Pull-ups on 16F88

[Steves]

Hi, I am monitoring some sensors which provide 0V to 10V output. I have two 4.7K resistors as a divider and all seemed OK but the accuracy was poor. I set the OPTION_REG bit 7 to turn off the internal pull-ups but no good. Some meassurements and calculations indicate the 10k internal resistor is still present.
With 8 bit resolution I get 255 at 10V but 51 at 0V. I am certain the 10K pull-ups are not diabled but after going over the data sheet a number of times I can't find how to disable them, other than option_reg. I am using portA to display on LCD and have tried ADCIN 5, val and ADCIN 6, val but both have pull-ups to Vdd.
Any suggestions please.
Steve

[Melanie]

You are aware that some 0-10v Sensors don't actually start at 0v (you should get 0v if they're unplugged). I usually use 1K or 1K5 1% Resistor values for this job (with a 1uF and 100nF in parallel across the PIC PIN to kill anything nasty arriving down the line).

[Steves]

Hi and thanks for the reply Melanie, your right some sensors are really modified from 4-20mA loops and don't go to 0V. However these do, also with the sensor unpluged (ie. PIC input is open circuit) the input goes to 5V which makes me sure that it is pulled up to rail.
As I understand the device there are no internal weak pull-ups on portA so I am going to try using that to see if I have everything else OK, but I need to find a way to use portB.
Hope you can provide another possible solution.
Thanks, Steve

[tenaja]

Any time you have problems with a port, be it digital or analog, always first look to see what else is connected to the port. In this case, RB6, T1OSO, PGC, and T1CKI all share AN5. Also on AN6 is RB7, PGD & T1OSI.

Without your code, we can't even begin to debug it. However, we can suggest you read the d/s to make sure that all of those functions are disabled. Also, it could be a hardware issue... if you are using ICSP, there may be interference with the ANx.

BTW, I don't see any reference to setting the TRIS properly... this is the most obvious issue.

[Steves]

Hi Tenaja,here is my code for the A/D part of my project. I have tried it and find that accuracy is better the further away from 0V the input voltage is.

INCLUDE "modedefs.bas"

@ DEVICE pic16F88, INTRC_OSC_NOCLKOUT ' system clock options
SI VAR PORTB.1 ' Serial output pin
SO VAR PORTB.2 ' Serial Input pin
dig_num var word
x var word
num var word
remain var word

TRISA = %00000001 'bit 0 (AN/0) is input
TRISB = %10000010 'bit 7 (AN6) is input bit 1 is serial in
OSCCON = %01101000 '4MHz clock
ANSEL = %01000001 'AN0 and AN6 are analog
ADCON0 = %11110001 'channel 6 is selected
ADCON1 = %01000000 'Ref is Vdd and Vss
OPTION_REG.7=1 'dissable weak pull-ups on portB?
pause 2000 'let power supply settle
sens_in:
serout2 SO,813,["Hello",13,10] 'test serial connection
'see if all registers are set OK
serout2 SO,813,["ADCON0 = ",#ADCON0,13,10]
serout2 SO,813,["ADCON1 = ",#ADCON1,13,10]
serout2 SO,813,["ANSEL = ",#ANSEL,13,10]
serout2 SO,813,["OSCCON = ",#OSCCON,13,10]
serout2 SO,813,["OPTION_REG = ",#OPTION_REG,13,10]
serout2 SO,813,["Before A/D in",13,10]
'check digital value of A/D before conversion
serout2 SO,813,["Digital num = ",#dig_num,13,10]
pause 5000

ADCIN 6, dig_num 'use chanel 6 (RB7/AN6) as A/D input dig_num is digital result
while ADCON0.2=1:WEND ' Check to make sure ADC conversion finished..
pause 250

serout2 SO,813,["After A/D in",13,10] 'let meknow A/D is finished
x = dig_num*39
num = x/1000
remain = (x//1000)/100
serout2 SO,813,["dig_num = ",#dig_num,13,10]
serout2 SO,813,["x = ",#x,13,10]
serout2 SO,813,["Result = ",#num,".",#remain,13,10]

pause 5000

goto sens_in
end

An open input results in a 10V reading. I have looked thru the D/S again as you suggested, but cannot find any ref. to internal pull-ups other than setting OPTION_REG.7, though measurement indicates that they exist.
Sometimes though you see what you expect to and my reg. settings may well be wrong.
Thanks for the help
Steve

