[Coco] Re: Disk controller summary?

[Mark Marlette]

As I recall, the high density clock for 3.5" disks is 500KHZ.

Go to www.atmel.com and look at the AVRs. Powerful little units. I did
at 4MHZ model here at work to read magnetic stripes for door control.
AVR was $2.38. Most instructions execute in one clock cycle, 32
registers, ram, flash. Dev kit from Digikey is $80 and comes with a few
AVRs. Hard to beat.....




Quoting KnudsenMJ at aol.com:

>
> In a message dated 3/9/05 6:47:39 PM Eastern Standard Time,
> jdaggett at gate.net writes:
>
> >Only  if you add an external PLL to demodulate the FM and MFM
> >signal off the  disk. I am not sure a digital PLL will work  or one
> in
> >software.  Though I maybe wrong on this point.
>
>
>
> I was once part of a reverse-engineering project where we had a
> competitor's
> 8" floppy disk, with no idea of its format or underlying OS.   We
> built an
> interface to just sample the flux transitions under the read head,
> which came
> thru as a string of 1's and 0's.  A friend built the hardware  that
> sampled
> these data and saved them as a file.
>
> I wrote a simple software state machine to track the flux transitions
> and
> decode into actual 0s and 1s in terms of the MFM encoding.  Didn't
> take  long to
> combine these into bytes, and figure out it was CP/M.
>
> The moral is: you don't need a PLL, just a system that can run 2 or 3
> times
> faster than the disk's clock rate, so you can oversample the flux
> changes.
> Your software must be smart enough to realize that some pairs of 1s
> or 0s mean
> that your ahead of the game, so you throw out the second 0 or 1 and
> re-synchronize your state machine.  That's the phase-locked part of
> it.
>
> Do any of these PICs, AVRs, or HC11s clock fast enough to sample the
> flux
> transitions?  ISTR they were in the 100s of KHz range, so probably
> can.
>
> BTW, never heard of the AVR before, but it sounds like a cool family
> of
> devices, maybe good for building MIDI routers?  --Mike K.
>
>
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