Date   : Sun, 25 Jul 2010 12:08:04 +0100
From   : info@... (Sprow)
Subject: Proposed allocation: Configuration RAM byte 8

In article <20100724170946.8iqy00gzs4k0o0s4@...>,
   J.G.Harston <jgh@...> wrote:
> I am thinking of using two bits of configuration RAM to store a default
> MIDI parameter. Looking at the list, byte 8, bits 6-7 would be suitable.
>
> Specifics:
> Most BBC MIDI interfaces (see http://mdfs.net/Info/Comp/BBC/MIDI) use a
> 6850 ACIA. This requires a clock input. Some interfaces (eg JGH, ETI, 
> M2000) derive this clock from the BBC's 1MHz signal, and so require the
> feed-in clock to be divided by 16 to get the MIDI 31.125KHz clock. Some 
> interfaces (eg EMR) have an on-board 2MHz clock, so need to divide by 64.

That only has two states, so wouldn't byte 18 bit 4 make more sense.

I also have notes to say that Edit uses bit 7 and the network software uses
bit 6 of byte 8, but no ROMs (handy) to back that up.

I also wonder if you could auto detect it rather than needing a
configuration value - 
  have a guess at the clock setting
  interrupts off
  write a NOP into the transmit register
  poll on the transmit complete interrupt (while counting)
  interrupts back on
  look at how long it took
choosing between x64 or x16 should be fairly clear based on the transmit
time,
Sprow.