Variable duty cycle clock generation circuits and methods and systems using the same
A signal generator generates an output signal with a programmable duty cycle and includes a first buffer which generates in response to an input signal an intermediate signal having a selected edge with a voltage slope selected to vary a length of a selected phase of the output signal. A second buffer having a selected input voltage threshold generates the output signal in response to the intermediate signal.
1. A signal generator for generating an output signs programmable duty cycle comprising:
a first buffer generating in response to an input signal an intermediate signal having a selected edge with a voltage slope selected to vary a length of a selected one of high and low phases of the output signal, the first buffer comprising a variable current buffer;
current control circuitry selectively controlling a current to the variable current buffer for varying the slope of the selected edge of the intermediate signal, the current control circuitry comprising:
a register for storing a value representing a selected duty cycle of the output signal;
a duty cycle detector for monitoring the output signal duly cycle; and
correction circuitry for varying the current to the variable current buffer to compensate for a deviation of the duty cycle monitored by the duty cycle detector from the selected duty cycle; and
a second buffer having a selected input voltage threshold for generating the output signal in response to the intermediate signal.
2. The signal generator of claim 1 wherein the selected edge is a rising edge of the intermediate signal.
3. The signal generator of claim 2 wherein the slope of the rising edge is decreased to lengthen a logic high phase of the output signal.
4. The signal generator of claim 1 wherein the selected edge is a falling edge of the intermediate signal.
5. The signal generator of claim 4 wherein the slope of the falling edge is decreased to lengthen a logic low phase of the output signal.
6. A method of generating an output signal with a selectable duty cycle comprising:
varying a drive current with a variable current buffer and current control circuitry controlling a current to the variable current buffer to vary a selected transition time of a selected edge of an intermediate signal, the transition time selected to vary in time generation of a corresponding edge of the output signal, varying the drive current comprising:
selecting an initial drive current;
monitoring a duty cycle of an initial output signal to detect a deviation from an expected duty cycle; and
in response to detection of a deviation from the expected duly cycle, stepping the drive current in binary weighted steps from the initial drive current to compensate for the deviation;
generating the intermediate signal from an input signal with the drive current; and
generating from the intermediate signal the output signal with a selected one of high and low phases varied by a time interval corresponding to the variation in time of the edge of the output signal.
7. The method of claim 6 wherein varying the drive current comprises decreasing the drive current to lengthen the transition time of the selected edge.
8. The method of claim 6 wherein varying the drive current comprises increasing the drive current to shorten the transition time of the selected edge.
9. The method of claim 6 wherein the selected edge of the intermediate signal is a rising edge and varying a drive current comprises varying a pull-up current for varying the rising edge of the intermediate signal.
10. The method of claim 6 wherein the selected edge of the intermediate signal is a falling edge and varying the drive current comprises varying a pull-down current for varying the falling edge of the intermediate signal.