediate clock signal comprises:
generating a delayed control signal by delaying the control signal by half its period; and
selectively combining the input clock signal with an inverted clock signal to produce the intermediate clock signal based on the control signal and the delayed control signal, wherein the inverted clock signal is complementary to the input clock signal.
18. The method of claim 17 , wherein generating the intermediate clock signal comprises:
generating an intermediate control signal by delaying the control signal by a duration equal to
⌊
N
2
⌋
×
T
,
where T is a period of the input clock signal;
generating the delayed control signal by delaying the intermediate control signal based on transitions of the inverted clock signal; and
selectively providing one of the input clock signal or the inverted clock signal as the intermediate clock signal based on respective states of the control signal and the delayed control signal.
19. The method of claim 18 , wherein the selectively providing comprises:
outputting the input clock signal as the intermediate clock signal when the delayed control signal is asserted; and
outputting the inverted clock signal as the intermediate clock signal when the control signal is asserted.
20. The method of claim 16 , wherein generating the output clock signal comprises:
generating the output clock signal by delaying an inverted output signal based on transitions of the intermediate clock signal, wherein the inverted output signal is complementary to the output clock signal.