Floppy controller questions

[Tony Duell]

> >  1) How do you vary the read/write data rate? Some 3.5" formats use MFM at
> >250kbits/sec, others use MFM at 500kbits/sec. What relation does the WRITECLK
> >input have to the data rate of the MFM-coded signal at the WRITE DATA output?
> >What about the 8MHz master clock?
> 
> BY prescaling the clock.  Master clock drive the internal cpu and
> after prescaling the various timers (step rate, Head load time, 
> head unload time) Possible useful rates for Clock is 8 and 4mhz.
> Use 8MHZ for fastest step rates, 4mhz for those drives (like sa400)
> that need really slow step rates. Also the rate that the master 
> status register is updated is affected due to internal microprocessors 
> speed.
> 
> Write clock sets the write data rate and should corrospond to the 
> rate needed for media. Writeclock is 2x data rate.  The master clock 
> is not synchronous to the write clock though it's easiest to derive
> both from the same master source. 

To avoid confusion. I think it would be clearer to say that Write clock 
_need not be synchornised_ to the master clock. As you go on to say, 
there is no problem if Write clock is derrived by dividing down the 
master clock signal (asusming that will give the data rate you want), in 
fact just about every floppy controller board does it that way.

[... Much good stuff deleted ...]

-tony