I have considered doing the SPI in software.  However, the SSP module can
transmit and receive at the same time.  Hopefully this means I can write 8
bits to the DAC whilst reading 8 bits from the ADC.  If this is correct then
with a bit of cunning I can save lots of cycles.  Can anyone confirm this or
not?

I still think things are going to be extremely tight as I have to implement
a PID controller in the remaining time.  At 20MHz I get a 200ns instruction
cycle.  At 50k samples/sec I have a loops time of only 20us which is only
100 instuctions.  Not possible IMHO.  I have haggled the sampling speed down
to 20k samples/sec which give me quite a bit more time, I still suspect it's
going to be difficult though.

Out of interest does the Hi-Tech C compiler support the Scenix chips, or can
the ASM output be hacked to make it compatible?

Regards

Mike Rigby-Jones

>    Mike, you need to provide a chip-select for each device on the SPI bus.
> I've found that it's much easier to do the SPI bus in software as many
> devices, such as 12-Bit converters, require more overhead with a hardware
> SPI bus.
>
>    - Tom
>
> At 10:50 AM 4/20/99 +0100, Michael Rigby-Jones wrote:
> >I call upon the mighty piclist once more....
> >
> >I've just been asked if I can help with a project involving a dual 12 bit
> >DAC (MAX144) and a dual 12 bit ADC (MAX5156) which are to be part of a
> >closed loop control system.  The system has to have  very fast response
> time
> >and the sampling speed will need to be in the region of 50k samples/sec.
> >This means I will need a part with a built in SPI interface.  No problem
> as
> >we have an emulator for the 16C63 etc.
> >
> >However, this system is to be controlled via a 68HC12 over an SPI bus.
> >Hmmm.  Now do any parts have 2 SPI interfaces?  Because I think that's
> what
> >I'm going to need unfortunately.  The PIC will (I suspect) be running
> pretty
> >well flat out performing it's control functions so a software
> implementation
> >is going to be tricky.
> >
> >Any suggestions?
> >
> >Regards
> >
> >Mike Rigby-Jones