Frequency spread spectrum (FSS) oscillator
In examples, an oscillator comprises a reference voltage generator circuit configured to increase and decrease a reference voltage responsive to each pulse of a clock signal based on a comparison of the reference voltage to upper and lower boundaries. The oscillator also comprises an oscillation circuit coupled to the reference voltage generator circuit and configured to provide the clock signal, the clock signal having a frequency that varies based on the reference voltage.
1 . An oscillator, comprising: a reference voltage generator circuit configurable to increase and decrease a reference voltage responsive to each pulse of a clock signal and a comparison of the reference voltage to a first voltage and a second voltage; and an oscillation circuit coupled to the reference voltage generator circuit and including a voltage generator, the voltage generator resettable by each pulse of the clock signal, and the oscillation circuit configurable to provide the clock signal responsive to a comparison between a third voltage at an output of the voltage generator and the reference voltage.
2 . The oscillator of claim 1 , wherein a frequency of the clock signal varies inversely with the reference voltage.
3 . The oscillator of claim 1 , wherein the reference voltage generator circuit comprises:
a comparator having a comparator output and first and second comparator inputs; and
an inverter having an inverter output and an inverter input, the inverter input coupled to the comparator output.
4 . The oscillator of claim 3 , wherein the inverter is configurable to pull the reference voltage on the first comparator input toward a voltage supply.
5 . The oscillator of claim 4 , wherein the comparator is configured to pull the voltage on the first comparator input toward ground.
6 . The oscillator of claim 3 , wherein the reference voltage is a first reference voltage, and wherein the second comparator input is configured to alternately receive the first and second voltages.
7 . The oscillator of claim 3 , wherein the second comparator input is coupled to a first switch having a first switch control terminal coupled to the inverter output and to a second switch having a second switch control terminal coupled to the comparator output.
8 . The oscillator of claim 3 , further comprising:
a current mirror having a first transistor coupled to a voltage supply and a second transistor coupled to a ground terminal;
a first switch coupled to the first comparator input and to a connection;
a second switch coupled to the first transistor and to the connection, the second switch having a second switch control terminal coupled to the inverter output; and
a third switch coupled to the second transistor and to the connection, the third switch having a third switch control terminal coupled to the comparator output.
9 . The oscillator of claim 8 , further comprising a capacitor coupled to the comparator output, to the first switch, and to the ground terminal.
10 . An oscillator, comprising:
a reference voltage generator circuit configurable to provide a first reference voltage across a first capacitor, the reference voltage generator circuit comprising:
a first comparator configurable to perform a comparison to compare the first reference voltage to second and third reference voltages and to provide a first comparator output signal based on the comparison; and
first, second, and third switches coupled to the first comparator, the first and second switches configurable to increase the first reference voltage based on the comparison, and the first and third switches configurable to decrease the first reference voltage based on the comparison, the first switch operable by a clock signal; and
an oscillation circuit coupled to the reference voltage generator circuit and comprising:
a second comparator coupled to a second capacitor, the second comparator configurable to compare a voltage across the second capacitor to the first reference voltage to provide a second comparator output signal;
a third capacitor configurable to be charged based on the second comparator output signal;
an inverter configurable to provide the clock signal by inverting a voltage across the third capacitor; and
a fourth switch configurable to control a voltage across the second capacitor based on the clock signal.
11 . The oscillator of claim 10 , further comprising a current mirror coupled to a voltage supply and to a ground terminal, the second switch coupled to the voltage supply and to a connection, the third switch coupled to the ground terminal and to the connection, and the first switch coupled to the connection and to an input of the first comparator.
12 . The oscillator of claim 11 , wherein the first capacitor is coupled to the input of the first comparator, the first switch, and the ground terminal.
13 . The oscillator of claim 11 , wherein:
the inverter is a first inverter;
the second switch has a second switch control terminal coupled to an output of a second inverter; and
the third switch has a third switch control terminal coupled to an output of the first comparator, the output of the first comparator coupled to an input of the second inverter.
14 . The oscillator of claim 10 , further comprising a current mirror coupled to a voltage supply, wherein an input to the second comparator is coupled to the voltage supply by way of the current mirror, the input to the second comparator also coupled to the second capacitor.
15 . The oscillator of claim 14 , further comprising a fifth switch coupled in parallel to the third capacitor and coupled to an output of the second comparator.
16 . The oscillator of claim 15 , wherein the inverter has an inverter output and an inverter input, the inverter input coupled to the third capacitor, the inverter output coupled to the fourth switch, the fourth switch coupled in parallel with the second capacitor.
17 . An oscillator, comprising:
a first comparator including a first comparator output and first and second comparator inputs, the first comparator input switchably coupled to multiple reference voltage terminals;
a first switch coupled to the second comparator input;
second and third switches coupled to each other and to the first switch, the second switch coupled to a power supply and the third switch coupled to a ground terminal;
a first capacitor coupled between the first switch and the second comparator input;
a first inverter having a first inverter output and a first inverter input, the first inverter input coupled to the first comparator output, the first inverter output coupled to the second switch;
a second comparator including a second comparator output and third and fourth comparator inputs, the third comparator input coupled to the power supply, the fourth comparator input coupled to the second comparator input;
a second capacitor coupled between the third comparator input and the ground terminal;
a fourth switch coupled in parallel to the second capacitor;
a fifth switch coupled to the second comparator output;
a third capacitor coupled in parallel to the fifth switch, the third capacitor and the fifth switch coupled to the power supply; and
a second inverter having a second inverter output and a second inverter input, the second inverter output coupled to the fourth switch and the second inverter input coupled to the fifth switch and the third capacitor.
18 . The oscillator of claim 17 , further comprising a current mirror through which the second switch is coupled to the power supply and through which the third switch is coupled to the ground terminal.
19 . The oscillator of claim 17 , wherein the oscillator is configurable to increase and decrease a reference voltage at each pulse of a clock signal based on a comparison of the reference voltage to one of a pair of secondary reference voltages at the multiple reference voltage terminals.
20 . The oscillator of claim 19 , wherein the oscillator is configurable to generate the clock signal to have a frequency that varies according to the reference voltage.