Measuring Pots With a 16F84A
Components shown will give 8-bit resolution. Highest reading for a 10k pot was e9h (233d). There's a small overhead to remove from this (about a 4-5 count). Bigger pots will need a correspondingly smaller cap, eg 100k pot = 15n pot. Not decreasing the cap would allow finer resolution, but it would take longer to go through the pots. The discharge loop is far too long - discharging the cap takes only a few microseconds, although I've yet to update this program. The code below is just a test program. A good quality plastic cap is recommended for stability, as well as using a stable regulated power supply.
The code for a 4051 MUX would be quite similar, even a little simpler. The 4017 way can be expanded indefinitely using the same 2-wire system, whereas a 4051 system needs an extra chip for the Inhibit (or Chip Select=0) lines
This could be useable in a robotics system as cheap or convenient (possibly more so than encoders) position sensing for something like a grasp or turntable
rs equ porta.0 ;o/p 4017 reset
pot equ porta.1 ;i/p o/p measure/discharge cap
clk equ porta.2 ;o/p 4017 clock
led equ porta.3 ;o/p finished
go equ porta.4 ;i/p start
orgdata ram
idx rb ;current position
ptn rb ;pot number to be done
count1 rb
count2 rb
count3 rb
entry movlw 12h ;...i ooio
tris porta
bcf clk
bcf rs
bcf led
bsf go
movlw 00h ;oooo oooo
tris portb
clrf portb
movlw 80h
option
movlw 10h ;store pot readings in RAM at address 10h up
movwf fsr
clrf idx
movlw 01h ;start with pot#1
movwf ptn
bcf led
wait btfsc go ;wait for PB release on a.4
goto wait
call msdelay
waitl btfss go
goto waitl
setpot call reset ;discharge cap, select pot
measure btfsc pot ;bump measure while pot pin is low
goto store ;else jump out and store count
incf count1,f
goto measure
; Suggestion by Scott Dattalo
;
; This cuts one instruction out of the loop. The consequence of this
; alteration is that the loop will reach 256 25% sooner. So the 150n
; capacitor needs to be lowered to around 120n, in order for the count
; to approach 256 at maximum resistance. The advantages are that the
; pot can be read faster, and the capacitor discharged faster, which
; may be helpful in multi-pot applications
; decf count1,f
;measure incf count1,f
; btfss pot ;bump measure while pot pin is low
; goto measure ;else jump out and store count
store call cstore ;store reading
incf ptn,f ;next pot#
movlw 04h ;limit = (number of pots)+1
xorwf ptn,w
btfss zero
goto setpot ;next pot
goto stop ;all done
cstore movf count1,w ;put count value in RAM
movwf indf
call stee ;and/or EEPROM (also from 10h up)
incf fsr,f
return
reset bsf rs ;reset 4017, q0=1, no pot selected
nop
nop
bcf rs
movlw 10h ;a.2 = o/p, discharge cap
tris porta
bcf pot
call msdelay
clrf idx
nextpot bsf clk
nop
nop
bcf clk
incf idx,f ;current 4017 position
movf ptn,w ;pot to be measured
xorwf idx,w
btfss zero
goto nextpot ;loop until 4017=pot#
movlw 12h
tris porta
clrf count1
return ;to measure new pot
msdelay movlw 0f6h ;delay about 1 millisecond @ 10MHz
movwf count1
clrf count2
inc incfsz count2,f
goto inc
incfsz count1,f
goto inc
return
stee movf fsr,w ;store pot reading in EEPROM
movwf eeadr
movf count1,w
movwf eedata
call write
return
write bsf rp0
bsf wren
movlw 55h
movwf eecon2
movlw 0aah
movwf eecon2
bsf wr
wrend btfss eeif
goto wrend
bcf eeif
bcf wren
bcf rp0
return
stop bsf led ;LED on, finished
wrst goto wrst ;wait for RESET to go again
More pot reading circuitry