Voltage level monitoring of an integrated circuit for production test and debug

1. A circuit, comprising:

a functional circuit;

a scan chain coupled to the functional circuit and configured to collect scan chain data from testing of the functional circuit and output a scan chain signal containing the scan chain data;

a voltage monitor circuit configured to compare a supply voltage to a threshold and assert a reset signal when the supply voltage crosses the threshold;

a flip flop circuit configured to receive the reset signal and change a logic state of a flip flop output signal in response to assertion of the reset signal; and

a logic circuit configured to block passage of the scan chain signal to an integrated circuit probe pad in response to the changed logic state of the flip flop output signal, wherein said integrated circuit probe pad is for probing by automated test equipment.

2. The circuit of claim 1 , wherein the flip flop circuit includes a clock input that receives the reset signal and a set input that receives a set signal.

3. The circuit of claim 2 , wherein the set signal is asserted to set the flip flop circuit in a first logic state before a test operation begins, and the asserted reset signal causes the flip flop circuit to reset to a second logic state.

4. The circuit of claim 1 , wherein the logic circuit comprises:

a multiplexer circuit having a first input that receives the scan chain signal, a second input and an output coupled to the probe pad; and

a logic pass gate having a first input that receives the scan chain signal, a second input that receives the flip flop output signal and an output coupled to the second input of the multiplexer.

5. The circuit of claim 4 , wherein the multiplexer includes a control input configured to receive a select signal, wherein a logic state of the select signal chooses which of the first and second inputs of the multiplexer is connected to the output of the multiplexer.

6. The circuit of claim 4 , the logic pass gate operating to pass the scan chain signal to the second input if the logic state of the flip flop output signal has not changed and further operating to apply a fixed logic level signal to the second input if the logic state of the flip flop output signal has changed.

7. The circuit of claim 4 , wherein the logic pass gate is a logic AND gate.

8. The circuit of claim 1 , wherein the logic circuit comprises:

a multiplexer circuit having a first input that receives the scan chain signal, a second input that receives a first code signal and an output coupled to the probe pad, the multiplexer further receiving a select signal including a bit corresponding to the flip flop output signal, said select signal being decoded to cause selection between the first and second inputs for connection to the output coupled to the probe pad.

9. The circuit of claim 8 , wherein the multiplexer includes a third input that receives an additional code signal, further comprising:

an additional voltage monitor circuit configured to compare an additional supply voltage to an additional threshold and assert an additional reset signal when the additional supply voltage crosses the additional threshold;

an additional flip flop circuit configured to receive the additional reset signal and change a logic state of an additional flip flop output signal in response to assertion of the additional reset signal;

wherein said select signal further includes an additional bit corresponding to the additional flip flop output signal, said select signal being decoded to cause selection between the first, second and third inputs for connection to the output coupled to the probe pad.

10. The circuit of claim 1 , further comprising a debug pad connected to the output of the flip flop circuit, wherein said debug pad is not for probing by automated test equipment during electronic wafer sort (EWS) but is available for connection to test equipment during package testing.

11. A method, comprising:

generating a scan chain signal containing the scan chain data from testing of a functional circuit;

comparing a supply voltage to a threshold and asserting a reset signal when the supply voltage crosses the threshold; and

blocking passage of the scan chain signal to an integrated circuit probe pad in response to assertion of the reset signal, wherein said integrated circuit probe pad is for probing by automated test equipment.

12. The method of claim 11 , further comprising:

setting a flip flop circuit before beginning a test operation which generates the scan chain data; and

resetting the flip flop circuit in response to assertion of the reset signal, wherein an output of the flip flop circuit when reset controls blocking passage of the scan chain signal to the integrated circuit probe pad.

13. The method of claim 11 , further comprising:

multiplexing the scan chain signal and selected passage of the scan chain signal in response to a select signal having a first logic state for multiplexed selection of the scan chain signal and a second logic state for multiplexed selection of the selected passage of the scan chain signal.

14. The method of claim 13 , wherein selected passage of the scan chain signal is controlled in response to the assertion of the reset signal.

15. The method of claim 11 , further comprising applying a fixed logic state signal to the integrated circuit probe pad if passage of the scan chain signal is blocked.

16. The method of claim 11 , further comprising outputting a signal indicative of the reset signal to a debug pad, wherein said debug pad is not for probing by automated test equipment during electronic wafer sort (EWS) but is available for connection to test equipment during package testing.

