Date   : Sun, 17 Feb 2008 11:15:19 +1100
From   : richard@... (Richard Wilson)
Subject: BBC FPGA Boots to BASIC... almost...

 

> -----Original Message-----
> From: 
> bbc-micro-bounces+richard=tassiecars.com.au@...
>  
> [mailto:bbc-micro-bounces+richard=tassiecars.com.au@...
> d9.co.uk] On Behalf Of Tom Walker
> Sent: Saturday, 16 February 2008 8:59 PM
> To: bbc-micro@...
> Subject: Re: [BBC-Micro] BBC FPGA Boots to BASIC... almost...
> 
> Actually, the correct design would have been to use some 
> variant of the DEN output, and probably a bit of voodoo, to 
> interrupt at the start of vertical blanking. Sadly this 
> wasn't done, though that's when the vblank interrupt happens 
> on the Electron and Archimedes.
> 
> As far as I was aware though, the vsync interrupt _did_ occur 
> at the start of vsync, rather than the end...

To do that you'd have to have some clever logic to make sure DEN was low for
more than about 20 cycles (1MHz), otherwise you'd get an interrupt every
scan line. The VIAs make it easy enough to do this anyway by setting up an
interrupt a set number of cycles after the VSYNC.

Richard