( ESNUG 335 Item 6 ) ---------------------------------------------- [11/3/99]
Subject: Making Latches Transparent For ATPG Generation In Test Compiler
> I am using the Synopsys Test Compiler for ATPG Generation in a design with
> latches. I tried to make the latches (which are not in the scanchains)
> transparent with the
>
> set_scan_transparent true -existing
>
> command. I assume that the ATPG generator tries to make the latches
> transparent by generating appropiate patterns which force the enable
> signal of the latch to '1', (i.e. the latch is transparent during parallel
> capture.) If Test Compiler fails to compute such a pattern, i.e. the
> latch is in holdmode during parallel capture, the output of the latch has
> to be assumed 'X'.
>
> O.K. now Test Compiler has produced testpatterns but during pattern
> resimulation the latches are NOT transparent BUT the input at the latch
> is assumed to be at the latch output in the following.
>
> Has anybody used the set_scan_transparent feature of the Synopsys
> Test Compiler successfully?
>
> I also asked the Synopsys Hotline and they said that Test Compiler ASSUMES
> the latches to be transparent although in the documentation it says that
> the ATPG generator will provide the patterns.
>
> - Friedrich Beckmann
> Infineon Technologies AG Muenchen, Germany
From: Frank Emnett <frank@aiec.com>
What the Synopsys person told you agrees with my experience. The -existing
option (which is _required_ ... go figure) tells Synopsys that you've
already added logic to make these transparent during scan. I manually added
logic to my circuit to force the latches transparent whenever a TEST_MODE
signal was asserted. Then I did a
set_test_hold TEST_MODE 1
set_scan_transparent true -existing {latches}
Syntax may not be quite right, I'm not at work.
The downside is that you don't get good coverage on the enable logic or
latch gate pins, but in my case, they're easily testable through functional
vectors. Scan vectors produced this way have run on the tester just fine.
- Frank Emnett
Automotive Integrated Electronics Corp.
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