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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: kamiga on March 04, 2014, 11:38:48 PM

Title: RTC double writes?
Post by: kamiga on March 04, 2014, 11:38:48 PM

When looking at the trapdoor memory interface on an A500, I notice that there are back to back double writes happening to the RTC clock chip on the A501 module.

With the four-bit databus that is being used, I highly suspect that there's some BYTE to nibble conversion happening in Agnus/Gary.

For instance, the RTC address will stay the same, but the databus will first write "wrong" data or "0"-data, and then the second write containing the actual correct value.

You see a low-going pulse for /XCLKWR, then a delay (217ns?), and then another low-going pulse for /XCLKWR.

Note that this works absolutely fine in practice, but something goofy is going on.  Unless there's some requirement that I can't find in the datasheet with the RTC chip to clear the value with "0" prior to writing it?

This is happening with each RTC register.

If I had access to the source code for the amigados "setclock" command, I could probably figure it out soon enough.  I could disassemble it, but not sure how cryptic that would be.

Thanks

Title: Re: RTC double writes?
Post by: kamiga on March 05, 2014, 08:04:02 PM

For reference, I've added a trace to my A501 Replacement project page.

http://www.techtravels.org/amiga/amigablog/?page_id=942#RTC_Traces