[Archangel]

Hi Steve,
Your config statement has noclockout option for osc, you might try INTRC_OSC, I am not too familiar with the PM assembler, in MPASM it says _INTRC_IO, and does not offer a NO_CLKOUT for this chip. Also the FCMEN_OFF should be in there as Device 2 as it is hooked up to the port you are using, put a scope on that port you might see a 32kc signal there . . . maybe.
the following is from the pm assembler's file about this chip:


; *** DEVICE Device Definitions
PIC16F88 equ 50081003h ; 14-Bit, 4K Code, 256 EE, ID = 3

; *** DEVICE Fuses Definitions
EXTRC_OSC_CLKOUT equ 3FEC0013h ; XX XXXX XXX1 XX11
EXTRC_OSC_NOCLKOUT equ 3FEC0012h ; XX XXXX XXX1 XX10
EXTRC_OSC equ 3FEC0012h ; XX XXXX XXX1 XX10
INTRC_OSC_CLKOUT equ 3FEC0011h ; XX XXXX XXX1 XX01
INTRC_OSC_NOCLKOUT equ 3FEC0010h ; XX XXXX XXX1 XX00
INTRC_OSC equ 3FEC0010h ; XX XXXX XXX1 XX00
EXTCLK_OSC equ 3FEC0003h ; XX XXXX XXX0 XX11
EC_OSC equ 3FEC0003h ; XX XXXX XXX0 XX11
HS_OSC equ 3FEC0002h ; XX XXXX XXX0 XX10
XT_OSC equ 3FEC0001h ; XX XXXX XXX0 XX01
LP_OSC equ 3FEC0000h ; XX XXXX XXX0 XX00
WDT_ON equ 3FFB0004h ; XX XXXX XXXX X1XX
WDT_OFF equ 3FFB0000h ; XX XXXX XXXX X0XX
PWRT_ON equ 3FF70000h ; XX XXXX XXXX 0XXX
PWRT_OFF equ 3FF70008h ; XX XXXX XXXX 1XXX
MCLR_ON equ 3FDF0020h ; XX XXXX XX1X XXXX
MCLR_OFF equ 3FDF0000h ; XX XXXX XX0X XXXX
BOD_ON equ 3FBF0040h ; XX XXXX X1XX XXXX
BOD_OFF equ 3FBF0000h ; XX XXXX X0XX XXXX
LVP_ON equ 3F7F0080h ; XX XXXX 1XXX XXXX
LVP_OFF equ 3F7F0000h ; XX XXXX 0XXX XXXX
CPD_ON equ 3EFF0000h ; XX XXX0 XXXX XXXX
CPD_OFF equ 3EFF0100h ; XX XXX1 XXXX XXXX
WRT_1FOURTH equ 39FF0000h ; XX X00X XXXX XXXX
WRT_HALF equ 39FF0200h ; XX X01X XXXX XXXX
WRT_3FOURTHS equ 39FF0400h ; XX X10X XXXX XXXX
WRT_OFF equ 39FF0600h ; XX X11X XXXX XXXX
DEBUG_ON equ 37FF0000h ; XX 0XXX XXXX XXXX
DEBUG_OFF equ 37FF0800h ; XX 1XXX XXXX XXXX
CCPMX_ON equ 2FFF0000h ; X0 XXXX XXXX XXXX
CCPMX_OFF equ 2FFF1000h ; X1 XXXX XXXX XXXX
PROTECT_ON equ 1FFF0000h ; 0X XXXX XXXX XXXX
PROTECT_OFF equ 1FFF2000h ; 1X XXXX XXXX XXXX

; *** DEVICE2 Fuses Definitions
FCMEN_OFF equ 3FFE0000h ; XX XXXX XXXX XXX0
FCMEN_ON equ 3FFE0001h ; XX XXXX XXXX XXX1
IESO_OFF equ 3FFD0000h ; XX XXXX XXXX XX0X
IESO_ON equ 3FFD0002h ; XX XXXX XXXX XX1X