17. A circuit, comprising:

a functional circuit;

a scan chain coupled to the functional circuit and configured to collect scan chain data from testing of the functional circuit and output a scan chain signal containing the scan chain data;

a first voltage monitor circuit configured to compare a first supply voltage to a first threshold and assert a first reset signal when the first supply voltage crosses the first threshold;

a first flip flop circuit configured to receive the first reset signal and change a logic state of a first flip flop output signal in response to assertion of the first reset signal;

a second voltage monitor circuit configured to compare a second supply voltage to a second threshold and assert a second reset signal when the second supply voltage crosses the second threshold;

a second flip flop circuit configured to receive the second reset signal and change a logic state of a second flip flop output signal in response to assertion of the second reset signal;

a first logic gate configured to logically combine the first and second flip flop output signals to generate a control signal;

a second logic gate having a first input configured to receive the scan chain signal and a second input configured to receive the control signal;

a multiplexer circuit having a first input configured to receive the scan chain signal and a second input configured to receive a signal output from the second logic gate; and

an integrated circuit probe pad connected to an output of the multiplexer circuit, wherein said integrated circuit probe pad is for probing by automated test equipment.

18. The circuit of claim 17 , wherein the multiplexer includes a control input configured to receive a select signal, wherein a logic state of the select signal chooses which of the first and second inputs of the multiplexer is connected to the output of the multiplexer.

19. The circuit of claim 17 , the logic pass gate operating to pass the scan chain signal to the second input of the multiplexer if the control signal has a first logic state and further operating to apply control signal to the second input of the multiplexer if the control signal has a second logic state.

20. The circuit of claim 17 , further comprising

a first debug pad for connection to test equipment during package test and connected to receive the signal output from the first flip flop circuit, and

a second debug pad for connection to test equipment during package test and connected to receive the signal output from the second flip flop circuit,

wherein neither of said first and second debug pads are for probing by automated test equipment during electronic wafer sort (EWS).

21. The circuit of claim 17 , wherein each of the first and second flip flop circuits is set to a first logic state before a beginning of a test operation, and individually reset to a second logic state in response to assertion of a corresponding one of the first and second reset signals.

22. A circuit, comprising:

a functional circuit;

a scan chain coupled to the functional circuit and configured to collect scan chain data from testing of the functional circuit and output a scan chain signal containing the scan chain data;

a first voltage monitor circuit configured to compare a first supply voltage to a first threshold and assert a first reset signal when the first supply voltage crosses the first threshold;

a first flip flop circuit configured to receive the first reset signal and change a logic state of a first flip flop output signal in response to assertion of the first reset signal;

a second voltage monitor circuit configured to compare a second supply voltage to a second threshold and assert a second reset signal when the second supply voltage crosses the second threshold;

a second flip flop circuit configured to receive the second reset signal and change a logic state of a second flip flop output signal in response to assertion of the second reset signal; and

a multiplexer having a first input coupled to receive the scan chain signal, a second input coupled to receive a first coded signal and third input coupled to receive a second coded signal, said multiplexer controlled by a multibit select signal, said multibit select signal being decoded to cause selection between the first, second and third inputs for connection to the output coupled to the probe pad.

23. The circuit of claim 22 , wherein the first coded signal identifies the first voltage monitor circuit as having asserted the first reset signal and the second coded signal identifies the second voltage monitor circuit as having asserted the second reset signal.

24. The circuit of claim 23 , the multiplexer further having a fourth input coupled to receive a third coded signal, wherein the third coded signal identifies both the first and second voltage monitor circuits as having asserted the first and second reset signals.

25. A method, comprising:

generating a scan chain signal containing the scan chain data from testing of a functional circuit;

comparing a first supply voltage to a first threshold and asserting a first reset signal when the first supply voltage crosses the first threshold;

comparing a second supply voltage to a second threshold and asserting a second reset signal when the second supply voltage crosses the second threshold; and

decoding logic states of the first and second reset signals to selectively choose between the scan chain signal, a first coded signal identifying that the first supply voltage crossed the first threshold and a second coded signal identifying that the second supply voltage has crossed the second threshold for output at an integrated circuit probe pad, wherein said integrated circuit probe pad is for probing by automated test equipment.

26. The method of claim 25 , wherein decoding further selectively chooses a third coded signal identifying that both the first supply voltage crossed the first threshold and the second supply voltage has crossed the second threshold for output at the integrated circuit probe pad.