[Coco] The MOOH board (MMU/SPI) for Dragon/CoCo1/2

[Tormod Volden]

On Sun, Mar 18, 2018 at 4:46 PM, Gene Heskett wrote:
>
> I've seen mention of an SPI interface, and am wondering what is the
> bandwidth achievable? I am currently running an 11" swing by 54" bed, 70

Hi Gene,

The on-board oscillator is 48 MHz, and the SPI interface has a
programmable clock divisor (from 2 to 32), it can also use the
computer E clock as a source to go slower (SD cards are initialized at
slow speed). At high speed settings a new byte is clocked in faster
than the 6809 is able to read from the port address, even when
"over-clocked".

The block read routine in FUZIX does (with divisor = 4):
 lda <SPIDATA
 ldb <SPIDATA
 std ,x++
in an unrolled loop.

I don't remember if I did the cycle math for 6309 TFM transfers, but I
think it also will be CPU-limited.

I chose ~48 MHz because it is fast enough to saturate the CPU, and
with the divisor goes down to where the E clock-derived speeds take
over.

Regards,
Tormod