[Coco] Re: Disk controller summary?

[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.