[Coco] 6x09 read and write strobes.

[John Kent]

Is Reset active high ?
I assume it's to protect against RD & WR being asserted during reset, in 
which case it should be Reset_n.
WR should really be gated with E rather than Q as data out from the CPU 
is valid on the falling edge of E.
(I hope I have Q around the right way).

I'm not sure what you mean by latching on the rising edge of WR.

E --\_______/----------\________/-----------\_______
Q _____/-----------\_______/--------------\_______/
D =>xxxxxxxxxx<====>xxxxxxxxx<=======>--------- Data from CPU valid 
during E,
RW xxx\_____________/-------------------------\XXXX
RD ______________________/------------\______
WR ________/----------\_____________________

John.

On 5/12/2011 2:12 AM, Phill Harvey-Smith wrote:
> Hi all,
>
> I am currently developing a SD/MMC interface for the Dragon and CoCo, 
> that uses a Xilinx CPLD to interface between the 6809 and the AVR on 
> the board.
>
> I am however having some problems with random lockups / crashes that 
> don't seem to be software related as I have tested the software in 
> mess and it will run flawless ly there.
>
> The thing I am unsure about is the read and write strobes and which 
> clocks they should be based upon.
>
> Currently in my Verilog I have :
>
> // Generate OE and WE signals, only do this if Reset is high !
> assign RD            = E & RW & Reset;
> assign WR            = Q & ~RW & Reset;
> assign nRD            = ~RD;
> assign nWR            = ~WR | RamWP;
>
> This way RD and WR are positive logic, so I am latiching into the CPLD 
> on the rising edge of WR and outputting to the 6809 bus while RD is 
> true. nRD and nWR are the read and write strobes for the onboard ROM 
> and RAM, the RamWP is in a latched register and allows me to write 
> protect the RAM.
>
> Is this the correct logic ?
>
> I have seen (on the DragonDos schematic, the read and write strobes 
> for the WD2797 are similar to the ones above based off both clocks, 
> however the write strobe for the drive/density select latch is based 
> off E.
>
> Comments anyone ?
>
> Cheers.
>
> Phill.
>

-- 
http://www.johnkent.com.au
http://members.optusnet.com.au/jekent