I am trying to control a digital potentiometer with a PIC18F14K50 and am having a random problem (potentiometer not remembering correct value) when I power off then back on. The X9313 datasheet states that to initiate a save of the digital pots wiper position into its own internal EEPROM the CS line must be taken from low to high while the INC line is pulled high. the INC line is on RC6 and the CS line is on RA5(CS is pulled to 5V thru a 10k to avoid unknown conditions on power up this was a late modification and is not shown on the schematic).
The problem is that when the volume is changed and the power is turned off and then back on the digital pot comes back on and has decremented one wiper position. This only happens when the volume is changed and then power is cycled the FIRST time. For example if I set the pot in the middle giving me an output of 1V at the wiper(using false numbers for simplicity) and each step is 50 mV when I cycle power the output at the wiper will be 950mV when I reapply power. If I leave the volume there and do not change it I can cycle power as many times as I want and it will always remember the 950 mV. But if I change it again to say 500 mV and cycle power, on the next power up I will have 450 mV at the wiper and that value will be remembered for as many power cycles as I have tested. It seems to be a walking wiper issue as discussed in this intersil app note but it seems strange that it only happens the first power cycle after the value has changed.
I added small portions of my code. the config bits and the portion where I am changing the value of the digital pot. If anyone wants to see the rest of the code let me know and I can get it to you. Can anyone see a problem with my code or the setup that would be causing this issue? I appreciate any input I can get.
CONFIG CPUDIV = NOCLKDIV ; No CPU System Clock divide
CONFIG USBDIV = OFF ; USB Clock comes directly from the OSC1/OSC2 oscillator block; no divide
CONFIG FOSC = IRC ; Internal RC oscillator, CLKOUT function on OSC2
CONFIG PLLEN = OFF ; Oscillator multiplied by 4
CONFIG PCLKEN = ON ; Primary clock enabled
CONFIG FCMEN = OFF ; Fail-Safe Clock Monitor disabled
CONFIG IESO = OFF ; Oscillator Switchover mode disabled
CONFIG PWRTEN = ON ; PWRT enabled
CONFIG BOREN = ON ; Brown-out Reset enabled in hardware only (SBOREN is disabled)
CONFIG BORV = 27 ; VBOR set to 2.7 V nominal
CONFIG WDTEN = OFF ; WDT is controlled by SWDTEN bit of the WDTCON register
CONFIG WDTPS = 1 ; 1:1
CONFIG MCLRE = ON ; MCLR pin enabled, RE3 input pin disabled
CONFIG HFOFST = OFF ; The system clock is held off until the HFINTOSC is stable
CONFIG STVREN = OFF ; Stack full/underflow will not cause Reset
CONFIG LVP = OFF ; Single-Supply ICSP disabled
CONFIG BBSIZ = ON ; 2kW boot block size
CONFIG XINST = OFF ; Instruction set extension and Indexed Addressing mode disabled (Legacy mode)
CONFIG CP0 = OFF ; Block 0 not code-protected
CONFIG CP1 = OFF ; Block 1 not code-protected
CONFIG CPB = OFF ; Boot block not code-protected
CONFIG CPD = OFF ; Data EEPROM not code-protected
CONFIG WRT0 = OFF ; Block 0 not write-protected
CONFIG WRT1 = OFF ; Block 1 not write-protected
CONFIG WRTB = OFF ; Boot block not write-protected
CONFIG WRTC = OFF ; Configuration registers not write-protected
CONFIG WRTD = OFF ; Data EEPROM not write-protected
CONFIG EBTR0 = OFF ; Block 0 not protected from table reads executed in other blocks
CONFIG EBTR1 = OFF ; Block 1 not protected from table reads executed in other blocks
CONFIG EBTRB = OFF ; Boot block not protected from table reads executed in other blocks
DIR_CCW
;
BSF UPDOWN ;Set up / down bit
BCF CS ;pulse CS line
NOP
BCF INC
NOP
NOP
BSF INC
NOP
;
MOVLW 0x64 ;100ms delay
MOVWF LOOP_COUNT
DLOOP CALL DELAY_ONE_MS
DECFSZ LOOP_COUNT
GOTO DLOOP
;
BSF CS
NOP
BCF ENCDR_FLG, 1
RETURN
;*********************************************************
DIR_CW
;
BCF UPDOWN ;Clear up / down bit
BCF CS ;pulse CS line
NOP
BCF INC
NOP
NOP
BSF INC
NOP
;
MOVLW 0x64 ;100ms delay
MOVWF LOOP_COUNT
DLOOP1 CALL DELAY_ONE_MS
DECFSZ LOOP_COUNT
GOTO DLOOP1
;
BSF CS
NOP
BCF ENCDR_FLG, 0
RETURN
Best Answer
I finally found the problem! While initializing the ports on the PIC the CS and INC lines on the digital pot were momentarily being cleared long enough to initiate a decrement command but not long enough for a store command. The correct value was being stored and the device would decrement that correct stored value each power cycle giving the illusion that a new value had been stored. By setting the CS and INC lines high before port initialization (didn't know I could do that even) I avoided the unwanted decrement.