1.7.6. Decreasing Pessimism in SSN Analysis
Worst-case conditions include all pins acting as aggressor signals on all possible victim pins and all aggressor pins switching with the worst possible timing relationship. The results of SSN analysis under worst-case conditions are very pessimistic. You can improve the results of SSN Analysis by creating group assignments for specific types of pins. Use the following group assignments to decrease the pessimism in SSN analysis results:
- Assign pins to an output enable group—All pins in an output enable group must be either all input pins or all output pins. If all the pins in a group are always either all inputs or all outputs, it is impossible for an output pin in the group to cause SSN noise on an input pin in the group. You can assign pins to an output enable group with the Output Enable Group logic option.
- Assign pins to a synchronous group—I/O pins that are part of a synchronous group (signals that switch at the same time) may cause SSN, but do not result in any failures because the noise glitch occurs during the switching period of the signal. The noise, therefore, does not occur in the sampling window of that signal. You can assign pins to an output enable group with the Synchronous Group logic option. For example, in your design you have a bus with 32 pins that all belong to the same group. In a real operation, the bus switches at the same time, so any voltage noise induced by a pin on its groupmates does not matter, because it does not fall in the sampling window. If you do not assign the bus to a synchronous group, the other 31 pins can act as aggressors for the first pin in that group, leading to higher QL and QH noise levels during SSN analysis.
In some cases, the SSN Analyzer can detect the grouping for bidirectional pins by looking at the output enable signal of the bidirectional pins. However, Altera recommends that you explicitly specify the bidirectional groups and output groups in your design.