A to D on the PIC16F687

[jblackann]

Hello, I am trying to just make a demo program so I can view the Ato D value on my LCD. I am not having good luck. I looked at another post that discussed problems with A to D on the 16F876. I was using this as a guide but something is not right.

I am trying to read a pot on RA4/AN3. I am reading a value but as I adjust my pot the numbers do not count up in a linear fashion. It will count from 0 to 1 to 2 for 7/8 of the single turn pot and then the numbers jump to 255 very quickly. This is linear pot and I should be gettng a linear increase or decrease.

Here is my code and the settings that I believe are relavant. The code is supposed to read the voltage from the pot and update a LCD to tell me the output. Thanks for any help.
----------------------------------------
AD_AN3 VAR PORTA.4
AD_AN3_VALUE VAR byte
DEFINE ADC_BITS 8 ' set number of bits in result
DEFINE ADC_CLOCK 3 ' set clock source
DEFINE ADC_SAMPLEUS 50 ' set sampling time for microseconds

MENU:
LCDOUT $FE, 1 ' Clear LCD screen
A_D_AN3:
ADCON0 = %00001101
PAUSEUS 50 ' wait 50 microsec
' ADCON0.2 = 1 'start conversion(bit 2= 1)
' WHILE ADCON0.2 = 1
' WEND ' wait for conversion
ADCIN 3, AD_AN3_VALUE ' read channel 3 to AD_AN3_VALUE
LCDOUT $FE, 1 ' Clear LCD screen
LCDOUT "A to D Value"
LCDOUT $FE, $C0, #AD_AN3_VALUE
PAUSe 100
BUTTON FUNCTION_BUTTON, 1, 255, 0, C2, 1, ON_TIME_FUNC
GOTO A_D_AN3

[schu4647]

where to begin. First you need to make port a an input. Second there are two AD registers 1 and 2. Third I would put the register setup before your main program. you only need to do it once. Try this.

DEFINE ADC_BITS 8 ' set number of bits in result
DEFINE ADC_CLOCK 3 ' set clock source
DEFINE ADC_SAMPLEUS 50 ' set sampling time for microseconds
ADCON0 = %10000000 'I think this formats the result
ADCON1 = %00000000 'I think this defines if ports are A or D. The first 0 may need to be a 1
trisa = %11111111 'Port A is input
AD_AN3 VAR PORTA.4
AD_AN3_VALUE VAR byte

pause 500 'LCD's need to initialise before sending data
LCDOUT $FE, 1 ' Clear LCD screen

A_D_AN3:
PAUSEUS 50 ' wait 50 microsec
ADCIN 3, AD_AN3_VALUE ' read channel 3 to AD_AN3_VALUE
LCDOUT $FE, 1 ' Clear LCD screen
LCDOUT "A to D Value"
LCDOUT $FE, $C0, #AD_AN3_VALUE
PAUSe 100
BUTTON FUNCTION_BUTTON, 1, 255, 0, C2, 1, ON_TIME_FUNC
GOTO A_D_AN3

If it still doesn't work its all about setting the bits in ADCON0 and 1 correctly. See the datasheet

[mister_e]

be sure your pot impedance meet the maximum impedance stated in the datasheet too.

ADCON1.7=1 ' Right justified results..

Also depending of your OSC speed, you may need to increase your sampling time, and you may want to add a tiny 0.01-0.1 uF capacitor at the PIC input to smooth the results.

[schu4647]

I just realized a mistake I told you. You may want to move your LCD off of port A since you are using that for your A to D. I don't know if it will work after making in input and analog. The book tells how to change it.

[psdayama]

Hi,
have U checked with multimeter that U are getting proper voltage on
wiper of POT? Unless that is confirmed then no use of banging head like
Mister_E on program and PIC. ;-)
It is possible that pot has mechanical problems at ground end and
after wiper moves few turns the ground end opens and U get full voltage
on wiper!!
As U are getting upto say number 10 then this is a probability.

[Acetronics2]

4 Errors found before compiling : ( between star lines ...)



DEFINE ADC_BITS 8 ' set number of bits in result
DEFINE ADC_CLOCK 3 ' set clock source
DEFINE ADC_SAMPLEUS 50 ' set sampling time for microseconds
ADCON0 = %10000000 'I think this formats the result
ADCON1 = %00000000 'I think this defines if ports are A or D. The first 0 may need to be a 1
trisa = %11111111 'Port A is input
AD_AN3 VAR PORTA.4
'************************************************* *********************
FUNCTION_BUTTON VAR PORTB.X ' TO BE DECLARED AS YOUR REQUIREMENTS !!!
'************************************************* ********************
AD_AN3_VALUE VAR byte
'************************************************* *********************
C2 VAR Byte 'TO BE DECLARED "
'************************************************* *********************
pause 500 'LCD's need to initialise before sending data
LCDOUT $FE, 1 ' Clear LCD screen

A_D_AN3:
PAUSEUS 50 ' wait 50 microsec
ADCIN 3, AD_AN3_VALUE ' read channel 3 to AD_AN3_VALUE
LCDOUT $FE, 1 ' Clear LCD screen
LCDOUT "A to D Value"
LCDOUT $FE, $C0, #AD_AN3_VALUE
PAUSe 100
'************************************************* ********************
C2 = 0 ' TO BE INITIALISED HERE !!!
'************************************************* ********************
BUTTON FUNCTION_BUTTON, 1, 255, 0, C2, 1, ON_TIME_FUNC
GOTO A_D_AN3

'************************************************* ********************
ON_TIME_FUNC: ' TO BE SOMEWHERE to be called !!!

' What is it supposed to do ???????????????????
'************************************************* *********************

END




NOW still hardware mix ... We continue : your LCD is "standard connected" ... so you MUST pass its RS pin to PORTB ( see defines list) to get RA4 free, as you want it an analog input. You also must declare ALL other PORTA pins as DIGITAL ( see ADCON bytes)

Sorry, I did not verify if defaults LCD connexions are the same for F687 or F84 ... but there's no reason for a difference in the pin names !!!


Now ... we will know if real ADC problems still exist ...

Alain