Accurate RC oscillator having peak - to - peak voltage control

• US 5,670,915 A
• Filed: 05/24/1996
• Issued: 09/23/1997
• Est. Priority Date: 05/24/1996
• Status: Expired due to Term

• Current Assignee: Microchip Technology Incorporated
• Original Assignee: Microchip Technology Incorporated
• Inventors: Ellison, Scott, Cooper, Russell E.
• Primary Examiner(s): Grimm, Siegfried H.
• Application Number:
  US08/644,917
• Time in Patent Office: 487 Days
• Field of Search: 331/108 D, 331/111, 331/143, 331/108 C
• US Class Current: 331/111
• CPC Class Codes: H03K 3/0231 Astable circuits H03K3/0315 takes precedence

Abstract

An accurate RC oscillator circuit located within a microcontroller chip for generating a signal of a predetermined frequency that accurately oscillates between two precise voltage levels, i.e., a low voltage (VL) and a high voltage (VH) is disclosed. The oscillator circuit uses first and second comparators having their outputs respectively coupled to set and reset inputs of a flip flop. The output of the flip flop is coupled to a series RC network for controlling the charging and discharging of the voltage across a capacitor of the RC network. The interconnection of the series RC network is coupled to an input of both the first and second comparators. The other input of the first comparator is coupled to a circuit for applying a modified high threshold version (VH'"'"') of the high voltage such that the signal generated by the oscillator circuit does not exceed the precise high voltage (VH). Similarly, the other input of the second comparator is coupled to a circuit for applying a modified low threshold version (VL'"'"') of the low voltage such that the signal generated by the oscillator circuit does not fall below that precise low voltage (VL). Additionally, means are provided to select different input voltages for the low voltage (VL) such that the desired output frequency of the oscillator may be adjusted to accurately oscillate between the high voltage (VH) and the selected low voltage (VL).

Claims

1. A circuit for generating an oscillatory signal of a predetermined frequency across a series resistor-capacitor (RC) network by ensuring that the oscillatory signal accurately oscillates between first and second voltages generated from a supply voltage source where the frequency of oscillation is determined by both the time constant of the RC network and the difference between the first and second voltages, comprising, in combination:

charging-discharging means for discharging a voltage across the capacitor commencing when voltage of the oscillatory signal exceeds a first threshold voltage and for charging said voltage across the capacitor commencing when voltage of the oscillatory signal falls below a second threshold voltage;

first sampling means coupled to said charging-discharging means for obtaining a first sampled voltage of the oscillatory signal upon commencement of discharging of said voltage across the capacitor;

first compensation means coupled to said first sampling means for adjusting said first threshold voltage to be the first voltage modified by a voltage difference between the first voltage and said first sampled voltage;

second sampling means coupled to said charging-discharging means for obtaining a second sampled voltage of the oscillatory signal upon commencement of charging of said voltage across the capacitor;

second compensation means coupled to said second sampling means for adjusting said second threshold voltage to be the second voltage modified by a voltage difference between the second voltage and said second sampled voltage; and

differential voltage setting means coupled to said second compensation means for selecting the second voltage from a plurality of different possible voltages and thereby selecting a voltage difference between the first and second voltages.

2. The circuit of claim 1 wherein said charging-discharging means comprises a flip flop having an output thereof coupled to the series RC network for controlling the charging and discharging of said voltage across the capacitor.

3. The circuit of claim 1 wherein said first sampling means comprises:

a negatively-triggered pulse generator having an input connected to an output of said charging-discharging means; and

first switch means coupled to an output of said negatively triggered pulse generator for momentarily coupling, in response to receipt of a pulse from said negatively-triggered pulse generator, said first compensation means to said first sampled voltage of the oscillatory signal.

4. The circuit of claim 1 wherein said second sampling means comprises:

a positively-triggered pulse generator having an input connected to an output of said charging-discharging means; and

second switch means coupled to an output of said positively-triggered pulse generator for momentarily coupling, in response to receipt of a pulse from said positively-triggered pulse generator, said second compensation means to said second sampled voltage of the oscillatory signal.

5. The circuit of claim 2 further comprising first and second comparators having their outputs respectively coupled to first and second inputs of said flip flop for setting and resetting said flip flop, said first comparator having a connection from its non-inverting input to a node between a resistor and the capacitor of the RC network, and said second comparator having a connection from its inverting input to said node.

6. The circuit of claim 5 wherein said first compensation means comprises:

an amplifier having first and second inputs and an output, said first input of said amplifier being coupled to receive the first voltage, and said output of said amplifier being coupled to an inverting input of said first comparator; and

a capacitor coupled at one end to the output of the amplifier and coupled at the other end to the second input of the amplifier.

7. The circuit of claim 6 wherein said second compensation means comprises:

an amplifier having first and second inputs and an output, said first input of said amplifier being coupled to receive the second voltage; and

said output of said amplifier being coupled to the non-inverting input of said second comparator; and

a capacitor coupled at one end to the output of the amplifier and coupled at the other end to the second input of the amplifier.

8. The circuit of claim 7 wherein said differential voltage setting means comprises a multiplexer coupled at an input thereof to the supply voltage source and having an output supplying the second voltage to said amplifier of said second compensation means.

