Date   : Fri, 07 Aug 2009 01:45:18 +0200
From   : rick@... (Rick Murray)
Subject: Acknowledging 1770 NMI - was Re: BBC B+ Econet

Michael Firth wrote:

> The question now is whether anyone can point me in the direction of the 
> minimum code needed to clear / acknowledge an interrupt from the disk
 > controller so that it won't hold the NMI line active on power on.

According to a text copy of the datasheet [*]:
   The 177x interrupts the CPU: *each time it finishes a command, *if a
   Force Interrupt condition is true, or *if the FDDC tries to write to a
   write-protected disk.  The 177x's interrupt output pin resets when the
   CPU reads the status register or writes to the command register,
   except after a Force Interrupt (immediate) command.


What I am wondering is if it is normal for it to want to interrupt on 
start-up?


It's a shame you can't just fiddle with link 9, but it must be *CLOSED* 
to disable the disc NMI so I think that could be 'unpleasant' for the 
FDC? Probably pulls it to a certain logic level, which the FDC will then 
want to override when asserting an interrupt... Mmmm...


Best wishes,

Rick.

* - http://modelb.bbcmicro.com/1772.txt


-- 
Rick Murray, irregular internet access at local library.
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...