[Coco] 6x09 read and write strobes.

[Phill Harvey-Smith]

On 16/12/2011 03:38, John Kent wrote:
> Falling edge of Q is it ?
> I posted a timing diagram, on 2011/12/05 but I must have had Q inverted.
>
> You can use a 7400 to generate read and write strobes, or half a 74139
> decoder.

Yep Dragondos uses both :), though it does use both Q and E :)

> Phil was using a CPLD and verilog to program it.

Phill :) :)

> The equations he used were:
>
> assign RD = E & RW & Reset;
> assign WR = Q & ~RW & Reset;
> assign nRD = ~RD;
> assign nWR = ~WR | RamWP;
>
> what it should have been I think was:
>
> assign RD = E & RW & nReset;
> assign WR = E & ~RW & nReset;

True I think that I need to change the name of my reset input to nReset,
I did try it with just E but it was even more flakey like that, however 
that was before I resolved the issue, which was caused elsewhere.. see 
below. So I may try with just E.

Cirtainly the just E makes sense, as things like the 6821 do it that 
way, and it matches the 6502 PHI NAND R/W combination, and since the 
6502 bus was based on the Motorola one, that also figures.

> I'm not sure if his reset was active high or active low.
> I have not used verilog much, so I'm not 100% sure of the syntax.
> Some peripherals such as the Z8530 assert both RD & WR to reset the
> peripheral, if my memory serves me correctly. I'm not sure if Phil was
> trying to do that or if he was trying to prevent random read/writes
> during reset.

Prevent random access during reset, not sure if it's needed but doesn't 
do any harm :)

> I didn't get a reply from Phil Harvey-Smith, so I'm not sure if he has
> sorted out his problem.

Yep got it sorted, and it took me several days of head scratching and 
waving a scope probe around.

Basically my board has several main components, A CPLD to do address 
decoding and buffering, an AVR to interface with the SD/MMC and some RAM 
and ROM for the firmware. Anyway to allow for field upgrades the AVR is 
also connected to the programming lines on the CPLD, what was happening 
was that the programming output pins on the AVR where not correctly 
initialised, and where chattering and causing the CPLD to randomly go 
into programming mode, which of course meant it stopped buffering and 
decoding....not good.

Anyway correctly initialising the programming lines has fixed this and 
all is now hopefully working.

There's some pictures on facebook :-

https://www.facebook.com/media/set/?set=a.2007692184197.2112104.1000448675&type=1&l=823e24b844

And a couple of videos on Youtube...

http://www.youtube.com/watch?v=Cr3C802q8Qo&context=C353e71aADOEgsToPDskJ6a_pXYcwK0M9M_GDEktA-

and

http://www.youtube.com/watch?v=qoLwJKakSDg&feature=context&context=C353e71aADOEgsToPDskJ6a_pXYcwK0M9M_GDEktA-

Cheers.

Phill.