Systems and methods for partial retention synthesis

1. A method for identifying retention status of registers of a circuit, the method comprising the steps of:

performing symbolic simulation for one or more simulation cycles starting at the beginning of the one or more simulation cycles;

replacing each register value with a multiplexer, a select of the multiplexer equal to 0 connecting to the register value and another input of the multiplexer connects to a symbol;

collecting a first symbolic trace at an output and building a first miter with a logic value of the first symbolic trace;

collecting a second symbolic trace in design registers and building a second miter with a logic value of each register value;

producing a Boolean function by implementing an OR logic gate, using a processor, that ORs the output of the first miter and the output of the second miter, if the output is 0 the circuit behavior has not changed, thereby identifying a non-retention register, and if the output is 1, the circuit behavior has changed, thereby identifying a retention register; and

determining a set of non-retention registers that allow the output to remain 0.

2. The method of claim 1 wherein the step of determining registers that allow the output to remain 0 comprises the steps of:

creating an empty set of non-retaining registers;

choosing one select line of the multiplexer of the Boolean function to be 1 and solve an output;

adding the select line of the multiplexer to the set of non-retaining registers if the output of the Boolean function is 0;

repeating the step of choosing one select line until all select lines of all multiplexers of the Boolean function have been chosen;

adding select lines not in support of the Boolean function to the set of non-retaining registers;

providing a final list of registers that correspond to the select lines in the set of non-retaining registers at the end of the analysis.

3. The method of claim 1 further comprising a method for determining non-retention candidates for a next interval, through temporal partitioning, comprising the steps of:

verifying that a register has not required retention at previous interval and if it does require retention at the previous interval, it is not a non-retention candidate;

determining, if the register has not required retention at the previous interval, whether the register is over-written yet, and if (a) it is over-written yet, it is not a non-retention candidate for the next interval and if (b) it is not overwritten-yet, it is a non-retention candidate for the next interval.

4. The method of claim 1 further comprising a method for determining non-retention status at an end of a sequence during simulation, as part of temporal partitioning, comprising the steps of:

determining if the register requires retention in any interval, and if (a) yes then the register needs retention, and if (b) no and the register has been over-written, then the register does not need retention, and if (c) no and the register has not been over-written yet, then the register's status is inconclusive; and

determining a retention status of the register at the final state to identify registers that require retention and registers that can be non-retention.

5. The method of claim 1 further comprising a method for determining non-retention status after analyzing all sequences, comprising the steps of:

verifying that a register has not required retention in any sequence and, if the register has required retention in any sequence then the register requires retention;

determining if there is non-retention in any sequence and (a) if there is no-retention in any sequence, then the register potentially does not require retention and (b) if there is not no-retention in any sequence, then the non-retention status is inconclusive and more tests need to be analyzed.

6. The method of claim 1 wherein a processor is configured to carry out the method steps.

7. A system for identifying retention status of registers of a circuit, the system comprising:

means for performing symbolic simulation for one or more simulation cycles starting at the beginning of the one or more simulation cycles;

means for replacing each register value with a multiplexer, a select of the multiplexer equal to 0 connecting to the register value and another input of the multiplexer connects to a symbol;

means for collecting a first symbolic trace at an output and building a first miter with a logic value of the first symbolic trace;

means for collecting a second symbolic trace in design registers and building a second miter with a logic value of each register value;

means for producing a Boolean function by implementing an OR logic gate, using a processor, that ORs the output of the first miter and the output of the second miter, if the output is 0, the circuit behavior has not changed, thereby identifying a non-retention register, and if the output is 1, the circuit behavior has changed thereby identifying a retention register; and

means for determining a set of non-retention registers that allow the output to remain 0.

8. The system of claim 7 wherein the means for determining registers that allow the output to remain 0 comprises:

means for creating an empty set of non-retaining registers;

means for choosing one select line of the multiplexer of the Boolean function to be 1 and solve an output;

means for adding the select line of the multiplexer to the set of non-retaining registers if the output of the Boolean function is 0;

means for repeating the step of choosing one select line until all select lines of all multiplexers of the Boolean function have been chosen;

means for adding select lines not in support of the Boolean function to the set of non-retaining registers;

means for providing a final list of registers that correspond to the select lines in the set of non-retaining registers at the end of the analysis.

9. The system of claim 7 further comprising a system for determining non-retention candidates for a next interval, through temporal partitioning:

means for verifying that a register has not required retention at previous interval and if it does require retention at the previous interval, it is not a non-retention candidate; and

means for determining, if the register has not required retention at the previous interval, whether the register is over-written yet, and if (a) it is over-written yet, it is not a non-retention candidate for the next interval and if (b) it is not overwritten-yet, it is a non-retention candidate for the next interval.

10. The system of claim 7 further comprising a method for determining non-retention status at an end of a sequence during simulation, as part of temporal partitioning, comprising:

means for determining if the register requires retention in any interval, and if (a) yes then the register needs retention, and if (b) no and the register has been over-written, then the register does not need retention, and if (c) no and the register has not been over-written yet, then the register's status is inconclusive; and

means for determining a retention status of the register at the final state to identify registers that require retention and registers that can be non-retention.

11. The system of claim 7 further comprising a system for determining non-retention status after analyzing all sequences, comprising:

means for verifying that a register has not required retention in any sequence and, if the register has required retention in any sequence then the register requires retention; and

means for determining if there is non-retention in any sequence and (a) if there is no-retention in any sequence, then the register potentially does not require retention and (b) if there is not no-retention in any sequence, then the non-retention status is inconclusive and more tests need to be analyzed.