Open-source floppy reader: the data separator/sync-er works!

Chuck Guzis wrote: Oh, no, not at all. The reason I added the data separator and sync detector was primarily because of Amiga format floppies. The track starts with a pair of $4489 sync words, and isn't synchronised to the index pulse. It just makes it easier to decode the data - if you know where the sync word is, you know where the SOT is, so you know where to start decoding data, which bits are data and which bits contain clocking information. If the disc doesn't use the $4489 sync word, you start reading at the leading edge of the index pulse, run one revolution, then stop reading at the second trailing edge. That leaves you with a RAM full of undecoded MFM data, that you can sync against manually. The sync detector lets you skip the synchronisation stage for discs written as an entire track - just take every second stored bit as a data bit. I'm quite tempted to make the sync word configurable, so you could sync on any 8-bit MFM sequence you liked. I know there's at least one other 'missing clock' sync word in common use (IIRC it's 0xC1 with a missing clock in the second nibble). The other 'interesting' bit is going to be handling read/write for hard-sectored discs. Reading shouldn't be too hard - AIUI, you'd wait for an index pulse, start reading, then wait for N+1 index pulses (where N=number of sectors -- n+1 provides one sector of overlap) and stop. Storing the index pulse state in the MSbit of the counter value byte would let you seek for a sector. The index pulse would need to write the current count to RAM and zero the counter every time an index was detected, otherwise writing the data back would be pretty difficult (the index->sector data timing skew would require correction). Writing back would involve a bit of 'fun' involving a primitive state machine. Probably something along the lines of checking to see if the MSbit was a 1, and waiting for an index pulse before sending the pulse and decrementing the counter. I can foresee all this circuitry filling two or three separate CPLDs... BTW, Chuck, thanks for posting the floppy drive pinout - it was very helpful. Thanks, -- Phil. | (\_/) This is Bunny. Copy and paste Bunny classiccmp at philpem.me.uk | (='.'=) into your signature to help him gain http://www.philpem.me.uk/ | (")_(") world domination.