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----- Message d'origine ----
De : murrayatuptowngallery
À : p...@yahoogroups.com
Envoyé le : Jeudi, 10 Avril 2008, 9h37mn 48s
Objet : [piclist] new here, some questions
Hi:

I'm an electronics tech but haven't had a uP class since an 8085 one
in 1979 and an 8088 one in maybe 1984.

I have a project I need to uC-ify and have been spinning my wheels
looking at all the choices.

I got two pieces of 16F688 (SOIC I think, it has to be small) because
it has enough memory for inefficient code AND expansion.

What I would like to do is find out where I can start reading some
simple examples that show some basics of configuration etc.

Any suggestions on simplest programming interface appreciated. I will
need to be able to modify parameters once the device is built.

I'm not asking how to design it, but a description follows so you can
tell me if I am expecting too much from an 8-bit uC (like the math
expectations) .

Basically, it needs to measure one or two voltages, depending what
mode it's in.

Single voltage mode, it needs to translate a linearly varying voltage
to a very specific unit scale and display it on a 3-digit LED
7-segment display. I got some Maxim Charlie-plexing LED drivers too to
make it easier. If not linear enough (3-5% req'd), I am wondering if
it makes more sense to build in a lookup table or, or is polynomial
curve fitting getting too bold for 8-bit?

In the other mode it needs to sample & hold (external hardware if
necessary is OK) two voltages, then (tell me if this is expecting too
much of an 8-bit uC), determine the ratio and then scale or multiply

Thanks

Murray Leshner
Holland MI

Hi sir,
Please would you check this link:
www.talkingelectronics.com
Best regards,

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Comments below...

> I got two pieces of 16F688 (SOIC I think, it has to be small) because
> it has enough memory for inefficient code AND expansion.
>
> What I would like to do is find out where I can start reading some
> simple examples that show some basics of configuration etc.

I think a good place to start is http://www.piclist.org . There's a good
collection of source code there.

>
> Any suggestions on simplest programming interface appreciated. I will
> need to be able to modify parameters once the device is built.
>

There are various programmers and debuggers available. I've had good luck
with the ICD-2, though it may be a bit expensive for hobbyist use. The
PicKit2 looks interesting. See
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en534451

I really suggest getting some sort of hardware debugger like the ICD-2 or
PicKit2.

On choice of chip, you might want to look at the PIC18 series. A student
version of the Microchip C compiler is available for free.
> I'm not asking how to design it, but a description follows so you can
> tell me if I am expecting too much from an 8-bit uC (like the math
> expectations) .
>
> Basically, it needs to measure one or two voltages, depending what
> mode it's in.
>
> Single voltage mode, it needs to translate a linearly varying voltage
> to a very specific unit scale and display it on a 3-digit LED
> 7-segment display. I got some Maxim Charlie-plexing LED drivers too to
> make it easier. If not linear enough (3-5% req'd), I am wondering if
> it makes more sense to build in a lookup table or, or is polynomial
> curve fitting getting too bold for 8-bit?

Sounds fine. Many PICs have 10 bit ADCs and are linear to something like
1/2 bit. Remember that your accuracy is no better than your reference
voltage.

Linearity of the ADC is very good. If you have a nonlinear transfer
function, I'd use a lookup table with linear interpolation. I did this to
linearize a precision thermistor in a cow thermometer
(http://www.gla-ag.com/).

>
> In the other mode it needs to sample & hold (external hardware if
> necessary is OK) two voltages, then (tell me if this is expecting too
> much of an 8-bit uC), determine the ratio and then scale or multiply
>

How close in time do you need to measure the voltages? In most cases, I
think you can use the single ADC in the PIC with the internal multiplexer.
the measurements will be pretty close to the same time. If you really need
two measurements at the same time, there are some PIC24 chips that have
two ADCs.
Good luck!

Harold
--
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