>I appreciate that this is an odd question to ask on a PIC list, but
>why did you opt for that part if you need to go to external memory ?

Because it was originally written for a '73 20MHz in MPLAB-C (which used
ALL ROM to the byte!).  I ported it to HiTech C, and ended up with 550 ROM
words free.  I added a ton of functionality to it and had it fitting in a
'77 (lots of unused I/O and about 400 ROM free), except for the need to
buffer messages (most RAM was used).

I investigated 68HC11, Z180, and others, but due to major timing
differences (portions of the code are _very_ time critical - it analyzes
NTSC video data) it would have meant a major rewrite of the code.  My
delivery date is too close to even think about new processor architecture
on top of new hardware.

I decided to take the easy way out - PIC17.

The '752 has a bunch of A/D and still more than enough I/O to do the job I
need.  External RAM (Cypress, message buffer) and Flash ROM are just too
convenient.

>An expanded 17C part seems to me to be all economically wrong with
>the port pins you loose, twice as many chips for external memory and

Because of the cost of windowed parts (I recently bought 850+ PIC16C73A/JW
parts, and priced 10000+ '77/JWs) it is _far_ more economical to go with
external ROMs, and because we wanted to update the firmware in the field
(not so firm any more - call it mushware <G>) we decided to go with AMD
FLash 5V parts.

Because we're doing everything with TQFP and TSSOP packages, space isn't a
big deal.  Because we don't need sockets for RAM or ROM, it's pretty small.
 So what - we added two '373 buffer chips (in SO format).  That's a lot
less space (and connections) than what it would take to get the I/O we
would need with other processors.

>expect that updating a flash with it would become quite a task
>(compared to more mainstream micros).

I'm making headway in this department.  If you have any ideas, I'd like to
hear them.

Hope this makes it clear.

Andy

==================================================================
                     Andy Kunz - Montana Design
          Go fast, turn right, and keep the wet side down!
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