Using a 3.5" 720k disk...

On 18 Apr 2009 at 22:54, Chris Elmquist wrote: one Chris, I did the same thing with a Microchip 12F629 (another 8 pin uC) but applied to 5.25" drives, passing sector marks through when present, but otherwise generating 10 sector simulation. I kept my sampling to 16 bits using roughly that precision for a 167 msec rev and even timed each rev for the next one as I generated pulses for the current one to keep track of any long-term speed variations. Sector pulses were exactly 4 msec in length. My finding (with the help of Andrew Lynch) was that the rev-to-rev speed variation on old 5.25" 100 tpi drives was nowhere near stable enough; if the speed error from rev to rev varied by more than a few msec., you were dead. With the old open-loop DC tach circuit found on old 5.25" drives, this was more than likely. I don't have any systems using HS diskettes here; if someone wants my code, they're welcome to it; it's PIC assembly and will do both 300 and 360 RPM drives with the ability to generate 32, 16 or 10 sector marks. The same chip also generates a READY signal for drives that don't have them. It was a work in progress and I'm happy that you found a solution using modern 3.5" drives. My evaluation of 5.25" drives indicates to me that it would be a far better idea to physically mark sectors on the hub flywheel, use a sensor to grab them and then use a PIC to delay them to conform with the positon of the index hole. --Chuck