Hello, before making the final casing for my Orwell computer I decided to try adding a 6581 SID to my machine. It is one pulled from a dead C64.
I am not having much luck though. I know these chips are notorious for failing but it would be good to check I am using it properly first.
The chip has only one /CS line. My machine is using Garth's address decoder from the primer. I am using an additional 74HC00 NAND for the extra decoding I need. I feed the /CS from the decoder into both inputs on one gate as an inverter. The output is feed into a second gate whose other input is the A8 address line. I think that should give me the correct /CS on the SID when POKEing to location $4100 (16640 decimal since my machine runs BASIC). If I check on the scope running a simple poke in a loop program I can see the correct negative pulse.
The SID though seems a bit suspect. It basically sits there making feeble noises all the time. But the noise is affected by whatever else is happening on the machine. So typing, running code, etc makes the noise change. It's a bit like all the computer in science fiction films who make noises when they are doing something.
I suspect the SID is dead (or badly wounded!) because even if I physically hold the /CS on it high it still makes the odd noises. The datasheet does say the reset needs to be 10 phi2 cycles to reset the chip registers to zero. I have a 100mS reset pulse and the machine runs at 1Mhz so that should be fine. That should make the chip silent on start-up.
It's as if the /CS isn't working so things are being written to it all the time making it do odd things and cause the strange noises I hear. Is that a common sort of failure with these chips? Or is there something amiss with my extra address decode I am missing?
I have ordered one of the Italian SwinSIDs off eBay to try (
http://www.ebay.com/itm/Commodore-64-swinsid-replacement-for-6581-8580-SID-/171728731586?ssPageName=ADME:L:COSI:US:1123). I unfortunately don't have, or know anyone with, a good C64 to try the SID in.
Simon