MCP4261 Digital Pot Control example - A quick little project

**mtripoli** · - 3rd March 2011, 20:45

I'm working on a project using the Microchip MCP4261 Digital Pots. Cute little parts...
Here's a code snippet and a schematic as example. The code is an example of reading a "real" pot via A/D and updating a "digital pot" with the value. I'm using the LAB-X2 board from meLabs for testing. The code uses the on-board POT1 for one channel (RA0) and a second off-board pot for channel two (RA1). Put a voltmeter on the wiper of the digital pot to see the values update. There are a few other features of the digital pot that can be used simply by changing the "address" value. There's code in there for writing the A/D value to a serial LCD display for testing - ignore it. Simple but maybe useful to someone...
Have fun.
MT
MCP4261.pdf

Code:

'******************************************************************************
'* Name    : MCP4261POT_000.bas                                               *
'* Compiler: PICBASIC PRO Compiler microEngineering Labs                      *
'* Author  : Mike Tripoli                                                     *
'* Notes:                                                                     *
'* Read an analog voltage on RA.0 and and RA.1                                *
'* copy the 8 bit value to a digital pot value register                       *
'* Display value on serial LCD display                                        *
'* 8-bit conversion yields 0-255 result for 0-5 volt input.                   *
'* Uses meLabs LAB-X2 demo board                                              *
'*                                                                            *
'*                                                                            *
'* Date    : 03/03/2011                                                       *
'******************************************************************************




'Setup some defaults for programming the device.
'High-speed OSC, Watch dog timer-OFF, Brown-out detect OFF, Low Voltage Program OFF, Protect code OFF
@ DEVICE HS_OSC,WDT_OFF,BOD_OFF,LVP_OFF,PROTECT_OFF

include "modedefs.bas"      'Shiftout mode definitions


' Define ADCIN parameters
Define  ADC_BITS        8	' Set number of bits in result
Define  ADC_CLOCK       3	' Set clock source (3=rc)
Define  ADC_SAMPLEUS    50	' Set sampling time in uS

ADC_VAL_0 Var     byte      ' Only using 8 bits of ADC Channel 0
ADC_VAL_1 Var     byte      ' Only using 8 bits of ADC Channel 1

POT0    var     word        ' WORD variable to for address (high byte) and pot value (low byte) POT0
POT1    var     word        ' WORD variable to for address (high byte) and pot value (low byte) POT1

LED1    Var     PORTB.0	    ' Alias PORTB.0 to LED1 on LAB-X2 board from meLabs
LED2    VAR     PORTB.1     ' Alias PORTB.1 to LED2
LED3    VAR     PORTB.2     ' Alias PORTB.2 to LED3

SCK     var     PORTC.3     ' Alias PORTC.3 to SCK (serial data clock)
SDO     var     PORTC.5     ' Alias PORTC.5 to SDO (serial data out)
CS      var     PORTC.2     ' Alias PORTC.2 to C_EN (chip enable)
LCD_DISP    var PORTB.3     ' LCD connected to PORTB.3 - LAB-X2 

TRISA   =   %11111111	    ' Set PORTA to all input
TRISB   =   %00000000       ' Set PORTB to all output
TRISC   =   %00000000	    ' Set PORTC to all output
ADCON1  =   %00000010	    ' Set PORTA analog
        
'Initialize registers and variables

        LOW     LED1    'Initialize LEDs OFF
        LOW     LED2    ' "
        LOW     LED3    ' "
        HIGH    CS                  'Chip enable high to start
        high     sck                'Mode 1,1 SCK idles high
;	    Serout2 LCD_DISP,396,[12]	' Clear the LCD display

  
;Initialize POT WORD
pot0.Highbyte = %00000000    'POT0 address in hex - easier for me to see which pot selected
pot0.lowbyte = 0             'POT0 value in DEC - initialize pot - can be any value

pot1.Highbyte = %00010000    'POT1 address in hex
pot1.lowbyte = 0             'POT1 value in DEC


mainloop:
        ADCIN 0, adc_val_0      	' Read channel RA.0 to ADC_VAL_0 - Uses onboard "POT1" on LAB - X2
        pot0.lowbyte = adc_val_0    'copy input voltage from pot to digital pot
        pause 10
        ADCIN 1, adc_val_1    	    ' Read channel RA.1 to ADC_VAL_1
        pot1.lowbyte = adc_val_1    'copy input voltage from pot to digital pot

;***** DELETE THIS SECTION - ONLY FOR TESTING ***************
;	    Serout2 PORTB.3,396,[12]	                            'Set to beginning of LCD display
;	    Serout2 PORTB.3,396,["ADC_VAL_0=",DEC3 adc_val_0,148]	' Display adc channel 0
;	    Serout2 PORTB.3,396,["ADC_VAL_1=",DEC3 adc_val_1,13]	' Display adc channel 1
;        PAUSE 10
;************************************************************
        GOSUB   write_pots
       
        goto    mainloop

WRITE_POTS:                                 ; Routine writes value to pots 
WR_POT_0:  
        low     CS                          ' Make chip active
        pause   1                           ' Delay after making chip active per data sheet - only need 60nS
        shiftout    sdo,sck,5,[pot0\16]     ' Mode 1,1 in MCP4261 DS / Mode 5 PBP - clock idles high, MSB shift out first
        high    CS                          ' MCP4261 DS recommends toggling /CS between commands
        pause   1                           ' Delay keeps from banging CS pin
WR_POT_1:
        low     CS                          'Make chip active
        pause   1                           'Delay after making chip active
        shiftout    sdo,sck,5,[pot1\16]
        high    CS
        RETURN  



        END 'ALL PROGRAMS NEED AN END STATEMENT!

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