set_input_delay Problem

set_input_delay

hi guys,

I have a funny porblem : I have a pair of lvds-receiver-pads - one for a clock signal, on for the data. The edges of the clock and the data are aligned within the buffer (with some additional skew detection cicuitry) - and the skew between the dges of the clock is smaller then 100ps - there is also some additional jitter (smaller then 100ps).

now I tried - just for a start - to synthesize just a flipflop for cpaturing the data. The funny thing is : when I use in the the dc-synthesis-script

"set_input_delay -clock clk 0.0 [all_inputs]"

and

"set_clock_latency 0 [get_clocks]"

- as it is obviously the case then I get the following timing violation :

min_delay/hold ('clk' group)

Required Actual
Endpoint Path Delay Path Delay Slack
-----------------------------------------------------------------
test_reg_1/out_reg/D 0.08 -0.09 f -0.17 (VIOLATED)

For the start I set the clock uncertainty to 0 - of course it shoul be at least 100ps due to jitter. But this would only even increase the problem ...

What I'm asking myself now - why the heck doesn't DC just infer some Buffer in the circuit or when will this happen ? (Do I have to do this by myself ?)

I'm puzzled - help me

DC wont infer buffer, untill you specify to fix the problem. In your case, you need to tell DC to fix the hold violations(set_fix_hold on all clocks).

Points to remember when applying input delay:
1) need to remove the clock ports from all inputs before applying input delay.
2) apply max and min dealy separatly. if we dont specify -min/max options, tool assumes same value for max and min, which is not the real scenerio.

Usually, we (Atleast our company) ignore hold violations at DC synthesis. The main reason is, hold violations are good to fix after CTS and layout.At Dc synthesis we will make sure, these hold violations are not due to bad constriants. Bad constrainte means,specify MCP for setup(multicylcle paths ) and forgot to correct the MCP for hold and then try to fix these violations in DC, area will blow up...

Good luck.