Subject:
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Re: Picking a PIC
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Tue, 27 Oct 1998 14:30:55 GMT
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Original-From:
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fthompson9@aol&Spamless&.com
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Viewed:
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1420 times
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Clem:
In a message dated 98-10-26 23:31:56 EST, you write:
<< I'm looking to offload the processing of IR proximity sensors and
ultrasound to a PIC(s). I'm hoping to connect the PIC to a HandyBoard via
the SPI.
<< I'm trying to decide which is the right PIC for the job.
The PIC16C7x (PIC16C76) series supports SPI, can source 25mA/pin, 2 PWM
channels, and has 22 I/Os with 5 ADCs. >>
Unless you need the A/D channels, look to the PIC 16C6x parts. The only
difference is the A/D and price.
<< Cool part! However, many people
seem to use the PIC16F84 because it's based on flash, so you don't have to
wait forever to erase them, but it doesn't have all the cool features of
the PIC16C7x, specifically SPI. >>
I don't find errase time to be a problem most of the time. I have been
working a lot lately with PICs. Generally I write some code, simulate it,
burn it, test it and after a while erase it. In the last 3 months of steady
progress, there have been only a couple of times where I was actually waiting
for the eraser to erase the chip so I could blow it again. (I have a weak
eraser which will only erase one chip at a time and it takes about 2 hours to
erase.) Most of your time should be spent in the simulator checking out your
code. Note though that it will not simulate the hardware functions of serial
I/O (I assume this is also true of the SPI channels). When you put a byte in
the TXREG (serial transmit out), it takes a certain amount of time for the
"TXREG empty" interrupt bit to go on, as though the letter had been shifted
out, but you do not get to see the I/O bit toggeling. Also I have not found a
good way simulate serial data coming. I just manually put the letter in the
RCREG and then manually set the "Receive data available" bit in the STATUS
register.
<< Emulating SPI in software seems like a silly waste of cycles. I'd really
like to overlap taking measurements with sending data, which looks like it
could be difficult, if it's manually clocking bits out on the serial
interface. >>
I have not done SPI myself yet, but plan to in the next couple of weeks.
But with the PIC hardware, it should be quite simple. Given what a PIC costs,
I would suggest using the hardware SPI.
<< What would you guys recommend as a good general purpose PIC? Digikey has
the PIC16C76/JW for $16.55 and the PIC16F84-04/P is $6.25. I'm planning
to use the PICProto18 or PICProto3 from microEngineering Labs. With PICs
being fairly cheap, it might be easier to just use multiple PICs rather
than handling the software complexity of doing everything in one chip. >>
I'd say the 16C77, same RAM and ROM as the 76 but you have more A/D, more
I/O available and it is 10 cents LESS. It is wider than the 77, and I see
that as its only drawback. This is not so much to do more on one chip, but
when you want ot move to a new project with different needs, you have the
ports. I have moved code from one chip to another with only changing two
lines of code (the ones at the top that says:
LIST p=16c74 ;chip type
INCLUDE P16C74.INC
The 16C67 is $1.32 less to loose the A/D, but that really isn't much, and
if you don't want A/D, just don't enable them.
<< Has anyone connected multiple PICs to a SPI bus, with the HC11 in master
mode and the PICs in slave mode? The 68HC11 docs I have are a little
sparse on how multiple slave mode works. Would I have one slave select
line to each PIC and then the HC11 code would have to poll the PICs by
selecting each one in turn with some sort of simple packet protocol? It's
ashame that the 68HC11 doesn't support I2C, because it seems that they
solve this problem with that interface...>>
Not me.
<< We've got a nice programmer at work, so I shouldn't have any trouble
programming either chip. Another complication is that I don't have a PC,
so I was hoping to do all my development using one of the free assemblers
on my Unix box.
>>
Check their web site:
www.microchip.com
If there is support for Unix boxes, it will be there. I am currently
running Windows 95, but I have run the assembler on an IBM XT (8088). The
simulator requires a minimum of x286. I have just started using MPLAB, which
is there full up integrated windows development environment. I'm still going
through growing pains with this, but it has a lot of features that I really
enjoy (click on a line in the error listing, and the source code comes up to
that line)... But the serial simulation does not seem to be as good as the
one I had with MPSIM. (I no longer get a "Transmit Buffer Empty" interrupt
after a time when sending letters.)
Let me know if you have any other questions. Also in all fairness I
should mention Ben Wirz (of Wirz electronics) preferrs the Flash stuff so you
might want to ask for his comments supporting this technology.
www.wirz.com
Hope this helps,
Pherd
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