Synopsys constraints: adding an external path

[lagoule]

Hello,

In a design for a class (a bad design IMHO, but I have to deal with it...), a chip access an external asynchronous memory.

Therefore, there is a path from the transition of addr rd/wr signals to the output pads to the async memory to the input_pad to the input registers in my chip. My problem is how to tell synopsys about that path?

When compiling the design, synopsys closes the timing from the internal signals to the output pads (according to the clock). It also closes the timing from the input pads to the internal registers. I just can't figure out how to tell it that both are related.

Thanks you,

Jonathan

 

[lostinxlation]

How do you know when the data comes back ?

if you have a strobe signal or similar kinds, you probably need to define it as a clock.

If the memory access happens in a cycle and no strobe signal is in use, you can just subtract the memory access time from the clock cycle and distribute the remainder to input delay and output delay.

 

[lagoule]

Hello,

The memory is totally simulated (and will remain, as this is what the professor wants). Thus, it has 0 access time and there is no strobe signal.

Your proposition to balance the clock period between input and output delays is so simple it works, thank you! However, the way to balance the delay must be chosen manually. Am I to understand that there is no way to make Synopsys chose how to balance this delay automatically?

By the way, I'm asking for my own enlightenment, this is way over the course's requirements. I just wish to learn how to use synopsys better.

Thanks again,

Jonathan

 

[lostinxlation]

it might be possible to budget the timing with the tool if the memory is actually a .db(or you can create a fake block with 0ns access time) that Synopsys tool can take to budget the timing and distribute it to the pins. That's widely done to define the input and output timing in the sub blocks. I forgot the name of the functions for characterization, but it was something with "characterize" and "context" with Prime Time. Search in the manual.