Any idea what is going on?

[fabbie]

ok. This is what im basically doing.

Using a 16F876 to monitor my temperature (LM35), current and speed of my R/C Car. A PC would be used to sent data to my PIC to control the movements. in order to confirm what data im receiving from the PC, i had the data received in the RCREG immediately displayed out through LEDS.
But everytime the LEDS lit up, my monitoring readings would fluctuate badly. Example, 26 celcius reading would suddenly jump to 35 celcius and maitain there until i off the LEDs. Another instance, i plug the ADC pin raeding my 'current' directly into the ground. Instead of displaying zero, it displays somekind of value (verysmall value).
This seems to happen everytime i output something or consume more current in the PIC.
Another thing to add is, by using my PIC to control my motor, the monitoring raedings would fluctuate very very badly. The motor is using another power source with the ground connected to the PIC ground of course.

Whenever i do not output anything from the PIC, everything seems to run smoothly. Is there any possiblity this is caused by an unregulated power supply ?
Any idea what is going on?
Thx

 

[Nigel Goodwin]

It sounds like your power supply isn't regulated?, so the extra current consumed by the LED's is pulling the supply down?.

What value series resisitors do you have feeding the LED's?.

As you're monitoring a voltage, I would suggest you should use an external reference for the A2D, as my tutorials do - this should help considerably.

There's also the possibility of poor layout causing the problem, particularly with the current monitoring.

You might try posting the complete circuit so we can see how you are wiring the system.

 

[fabbie]

yup im currently not using a regulated supply. The reason is my college's so called 'regulated supply' causes my LEDs displaying the RCREG data to flicker uncontrollably. After changing to the unregulated supply, my UART seem to be far more stable. However, now im having this problem.

I was just wondering, can the 7805 be considered a reliable regulated supply?
i havent tried that yet

 

[Nigel Goodwin]

I was just wondering, can the 7805 be considered a reliable regulated supply?
i havent tried that yet

Yes, it's reliable - presumably you're not using a seperate voltage reference either?, so it's not suprising that the readings aren't reliable.

I would suggest using a 7805 (with the required two capacitors) to feed the PIC, and a seperate 2.5V precision reference for the A2D.

 

[fabbie]

If i use a different voltage reference, the -Vref would still be the PIC's Vss i assume?

I dont really understand, if i use the Vdd (7805 5V output) as the +Vref, why wouldnt it be reliable?although the 7805 isnt accurately 5V, its stable right? My main concern is to monitor the operations in a stable condition.

 

[Nigel Goodwin]

If i use a different voltage reference, the -Vref would still be the PIC's Vss i assume?

I dont really understand, if i use the Vdd (7805 5V output) as the +Vref, why wouldnt it be reliable?although the 7805 isnt accurately 5V, its stable right? My main concern is to monitor the operations in a stable condition.

The 5V output from a 7805 is 'reasonably' stable, but it's not really intended to be used as a voltage reference. A precision voltage reference is intended for just that, and is MUCH more stable than the output from a 7805 (particularly as it's fed from a 7805, so has a fairly decent source to start with!).

But you could certainly try it from a 7805 to begin with, and see how it works?.