Help interfacing with a MCP4261 digital pot (SPI)

achilles03

I'm trying to interface with a dual channel digital pot (MCP4261), and can't get it to work. I've accessed just about everything else that uses SPI, but for some reason I'm having trouble with this chip:

http://ww1.microchip.com/downloads/en/DeviceDoc/22059b.pdf
(The meat of the SPI info is between pages 42 and 47)

I keep measuring 100k resistance across one side of the tap, and 0 across the other (both channels). I can't figure out what I'm doing wrong. The chip is connected to the pins via 10k resistors, and the LED works as it should. Vss and Vdd are connected, and everything else is floating. The BS2 pins aren't burned out, and I've tried a different stamp with the same results. Here's my code:

---

res var byte
Command var WORD

CS con 1
CLK con 2
SI con 3
LED con 4

high CS
high LED
pause 500
low LED
pause 500
high LED

res=24

repeatit:
command.highbyte=0
comand.lowbyte=res
low CS
shiftout SI,CLK,MSBFIRST,[noparse][[/noparse]command\16]
high CS

pause 10

Command.HighByte=%00010000
low CS
shiftout SI,CLK,MSBFIRST,[noparse][[/noparse]command\16]
high CS

pause 500
res=res+32

goto repeatit

---

No change in resistance while this program is running.

Any advice is greatly appreciated!

Dave

metron9

I think you forgot about the mode in the shiftout command. Check your idle state of the sck pin.

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

achilles03

I think I figured out the problem. I switched the 10k resistors with 500 ohm resistors and viola! It worked. There must be some internal pull-up or pull-down resistors on the digital pot that prevented the pins from reaching the appropriate logic states with the 10k resistors.

Dave