ADC with PIC16F88

The data sheet for the PIC16F88 tells you just about everything you=20 need to know about the A/D, except that it is very sensitive to=20 voltages outside the power supply range.

Do you need to know how to interface the temperature sensor to the=20 A/D? Since the sensor has an absolute voltage for a given=20 temperature, you may need to reference the A/D to an absolute voltage,=20 instead of using the power supply as the full scale reference. Then=20 you need to amplify the temperature signal (with an op amp, perhaps)=20 to move and expand the signal to use most of the A/D input voltage=20 range, so that you can take advantage of the 1024 levels of the A/D=20 for something like .16 degree resolution, on average (higher=20 resolution if you don't need the whole temperature range).

"John Popelish" wrote

Sounds like the OP is using an LM34 or LM35 temp sensor. I have played with these and using a 1.024V Vref (outside the datasheet specs for min Vref I know, but it worked fine for me using a 16F88). This allowed me to have .1F resolution, but also limited the max temperature to 102.3F. I know that the proper way would have been to use a higher Vref and an op-amp to boost (double) the output of the temp sensor, but hey it was purely for grins to see if it would work.

To the OP, I have some code that will initialize the 16F88 and take a sample. I can post it if you'd like.

Thanks, if you can post your code ...

My idea is to keep the circuit very simple, so without Amplifier: i think i have a signal of about 1.7 Volt (from -40 =B0C to 125 =B0C) so with 1700mV i can detect 0.1=B0C... but how can i adjust with the 1024 levels ?

If you don't care too much about resolution you can just connect it directly to the input. With Vref=Vcc=5V you'll get about 0.5°C resolution. If you use a TL431 for the reference (the minimum datasheet value of 2.5V for full temp range operation) then that will be halved to more like 0.25°C (or +/- 0.13°C in marketingspeak).

You can easily do a bit of integer math to calculate the temperature with an arbitrary reference, it's not like it's going to change very fast.

Best regards, Spehro Pefhany

"merco" wrote

Here is my __CONFIG stuff:

;Program Configuration Register 1 __CONFIG _CONFIG1, _CP_OFF & _CCP1_RB3 & _DEBUG_OFF & _WRT_PROTECT_OFF & _CPD_OFF & _LVP_OFF & _BODEN_OFF & _MCLR_OFF & _PWRTE_ON & _WDT_OFF & _INTRC_IO

;Program Configuration Register 2 __CONFIG _CONFIG2, _IESO_OFF & _FCMEN_OFF

This is how I set up my 16F88 that does ADC as well as I2C and RS232 serial communications. You will want to check the pin directions for RA1 and RA0. I also apply an external Vref on RA3. I think that RA5 is stuck as a 1 when you use _MCLR_OFF, but I set it anyway.

bsf STATUS, RP0 ;Switch to Bank 1

movlw b'00111011' ;Define I/O on Port A (RA5 mclr input, RA4 ADC input, RA3 Vref+ input, RA1 SDA, RA0 SCL, all others outputs) movwf TRISA & 0x7F movlw b'00000101' ;Define I/O on Port B (mostly output, RB2-RX input, RB0 input) movwf TRISB & 0x7F

movlw 0x00 ;All pins digital i/o for now movwf ANSEL & 0x7F

movlw b'01110000' ;Set internal osc to 8Mhz movwf OSCCON & 0x7F

; movlw 0x07 ;Turn off the comparators (unnecessary on 16F88) ; movwf CMCON

bcf STATUS, RP0 ;Back to Bank 0

call ADC_Init

Here is code to do an analog to digital conversion.

bsf ADCON0, ADON ; Turn the ADC back on

call Wait_100mS ;Let things stabilize for a real long time

bsf ADCON0, GO_DONE ;Start a conversion _ck_conversion btfsc ADCON0, GO_DONE ;Is it done yet? goto _ck_conversion ; nope, check again

movfw ADRESH movwf CurrentTempH ; save all 10 bits as temp (precision to .1 degrees w/1.024V Vref) bsf STATUS, RP0 ;Bank 1 for lower bits movfw ADRESL & 0x7F bcf STATUS, RP0 ;Bank 0 movwf CurrentTempL

Here is the ADC_Init subroutine.

ADC_Init

bsf STATUS, RP0 ; Select BANK 1

movlw b'00011000' ; ANSEL ; |||||||+-------------- ANS0 (RA0) ; ||||||+--------------- ANS1 (RA1) ; |||||+---------------- ANS2 (RA2) ; ||||+----------------- ANS3 (RA3) Vref+ ; |||+------------------ ANS4 (RA4) AN4 ; ||+------------------- ANS5 (RB6) ; |+-------------------- ANS6 (RB7) ; +--------------------- Unused movwf ANSEL & 0x7F

movlw b'11100000' ; ADCON1 ; |||||||+-------------- Unused ; ||||||+--------------- Unused ; |||||+---------------- Unused ; ||||+----------------- Unused ; |||+------------------ VCFG0 (Vref-) ; ||+------------------- VCFG1 (Vref+) ; |+-------------------- ADCS2 ; +--------------------- ADFM (0=left, 1=right justification) movwf ADCON1 & 0x7F

bsf TRISA & 0x7F, 4 ; Make RA4 input (Temp Sensor) bsf TRISA & 0x7F, 3 ; Make RA3 input (Vref+)

bcf STATUS, RP0 ; Select BANK 0

movlw b'01100001' ; ADCON0 ; |||||||+-------------- ADON ; ||||||+--------------- Unused ; |||||+---------------- GO/DONE ; ||||+----------------- CHS0 ; |||+------------------ CHS1 ; ||+------------------- CHS2 ; |+-------------------- ADCS0 ; +--------------------- ADCS1 movwf ADCON0

return

I cut and pasted it all, so hopefully everything you need is present except for my generic time delay/waste routines.