Date   : Sat, 16 Feb 2008 09:59:26 +0000 (GMT)
From   : tommowalker@... (Tom Walker)
Subject: BBC FPGA Boots to BASIC... almost...

--- Mark McDougall <msmcdoug@...> wrote:
> The keyboard is "no event" atm - I know that because it reads the option
> links for MODE 6 correctly, and I just fixed a bug in which it looped
> forever waiting for the user to release the CTRL key...
> 
> IIRC the joystick fire buttons are active low, but I'm not sure of the
> sense
> of the two speech inputs... since the schematics don't designate active low
> naming I'm assuming they're active high??? I guess I should check the MESS
> source further...

Speech inputs are active low also.
 
> I also notice in the MOS disassembly the comment that the VIA is set to
> interrupt on the negative edge of VYSNC and the VSYNC output of the 6845 is
> active high - which of course means that the interrupt happens on the
> falling edge of VSYNC. I would've thought you'd design it to interrupt on
> the leading edge, to give a few more scanlines of time in an ISR before the
> video started to redraw...?!?

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...

Tom


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