Strange power numbers w/ ASAP7 #6320

evmanz · 2024-12-06T21:01:40Z

evmanz
Dec 6, 2024

Hello everyone!

I am running OpenRoad flow for my design and I see some strange numbers for reported power.

Here is a snippet of two cells on the timing path:

      Cap      Slew     Delay      Time              Description
-------------------------------------------------------------------------------
                                          _09534_ (net)
            39.1912    0.5733 2766.8992 ^ _35759_/A (XNOR2x1_ASAP7_75t_R)
   1.6660   23.6009   27.2220 2794.1211 ^ _35759_/Y (XNOR2x1_ASAP7_75t_R)
                                          _09851_ (net)
            23.6031    0.1195 2794.2407 ^ _35823_/A (XNOR2x1_ASAP7_75t_R)
   0.9752   15.4225   22.0917 2816.3323 ^ _35823_/Y (XNOR2x1_ASAP7_75t_R)
                                          _09914_ (net)

They are both the same type, both with reasonable input transition and output capacitance.
However, both have insane power consumption (more than 1mW each!):

   Internal  Switching    Leakage      Total
      Power      Power      Power      Power (Watts)
--------------------------------------------
   9.74e-04   4.72e-04   1.67e-10   1.45e-03 _35823_
   6.39e-04   5.19e-04   1.67e-10   1.16e-03 _35759_

What is strange is that the second cell in the timing path (35823) consumes more power than its driver (35759) while having better input transition & output capacitance.

I checked .lib[1], and for those operating conditions the cells should consume about 0.24 - 0.5 pW (depending on rise or fall transition, and the state of other pins), which is VERY far from the reported values.

As a result, I have designs with 100k cells, which consume 2kW of power, which makes zero sense.
The designs have small WNS (say -10ps), and TNS at most -100ps after PnR. Length of the timing path above is roughly 100 cells.

What do I miss?