I'm trying to disassemble some code and cannot reconcile it with my interpretation of the datasheet.

During the initialization we have



I'm guessing this is attempting to disable interrupts but seems wrong.

I don't see what writing all zeros will achieve, the Rockwell datasheet reads



The write to the IER looks equally odd, the datasheet says



So this looks like a no-op.

Am I misunderstanding the datasheet or is the code nuts?

I agree.

Of course the IFR and IER are cleared anyway by a reset. So maybe the code snippet was included "to be on the safe side," and the author never realized it accomplished nothing.

-- Jeff

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Yep. You clear the enables by reading them and writing back the value

Writing to the flag register doesn't always work. Some of the interrupt flags can only be cleared by updating other registers.

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Andrew Jacobs
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My interpretation of the datasheet is that if you want to disable all interrupts on the VIA, then you actually would write:


This can't be right. Reading the IER wouldn't achieve this because the datasheet says it will _always_ return a 1 for bit 7 and writing 0s _always_ ignores that particular bit. So, for example, if you had all interrupts enabled: