[Coco] MPI writing to $c000

[Darren A]

On Mon, Jan 25, 2016 at 8:47 PM, RETRO Innovations wrote:


> The final equations that worked:
>
>  * CE was not needed (can be tied low)
>  * When WE goes low, bring SLENB low, otherwise, SLENB should be HI-Z
>    (so other carts can take it low)
>  * OE = CTS
>  * My WE equation is:  !(q& !r_w & address[15] & address[14] &
>    !address[13] & write_enable);
>
> My equations differ from Darren's, in that I used Q and not E.  The 6809
> datasheet states that addresses are valid on rising edge of Q and data to
> write it valid on rising edge of Q.  It seemed prudent to latch the data on
> the falling edge of Q before E falls, but I can change it to use the E
> clock instead if I am missing something.




Using Q would be okay if your SRAM and decoding logic is fast enough.  You
should verify how much Address Setup Time is needed. The datasheet for the
68B09 lists a minimum of only 15ns Valid Address Time before the rising
edge of Q. Most CoCo peripherals (including the Disk Controller and Serial
Pak) use only E. This gives plenty of setup time. The datasheet also shows
30ns of Write Data Hold Time after the falling edge of E.

- Darren