9 Dec
2009
9 Dec
'09
6:55 a.m.
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