Duty cycle calibration for phase-locked loop
Technologies directed to phase-locked loop (PLL) duty cycle calibration are described. A circuit includes an amplifier to generate a level-shifted clock with a duty cycle. The circuit further includes comparison logic coupled to an output of the amplifier that compares the duty cycle to a target duty cycle and generates a difference value. Tuning logic coupled to the amplifier adjust the duty cycle based on the difference value.
1 . A circuit, comprising:
an amplifier to receive an oscillator clock with a first duty cycle and output a level-shifted clock with a second duty cycle;
tuning logic coupled to the amplifier; and
comparison logic coupled between an output of the amplifier and the tuning logic, wherein the comparison logic is to generate a difference value based on a difference between the second duty cycle and a target duty cycle;
wherein the tuning logic is to adjust, based on the difference value, the second duty cycle generated by the amplifier; and
wherein the comparison logic comprises a low pass filter (LPF) to receive the level-shifted clock and generate a first voltage proportional to the second duty cycle, and wherein the comparison logic is to compare the first voltage to a reference voltage associated with the target duty cycle.
2 . The circuit of claim 1 , wherein the first voltage generated by the LPF is a first portion of a supply voltage, and wherein the first portion corresponds to the second duty cycle.
3 . The circuit of claim 1 , wherein the comparison logic further comprises:
a low-offset comparator comprising a first input terminal and a second input terminal, wherein the low-offset comparator is to compare voltages on the first input terminal and the second input terminal; and
a switching circuit coupled between the LPF and the low-offset comparator, wherein the switching circuit is configured to alternately switch the first voltage and the reference voltage between the first input terminal and the second input terminal.
4 . The circuit of claim 1 , wherein the target duty cycle comprises between 48 to 52 percent.
5 . The circuit of claim 1 , further comprising a feedback resistor coupled between the output and an input of the amplifier, the feedback resistor being tapped to receive a bias voltage, and wherein the tuning logic is configured to generate the bias voltage based on the difference value.
6 . The circuit of claim 1 , further comprising a feedback resistor coupled between the output and an input of the amplifier, the feedback resistor being tapped to receive a bias voltage, and wherein a location where the feedback resistor is tapped is adjusted based on the difference value.
7 . A phase-locked loop (PLL), comprising:
an oscillator to generate an oscillator clock with a first duty cycle;
an amplifier coupled to the oscillator, wherein the amplifier is to receive the oscillator clock and output a level-shifted clock with a second duty cycle;
tuning logic coupled to the amplifier; and
comparison logic coupled between an output of the amplifier and the tuning logic, wherein the comparison logic is to generate a difference value based on a difference between the second duty cycle and a target duty cycle;
wherein the tuning logic is to adjust, based on the difference value, the second duty cycle generated by the amplifier; and
wherein the comparison logic comprises a low pass filter (LPF) to receive the level-shifted clock and generate a first voltage proportional to the second duty cycle, and wherein the comparison logic is to compare the first voltage to a reference voltage associated with the target duty cycle.
8 . The PLL of claim 7 , wherein the first voltage generated by the LPF is a first portion of a supply voltage, and wherein the first portion corresponds to the second duty cycle.
9 . The PLL of claim 7 , wherein the comparison logic further comprises:
a low-offset comparator comprising a first input terminal and a second input terminal, wherein the low-offset comparator is to compare voltages on the first input terminal and the second input terminal; and
a switching circuit coupled between the LPF and the low-offset comparator, wherein the switching circuit is configured to alternately switch the first voltage and the reference voltage between the first input terminal and the second input terminal.
10 . The PLL of claim 7 , wherein the target duty cycle comprises between 48 to 52 percent.
11 . The PLL of claim 7 , further comprising a feedback resistor coupled between the output and an input of the amplifier, the feedback resistor being tapped to receive a bias voltage, and wherein the tuning logic is configured to generate the bias voltage based on the difference value.
12 . The PLL of claim 7 , further comprising a feedback resistor coupled between the output and an input of the amplifier, the feedback resistor being tapped to receive a bias voltage, and wherein a location where the feedback resistor is tapped is adjusted based on the difference value.
13 . The PLL of claim 7 , wherein the oscillator is a ring oscillator.
14 . A method of operating a phase-locked loop (PLL) circuit comprising an amplifier, an oscillator coupled to the amplifier, tuning logic coupled to the amplifier, and comparison logic coupled between the amplifier and tuning logic, the method of operating the PLL circuit comprising:
generating, by the oscillator, an oscillating clock with a first duty cycle;
receiving, by the amplifier, the oscillating clock;
outputting, by the amplifier, a level-shifted clock with a second duty cycle;
receiving, by the comparison logic, the level-shifted clock;
generating, by the comparison logic, a difference value based on a comparison of the second duty cycle to a target duty cycle, wherein generating the difference value comprises:
inputting the level-shifted clock to a low pass filter (LPF),
outputting, by the LPF, a first voltage proportional to the second duty cycle, and
comparing the first voltage to a reference voltage associated with the target duty cycle; and
adjusting, by the tuning logic, the second duty cycle generated by the amplifier towards the target duty cycle.
15 . The method of claim 14 , wherein the comparison logic further comprises a switching circuit and a low-offset comparator comprising a first input terminal and a second input terminal, and wherein comparing the first voltage to the reference voltage comprises:
comparing, by the low-offset comparator, voltages on the first input terminal and the second input terminal;
receiving, by the switching circuit, the first voltage and the reference voltage; and
alternately switching, by the switching circuit, the first voltage and the reference voltage between the first input terminal and the second input terminal.
16 . The method of claim 14 , wherein the target duty cycle comprises between 48 to 52 percent.
17 . The method of claim 14 , wherein the PLL circuit further comprises a feedback resistor coupled between the output and an input of the amplifier, the feedback resistor being tapped to receive a bias voltage, and wherein adjusting the second duty cycle towards the target duty cycle comprises generating, by the tuning logic, the bias voltage based on the difference value.