[Eda-dev] Experiment with gscl45nm with Verilog output from an LRU implemented in Chisel

Øyvind Harboe oyvind.harboe at zylin.com
Fri Jul 5 23:58:31 EDT 2019

 I've implemented a very simple 4x4 LRU in Chisel, which I thought would be
a good smoketest for qflow. The resulting Verilog is attached. All output
from Chisel is very similar, but usually MUCH bigger.

The LRU is a classic nxn matrix implementation. Upon access of n, set row
to all 1's and column to all 0's and 0 in the intersection. n with the most
zero's is least recently used.

I was surprised to see an fMax of 108MHz. This is a very simple circuit, I
would have expected it to be pretty much the closest to fMax for any
non-trivial design at gscl45nm.

I believe I'm doing something silly w.r.t. how I've set up this "smoketest"
of fMax for gscl45nm or that I'm not reading it right. What I'm looking for
in qflow is to do P&R of individual modules of my design to give me
pushback on the design. My thinking is that fMax of the design will be
lower than the fMax of an individual tiny module.

I'm unsure on how to read the output. I can find matrix_0_1 in the Verilog,
but where's the "other end" to this "tallest pole in the tent"?

# qflow build --tech gscl45nm LRU
Top 20 maximum delay paths:
Path _321_/CLK to _343_/D delay 9180.4 ps
      0.0 ps  clock_bF$buf3: CLKBUF1_insert1/Y -> _321_/CLK
    108.8 ps     matrix_0_1:           _321_/Q -> _235_/A
    152.5 ps           _67_:           _235_/Y -> _238_/A
    225.7 ps           _70_:           _238_/Y -> _256_/B
    388.2 ps           _88_:           _256_/Y -> _257_/A
    865.9 ps           _89_:           _257_/Y -> _258_/B
   1058.8 ps           _90_:           _258_/Y -> _267_/A
   1447.9 ps           _99_:           _267_/Y -> _268_/A
   1457.6 ps          _100_:           _268_/Y -> _269_/B
   1906.0 ps            _1_:           _269_/Y -> _343_/D

   clock skew at destination = 10.697
   setup at destination = 7263.67
Computed maximum clock frequency (zero margin) = 108.928 MHz

Øyvind Harboe, General Manager, Zylin AS, +47 917 86 146
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