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[pwm,rtl] Rewrite two write-enables more explicitly #25062

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[pwm,rtl] Rewrite two write-enables more explicitly #25062

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Commits on Nov 12, 2024

  1. [pwm,rtl] Rewrite a pair of write-enables more explicitly 

    The behaviour will be the same, but the first change avoids an
expression coverage hole being reported for `blink_en_i == 0` and
`blink_ctr_q == 1`. This can't actually happen: `blink_ctr_q` can only
be updated by being set to `blink_ctr_d` (on this line). If
`blink_en_i` is false then `blink_ctr_d == 0`.

The only other way to reach the item would be to have incremented
`blink_ctr_q` and then disabled blink. But disabling blink needs a
register write, which will set a bit in `clr_blink_cntr` in
`pwm_core`, which means that `clr_blink_cntr_i` is true, zeroing
`blink_ctr_q`.

Coding this write-enable more explicitly gets rid of the expression
coverage item.

The second change covers two further coverage holes, that have
htbt_ctr_q and the write predicate false (either because blink_en_i is
false or htbt_en_i is false).

As before, this isn't possible. If the predicate is not true then
htbt_ctr_d will be zero (so htbt_ctr_q cannot become nonzero). If it
was nonzero before and we are changing the predicate to become false,
the register write will cause clr_blink_cntr_i to go high, zeroing
htbt_ctr_q.

Signed-off-by: Rupert Swarbrick <[email protected]>
    @rswarbrick
    rswarbrick committed Nov 12, 2024
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