At 04:10 PM 1/9/2004 -0600, you wrote:
>Hello all,
>
>I seem to have my RS232 problem fixed but now I have another problem that
>seems more strange to me.  I have an LT1790 1.25V Voltage reference
>connected to the Vref+ of the PIC A/D converter and powered from RD3
>(PIC16F877A) in my temperature logging circuit.  I can adjust the sampling
>interval from the PC over the RS232 link and when I set it to 1 or 2 seconds
>a strange problem arises.
>
>The LT1790 along with the LM60 temp sensor are switched on with the PIC pins
>when they are needed to save power.  With the sample interval set to 1 or 2
>seconds the voltage reference doesn't power on properly every other sample.
>Looking with a scope with the interval set to 1s I see the power to the
>reference IC rise to the full value every second.  I can't see any
>difference from one pulse to the next.  However the output of the reference
>is correct one time and then barely rises the next.  It consistently
>alternates between accurate output and barely rising above ground.  I want
>to stress that this doesn't appear to be random - in the few hundred samples
>I have taken and looked at it either works fine or alternates 1 correct 1
>wrong.
>
>When I set the sample interval to 4 seconds it appears to work fine.  And
>just now I set it to 2 seconds and it worked OK for the few samples that I
>watched.  But set it to 1 second and it consistently shows the odd behavior.
>
>Any ideas?

Try tri-stating (changing to input) RD3 rather than setting it to low.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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