Floppy drive interfacing

Jules Richardson wrote: Oh, I'm going to do most of that in software too. I just want to get MFM sync-word detection and maybe pre-decoding working. SWD would be handy for reading Amiga format discs, on the grounds that the data can start anywhere on the track, not just at an index pulse. AIUI, with uPD765-written discs, you'd start reading when the index pulse goes active, read a whole track, then stop reading when it goes active then inactive, i.e.: __ __ INDEX ___| |_____________________| |___ : : : : Start point Stop point This gives you enough overlap (assuming the index sensor is fairly accurate) to be reasonably sure you've got the whole track. Then you look for the sync markers and decode it in software. With Amiga discs, because the data is stored with no index-pulse sync, you look for the MFM codeword 0x4489 which decodes as 0xA1, but isn't a valid MFM encoding (there's a missing clock pulse -- see last paragraph and bottom diagram of <http://en.wikipedia.org/wiki/Modified_Frequency_Modulation>). The whole reason the datasep is needed is to figure out where the data bits lie. To detect the sync word, you shift data bits into a 16-bit shift register according to the transitions (positive *and* negative) of the Data Window line. If there was a pulse on DATA during a Data Window cycle, you clock in a one, else you clock in a zero. Then you add a comparator (basically an AND gate and a bunch of inverters) to signal when the shift register contains 0x4489. You use that signal to trigger the start of an acquisition cycle, then stop shortly after the sync word is detected again. The beauty of the sync word is that because it's not valid MFM (but it's close enough to be picked up by a data separator), it can never appear in normal data, so you don't have to worry about what to do if the track contains a 0xA1 byte somewhere in a sector (or sector header). If you want to be ultra-pedantic, you could search for the whole Amiga sync sequence -- 0xAAAA 0xAAAA 0x4489 0x4489 -- but then you need a 64-bit shift register instead of a 16-bit shift register. I'd love to try my idea out, but I don't have any Amiga floppies to play with, nor do I have an Amiga to create them... -- 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.