9. The circuit of claim 8 wherein said multiplexer has a plurality of input taps each supplying a different voltage from said multiplexer to said amplifier of said second compensation means when selected.

10. The circuit of claim 9 wherein said multiplexer includes selection means for selecting one of said plurality of input taps, and wherein said circuit is located on a chip with a microcontroller.

11. A method for generating an oscillatory signal of a predetermined frequency across a series resistor-capacitor (RC) network by ensuring that the oscillatory signal accurately oscillates between first and second voltages generated from a supply voltage source where the frequency of oscillation is determined by both the time constant of the RC network and the difference between the first and second voltages comprising the steps of:

providing charging-discharging means for discharging a voltage across the capacitor commencing when voltage of the oscillatory signal exceeds a first threshold voltage and for charging said voltage across the capacitor commencing when voltage of the oscillatory signal falls below a second threshold voltage;

providing first sampling means coupled to said charging-discharging means for obtaining a first sampled voltage of the oscillatory signal upon commencement of discharging of said voltage across the capacitor;

providing first compensation means coupled to said first sampling means for adjusting said first threshold voltage to be the first voltage modified by a voltage difference between the first voltage and said first sampled voltage;

providing second sampling means coupled to said charging-discharging means for obtaining a second sampled voltage of the oscillatory signal upon commencement of charging of said voltage across the capacitor;

providing second compensation means coupled to said second sampling means for adjusting said second threshold voltage to be the second voltage modified by a voltage difference between the second voltage and said second sampled voltage; and

providing differential voltage setting means coupled to said second compensation means for selecting the second voltage from a plurality of different possible voltages and thereby selecting a voltage difference between the first and second voltages.

12. The method of claim 11 wherein said charging-discharging means comprises a flip flop having an output thereof coupled to the series RC network for controlling the charging and discharging of said voltage across the capacitor.

13. The method of claim 11 wherein the step of providing said first sampling means comprises the steps of:

providing a negatively-triggered pulse generator having an input connected to an output of said charging-discharging means; and

providing first switch means coupled to an output of said negatively triggered pulse generator for momentarily coupling, in response to receipt of a pulse from said negatively-triggered pulse generator, said first compensation means to said first sampled voltage of the oscillatory signal.

14. The method of claim 11 wherein the step of providing said second sampling means comprises the steps of:

providing a positively-triggered pulse generator having an input connected to an output of said charging-discharging means; and

providing second switch means coupled to an output of said positively-triggered pulse generator for momentarily coupling, in response to receipt of a pulse from said positively-triggered pulse generator, said second compensation means to said second sampled voltage of the oscillatory signal.

15. The method of claim 12 further comprising the step of providing first and second comparators having their outputs respectively coupled to first and second inputs of said flip flop for setting and resetting said flip flop, said first comparator having a connection from its non-inverting input to a node between a resistor and the capacitor of the RC network, and said second comparator having a connection from its inverting input to said node.

16. The method of claim 15 wherein the step of providing said first compensation means comprises the steps of:

providing an amplifier having first and second inputs and an output, said first input of said amplifier being coupled to receive the first voltage, and said output of said amplifier being coupled to an inverting input of said first comparator; and

providing a capacitor coupled at one end to the output of the amplifier and coupled at the other end to the second input of the amplifier.

17. The method of claim 15 wherein the step of providing said second compensation means comprises the steps of:

providing an amplifier having first and second inputs and an output, said first input of said amplifier being coupled to receive the second voltage, and said output of said amplifier being coupled to the non-inverting input of said second comparator; and

providing a capacitor coupled at one end to the output of the amplifier and coupled at the other end to the second input of the amplifier.

18. The method of claim 17 wherein said differential voltage setting means comprises a multiplexer coupled at an input thereof to the supply voltage source and having an output supplying the second voltage to said amplifier of said second compensation means.

19. The method of claim 18 wherein said multiplexer has a plurality of input taps each supplying a different voltage from said multiplexer to said amplifier of said second compensation means when selected.

20. A circuit for generating an oscillatory signal of a predetermined frequency across a series resistor-capacitor (RC) network by ensuring that the oscillatory signal accurately oscillates between first and second voltages generated from a supply voltage source and where the frequency of oscillation is determined by both the time constant of the RC network and by the difference between the first and second voltages, the circuit including first and second comparators having their outputs respectively coupled to first and second inputs of a flip flop for setting and resetting the flip flop, an output of the flip flop being coupled to the series RC network for controlling the charging and discharging of voltage across the capacitor, the interconnection of the series RC network being coupled to an input of both the first and second comparators, the improvement comprising:

first voltage modifying means for receiving the first voltage and for applying a modified version of the first voltage to the other input of the first comparator such that the first comparator switches before the oscillatory signal actually reaches the first voltage thereby ensuring that the oscillatory signal precisely reaches the first voltage by the time the flip flop switches;

second voltage modifying means for receiving the second voltage and for applying a modified version of the second voltage to the other input of the second comparator such that the second comparator switches before the oscillatory signal actually reaches the second voltage thereby ensuring that the oscillatory signal precisely reaches the second voltage by the time the flip flop switches;

differential voltage setting means coupled to said second voltage modifying means for selecting the second voltage from a plurality of different possible voltages and thereby selecting a voltage difference between the first and second voltages; and

said circuit being located on a chip with a microcontroller.