MCP4231 vs. MCP4251?

[keithv]

I was working with the 7-bit MCP4231. Everything was going fine, but I eventually found that I needed the finer resolution of the 8-bit MCP4251. I had assumed that the two were essentially pin-for-pin, and that even the code on the MCU didn't need to be changed (the wiper would just move half as far on the 4251). Below is an example practice code I used to get started working with the 4231.




' ' Hardware configuration ' ====================== TRISC=0 ' ' Hardware connection ' =================== CS VAR PORTC.0 CLK VAR PORTC.1 SI VAR PORTC.2 ' ' Variables definition ' =================== Wiper var byte ' ' Constants definition ' ==================== MSBFIRST CON 1 WriteOnPot0 con 000001 WriteOnPot1 con 010001 ' ' Software/Hardware initialisation ' ================================ cs = 1 ' disable MCP PAUSE 100 ' safe settle time Start: for wiper = 0 to 126 cs=0 ' enable shiftout si,clk, MSBFIRST,[WriteOnPot0, WIPER] ' Write to MCP first address shiftout si,clk, MSBFIRST,[WriteOnPot1, WIPER] ' Write to MCP second address cs = 1 ' Disable pause 10 next goto start






when attached to an LED, this program makes an LED get brighter and then stay fully lit. I had assumed that if I simply replace the 4231 with the 4251, that the program would still work, but the LED would only be lit half way, and that if I changed "for wiper = 0 to 255", that it would make the LED go to full brightness, but take twice as long. See page 38 of datasheet.

What am I missing here?

[keithv]

Sorry. I didn't post the code correctly last time. Here it is again. Basically what I want to know is why does this work just fine on the MCP4231, but not on the MCP4251?



'
' Hardware configuration
' ======================
TRISC=0

'
' Hardware connection
' ===================
CS VAR PORTC.0
CLK VAR PORTC.1
SI VAR PORTC.2

'
' Variables definition
' ===================
Wiper var byte

'
' Constants definition
' ====================
MSBFIRST CON 1
WriteOnPot0 con 000001
WriteOnPot1 con 010001

'
' Software/Hardware initialisation
' ================================
cs = 1 ' disable MCP
PAUSE 100 ' safe settle time

Start:
for wiper = 0 to 255
cs=0 ' enable
shiftout si,clk, MSBFIRST,[WriteOnPot0, WIPER] ' Write to MCP first address
shiftout si,clk, MSBFIRST,[WriteOnPot1, WIPER] ' Write to MCP second address
cs = 1 ' Disable
pause 10
next
goto start

[tumbleweed]

The way I read the datasheet it doesn't look like the first byte you're sending is correct (WriteOnPot0/WriteOnPot1).

The first 8-bits of the 16-bit command are: AD3 AD2 AD1 AD0 C1 C0 D9 D8
where D9 and D8 are the upper parts of the 10-bit wiper data, followed by the lower 8-bits of data D7-D0 in the second byte

The 7-bit part wiper settings are from 000H to 080H (full-scale), with 081H to 3ffH reserved
The 8-bit part wiper settings are from 000H to 100H (full-scale), with 101H to 3ffH reserved

I'm assuming you meant to use the binary constants


WriteOnPot0 con %000001
WriteOnPot1 con %010001

If so, you're always setting data bits D9 and D8 to '01' which means the wiper data is always from 100H to 1ffH


Try these instead


WriteOnPot0 con %00000000
WriteOnPot1 con %00010000

[keithv]

The way I read the datasheet it doesn't look like the first byte you're sending is correct (WriteOnPot0/WriteOnPot1).

The first 8-bits of the 16-bit command are: AD3 AD2 AD1 AD0 C1 C0 D9 D8
where D9 and D8 are the upper parts of the 10-bit wiper data, followed by the lower 8-bits of data D7-D0 in the second byte

The 7-bit part wiper settings are from 000H to 080H (full-scale), with 081H to 3ffH reserved
The 8-bit part wiper settings are from 000H to 100H (full-scale), with 101H to 3ffH reserved

I'm assuming you meant to use the binary constants


WriteOnPot0 con %000001
WriteOnPot1 con %010001

If so, you're always setting data bits D9 and D8 to '01' which means the wiper data is always from 100H to 1ffH


Try these instead


WriteOnPot0 con %00000000
WriteOnPot1 con %00010000


Thanks! That fixed it! I don't know why that previous code worked just fine for the 7-bit version, but it did. The 8 bit version didn't like it at all.