Re: 1571/1581 Sources: Macro WDTEST inserting NOP before reading WD177x status register / writing command register

On Sat, Mar 12, 2022 at 10:07 AM smf <smf_at_null.net> wrote:
>
> If your thought is true, then isn't that a justifiable cost tradeoff?
>

no idea, an hardware error might be "corrected" with a software
hack, but there may be no software hack to correct it, unless you
first check that you can correct it in every case.
As an engineer I don't justify "cost tradeoffs" as they are the root
of all evil.
Unless you can prove the hack has no side effects of course.

> My initial thought was that there was some speed issue accessing the
> chip and the macro was written based on a guess that happened to work.

there's no speed issue at 2 MHz clock to a WD1770.
On the other hand, using low phase of phi1 to qualify for high phase of phi2
is a well known error and leads to unpredictable errors since you're selecting
a device too early.
As reported in the 6500 hardware manual, high phases of phi1 and phi2 are
the only ones to be used, if you need them inverted, you have to invert them.

>
> Unless you prove what the issue is, then you can't tell whether the
> address bits are important or it just randomly works if you insert nops
> based on that heuristic.

it's easy, just remove the NOP at A0=A1=0 and see what happens, if you have
spurious reads and writes to the WD1770, then it only may be because
of the wrong
select clock phase has been used.

Frank