Processor based integrated circuit with a supply voltage monitor using bandgap device without feedback

1. An integrated circuit with a voltage monitor circuit, for monitoring the supply voltage of the integrated circuit, which integrated circuit is processor based, comprising:

a processor;

an bandgap circuit having first and second pn junctions, said bandgap circuit driven by the supply voltage to be monitored;

a first node associated with said first pn junction for providing a first voltage and driven by the supply voltage to be monitored, which said first voltage varies as a function of the voltage to be monitored at a first rate;

a second node associated with said second pn junction that provides a second voltage and driven by the supply voltage to be monitored, which said second voltage varies as a function of the voltage to be monitored at a second rate different than said first rate; and

a comparator circuit having a first input coupled to a voltage produced by said first node, and a second input coupled to a voltage produced by said second node to determine when said first and second voltages are within a predetermined separation and polarity, and provide as an output a reset signal to the processor.

2. The integrated circuit of claim 1 , wherein said first node comprises a junction between a resistor and a device having said first pn junction, and said second node comprises a junction coupling two series-connected resistors together in series with a device having said second pn junction.

3. The integrated circuit of claim 1 , wherein said bandgap circuit includes a circuit for scaling the supply voltage to be monitored.

4. The integrated circuit of claim 3 , wherein said scaling circuit comprises a respective resistor bridging each said pn junctions.

5. The integrated circuit of claim 1 , wherein said comparator circuit has no feedback circuit between an output thereof and an input thereof.

6. The integrated circuit of claim 1 , wherein said bandgap circuit includes a resistor having one terminal connected to the supply voltage to be monitored, and a second terminal coupled so as to provide current to both pn junctions.

7. The integrated circuit of claim 1 , wherein said comparator circuit includes a first comparator having inputs coupled to said bandgap circuit, and a second comparator providing a logic output when said first comparator fails to operate properly as a result of an inadequate supply voltage.

8. The integrated circuit of claim 7 , wherein said second comparator comprises a single ended amplifier.

9. The integrated circuit of claim 1 , further including an enable/disable circuit responsive to a signal for enabling and disabling operation of said bandgap circuit.

10. The integrated circuit of claim 9 , further including circuits responsive to said signal for driving an output of said voltage monitor circuit to a predefined state.

11. The integrated circuit of claim 10 , wherein said circuits drive an output of the voltage monitor circuit to a state indicating that the voltage to be monitored is within a specified limit, when indeed the voltage to be monitored is not within the specified limit.

12. A method of monitoring a supply voltage on an integrated circuit, which integrated circuit is processor based, comprising the steps of:

applying to a supply input on the integrated circuit a voltage to be monitored as the supply voltage to a bandgap circuit;

generating by the bandgap circuit a first voltage associated with current driven through a first nonlinear device and a second voltage associated with current driven through a second nonlinear device that each vary with the voltage to be monitored; and

comparing with a comparator circuit the first voltage with the second voltage, and providing an output indicating a condition of the voltage to be monitored, and applying it to a processor on the integrated circuit as a reset signal.

13. The method of claim 12 , further including carrying out the comparing step using a comparator without feedback coupled between an input and output of the comparator.

14. The method of claim 12 , further including determining whether the voltage to be monitored is above a given threshold.

15. The integrated circuit of claim 1 , wherein the point at which said first and second voltages are within a predetermined separation and polarity is substantially temperature independent.

16. The integrated circuit of claim 1 , wherein said predetermined separation and polarity is substantially zero volts.

17. The method of claim 14 , wherein the first and second nonlinear devices each have associated therewith a semiconductor junction.

18. The method of claim 14 , wherein the condition of the supply voltage to be monitored is where the difference between the first and second voltages is substantially temperature independent.