[Coco] I2C - Is it possible?

[John Kent]

On 25/07/2011 7:35 AM, Bob Devries wrote:
> John,
>
> the MM/1 computer has an I2C port, but to the best of my knowledge no 
> software was ever produced for it.
>
> Are you aware of anything that uses the 68070 CPU's built-in I2C? It 
> would be great to be able to use that port to read a clock, since the 
> clock in my MM/1 has now completely failed.
>
> Regards, Bob Devries
> Dalby, QLD, Australia
>
Hi Bob,

Back in the 1980s I designed a 68000 board with 68901 Multi Function 
Peripheral chip. The 68000 could run at 8MHz, but the 68901 MFP was only 
rated at 4MHz unfortunately, which meant you had to slow the clock rate 
down to it. The 68901 had an 8 bit I/O port that could also double as 
interrupt inputs as well as 4 timers and a serial port. I was looking 
very enviously at the 68070 back then as it had more integrated 
peripherals, however I think it was only supplied by Philips and there 
was some reason for not using it. Whether it was because of lack of 
emulator support or whatever I can't remember.

There are datasheets on the 68070 here as well as information on the I2C 
interface.

http://www.datasheetarchive.com/SCC68070-datasheet.html

Section 10 of the SCC68070 describes the I2C interface
The actual code to control the I2C interface would be fairly trivial I 
would have thought
The I2C interface is mapped at $80002000 to $8000200F
It consists of
$8000 2001 data register
$8000 2003 slave address register
$8000 2005 status register
$8000 2007 control register
$8000 2009 clock control register

Note that the registers are mapped on lower (odd) byte boundaries.

The bit definitions of the I2C interface registers are in the data sheet.
It looks like the I2C interface in the 68070 is very basic.

Looking up Wikipedia I see that the MM/1 refers to the Multi Media 1.

http://en.wikipedia.org/wiki/TRS-80_Color_Computer

There is a 68000 FPGA core called Minimig but I have not really looked 
at it very closely.
Mark McDougall might have it in his FPGA project SVN.
I think it might be missing interrupts and exception processing.
I had an idea that the 68070 had a few additional instructions compared 
to the 68000.
I could design my own FPGA 68000 but that might be detracting from the 
minimig project.

John.

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