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'PIC using AD7714'
1994\10\26@202811 by crocontroller discussion list

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Hello PIC users,

  I have a real question as well as using this as a test message for
the PIC e-mail list which seems to have low volume currently.

  I have a thermal control system (crystal oscillator oven) that uses
an Analog Devices front end, an AD7714, which is a signal conditioning
ADC.  The 7714 digitizes the output of a thermistor bridge (a voltage
level of +/- 1.25 volts) and the output of the 7714 is a 24-bit serial
word.  The goal here is to have the oven control done by a PIC.  I'm
wondering if anyone has the driver code which will permit communication
with an AD7714 from a PIC.  I've never coded assembly language before so
I'm starting at ground zero.  I have the system running from BASIC, but
now we want to decrease the physical size of everything from an entire
benchtop to a small pc board.

  Has anyone used a PIC to speak to this Analog Devices part?  If so,
I'm hoping for some getting-started hints!

  Thanks,

  Jim Johnson
  spam_OUTjjohnsonTakeThisOuTspamhpl.hp.com

1994\10\27@001836 by crocontroller discussion list

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On Wed, 26 Oct 1994, James L. Johnson wrote:

>    I have a thermal control system (crystal oscillator oven) that uses
> an Analog Devices front end, an AD7714, which is a signal conditioning
> ADC.  The 7714 digitizes the output of a thermistor bridge (a voltage
> level of +/- 1.25 volts) and the output of the 7714 is a 24-bit serial
> word.  The goal here is to have the oven control done by a PIC.  I'm
> wondering if anyone has the driver code which will permit communication
> with an AD7714 from a PIC.  I've never coded assembly language before so
> I'm starting at ground zero.  I have the system running from BASIC, but
> now we want to decrease the physical size of everything from an entire
> benchtop to a small pc board.
>
>    Has anyone used a PIC to speak to this Analog Devices part?  If so,
> I'm hoping for some getting-started hints!

I currently don't have my PIC books with me at home, but in one of the
app notes they have the code for running a motor that has an A/D serial
output chip being used as feedback.  When I go back to the lab, I can
look it up and dig out some of my comm code that I have for the PIC.

Darryl
P.S. I think they have RS232 half and full-duplex as an app in the
embedded controls book.

1994\10\27@195534 by crocontroller discussion list

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On Wed, 26 Oct 1994, James L. Johnson wrote:

> Hello PIC users,
>
>    I have a real question as well as using this as a test message for
> the PIC e-mail list which seems to have low volume currently.
>
>    I have a thermal control system (crystal oscillator oven) that uses
> an Analog Devices front end, an AD7714, which is a signal conditioning
> ADC.  The 7714 digitizes the output of a thermistor bridge (a voltage
> level of +/- 1.25 volts) and the output of the 7714 is a 24-bit serial
> word.  The goal here is to have the oven control done by a PIC.  I'm
> wondering if anyone has the driver code which will permit communication
> with an AD7714 from a PIC.  I've never coded assembly language before so
> I'm starting at ground zero.  I have the system running from BASIC, but
> now we want to decrease the physical size of everything from an entire
> benchtop to a small pc board.
>
>    Has anyone used a PIC to speak to this Analog Devices part?  If so,
> I'm hoping for some getting-started hints!
>

 What kind of serial interface is there to the AD7714? Is it data and
clock, and maybe a chip select? Shouldn't be a problem reading the data
into PIC's RAM.  What do you want to do with the info once the PIC has it?

 Some useful code to look at would probably be the app notes on
communicating with Microchip's serial eeprom parts.

  Brian

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1994\10\28@162305 by crocontroller discussion list

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Hello,

 My thanks to responders Bryan Crotaz, Gary Gaskell, and to Brian who
writes:

>
>   What kind of serial interface is there to the AD7714? Is it data and
> clock, and maybe a chip select? Shouldn't be a problem reading the data
> into PIC's RAM.  What do you want to do with the info once the PIC has it?
>

   There are separate data, clock, and chip select lines in the AD7714.
It doesn't look like there will be any problems reading the data into
the PIC, I just haven't jumped into the details yet.  A description of
my project follows:

   We have a copper oven mass, which is about the size of a package of
cigarettes and weighs about 1.1 kilograms.  We are using it to keep a
quartz crystal at a constant temperature, about 84 deg-Centigrade.
There are thermistors buried in the oven mass and they are part of a
bridge circuit used to measure the temperature of the oven mass.  When
the bridge is balanced, the oven mass is at the correct temperature.
The AD7714 digitizes the two arms of the bridge and puts out a serial
24-bit word which is the value of the voltage across the bridge.  The
PIC reads the output of the AD7714 and implements a "PID" (proportional,
integral, and differential) type of controller.  Then the PIC drives a
D/A converter which drives a DC-DC converter connected to the oven mass
heater.  The heater heats the oven mass, completing the loop back to the
thermistor bridge.  Right now, we use an HP computer running BASIC as
the controller of this loop.  The HP computer drives an HP D/A converter
box, which drives a big DC amplifier, which drives the heaters on the
oven mass.  It all works quite well, the problem being that the whole
thing is about the size of a refrigerator!  So we want to use the PIC as
the computer and some small, pc board mounted components like D/A
converters and DC-DC converters such as those from Linear Technology or
Maxim or National Semiconductor, etc.

  Thanks again to all responders!

  Jim Johnson
  jjohnsonspamKILLspamhpl.hp.com

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