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PICList Thread
'PIC17 Dev Tools'
1998\04\17@120343 by Andy Kunz

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I'm about to start on a PIC17C752-based project with external ROM and RAM
(table read/write).

I'm new to the 17C stuff, so any info on hardware support would be
appreciated.  I'll be using HiTech C for it.

Thanks!

Andy

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Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
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1998\04\17@204735 by Steve Baldwin

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> I'm about to start on a PIC17C752-based project with external ROM and RAM
> (table read/write).

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 ?

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
just the two instructions for getting at program code space. I would
expect that updating a flash with it would become quite a task
(compared to more mainstream micros).

I don't have any problem with people sticking to an architecture they
are familiar with. If that's the reason, that's fine. If this part
was chosen from a choice of many, I'd be interested in why.

Steve.


======================================================
 Very funny Scotty.  Now beam down my clothes.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: spam_OUTstevebTakeThisOuTspamkcbbs.gen.nz   fax +64 9 820-1929
======================================================

1998\04\18@065533 by Andy Kunz

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>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

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