Disc reader project -- quick status update

Eric Smith eric at brouhaha.com
Wed Dec 9 00:55:59 CST 2009


Keith M wrote:
> Let me play devil's advocate.  8x500kbps would be 4mhz.  Which means 
> that your samples are every 250ns.  But the pulses come from the drive 
> are usually 250-300ns wide.  _You could potentially miss one._  Now 
> the pulse WIDTH is fixed and doesn't matter, so it's not like you need 
> multiple samples per pulse, but I would just have to wonder about 
> pulse placement accuracy.  (ie when did that pulse really occur, did 
> it happen at t=500ns, or t=750ns)  Seems like a huge difference to me.
The leading edge of the pulse is what you care about.  The floppy drive 
electronics use a one-shot to generate the pulse, and you don't care how 
wide the pulse is.  If your sampling rate is too low and might miss 
pulses, you can put the pulse into the async preset input of a D 
flip-flop, and tie the D input low.  For example, using a 74LS74 or 
equivalent, on every rising clock edge the FF would be set to 0, and the 
leading edge of a pulse from the floppy drive would set it.  A second D 
FF should be used to register the result from the first one.

Eric




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