Temperature with the LM34C

[StoneColdFuzzy]

In the project by Reynolds Electronics here, what's a good size potentiometer to use for calibration?

~ Dave

[Archangel]

That is a simple voltage divider. 5v supply /5k ohm = .001 Amp
5v / 10k ohm = .0005 amp how much current do you want to waste ?
Assuming zero current flow into the PIC (impossible I know) the voltage will be linear across either one. It gets less true as the PIC begins to load the voltage divider, and as this parasitic loss increases the more suitable the lower resistance becomes.

[mister_e]

Look in your PIC datasheet in the analog to digital converter section. There you will find the maximum impedance you may use with your Specific PIC model.

Usually a simple small trim-pot (single or multi-turn) work pretty well... You could also use a already made voltage reference, this doesnt require any calibration and tend to be more stable on the long run.

[StoneColdFuzzy]

OK, I'm using a PIC16F877a instead of the PIC16F877. I have a 4 MHz ceramic osc and I'm using the direct connection as on the Rentron page. I don't know if I need to do this or not, but I have the VSS and VDD pins tied together, respectively. I checked the connections and tested the LM34 with 5 volts. The LM34 changes output voltage fine I don't know if I goofed the code up or not, but I'm getting nothing. Here's the code:

@ device  pic16F877A, xt_osc, wdt_off, lvp_off, protect_off

DEFINE loader_used 1    ' Boot loader is being used
DEFINE debug_mode  1    ' Debug sending Inverted serial data
DEFINE debug_reg portc  ' Debug Port = PortC
DEFINE debug_bit 6      ' Debug.bit = PortC.6
DEFINE debug_baud 9600  ' Default baud rate = 9600
DEFINE osc 4            ' We're using a 4 MHz oscillator
DEFINE ADC_BITS 8       ' Set A/D for 8-bit operation
DEFINE ADC_CLOCK 1      ' Set A/D clock Fosc/8
DEFINE ADC_SAMPLEUS 50  ' Set A/D sampling time @ 50 uS
samples VAR WORD        ' Multiple A/D sample accumulator
sample  VAR BYTE        ' Holds number of samples to take
temp    VAR BYTE        ' Temperature storage
samples = 0             ' Clear samples accumulator on power-up

    TRISA = %11111111   ' Set PORTA to all input
    ADCON1 = %00000011  ' Set PORTA.0,1,2,5 = A/D, PortA.3 = +Vref
    PAUSE 500           ' Wait .5 second

loop:
    FOR sample = 1 TO 20    ' Take 20 samples
        ADCIN 0, temp       ' Read channel 0 into temp variable
        samples = samples + temp ' Accumulate 20 samples
        PAUSE 250           ' Wait approximately 1/4 seconds per loop
    NEXT sample
    temp = samples/20
    DEBUG "Temperature is: ",DEC temp," Deg F",10,13
    samples = 0             ' Clear old sample accumulator
    GOTO loop               ' Do it forever

    END

[mackrackit]

What do you have connected to AN3? If nothing then try
ADCON1 = %00000100

[StoneColdFuzzy]

What do you have connected to AN3? If nothing then try
ADCON1 = %00000100

I have the wiper of a pot connected and set at 2.55 volts.

~ Dave