So I recall when an assertion triggers the data signals in the assertion are from just prior to the clock edge.
I'd like to put a helpful error message in my assertion to let the user know what went wrong, but by the time $display() happens the data has already changed.
As an example the below assertion will fail whenever data=0 when a ready/valid handshake occurs. However the $display that is printed will show data=1 if there is coincidentally a rising edge of data when the assertion fired.
default clocking CLK @(posedge clk);
endclocking
//Assertion triggers an error if data is not = 1 when new data is clocked into the DUT.
POS_DATA: assert property ((ready & valid) |-> data) else $display("[%t] POS_DATA fail data:%d",$time,data);
Is there a better way to code this assertion?
Created an EDAPlayground Example and looks like all the simulators do the same thing, so seems like it's my fault.
All you need to do is add $sampled(data)
POS_DATA: assert property ((ready & valid) |-> data) else $error(" POS_DATA fail data:%d",$sampled(data));
And you should be using $error instead of $display. You'll thank me later when you start running regressions and getting coverage reports.