Help with an SPI pot

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi guys,
Im new to arduino and to these sorts of things in general (meaning SPI etc). In fact, i just received my arduino the other day. I have to say, i am very happy with it so far.

Sorry to bug, though, but i need a little hand with an SPI pot i am working with.

The pot is an MCP42100, you can find a datasheet here: http://futurlec.com.au/SFMicrochip/MCP42100.jsp

I am sure that i had it working late last night, but when i got up this morning it wasn't working. How frustrating! So i was hoping somewhere here could tell me what i might be doing wrong. maybe i have made a silly mistake?

Thanks for your help!

Here is the Arduino code i am using (for 0007 version):

#define SLAVESELECT 10//ss
#define SPICLOCK 13//sck
#define DATAOUT 12//MOSI
#define DATAIN 11//MOSI

// command bytes

byte cmdWritePot1 = B00010001; // write data, to pot 1
byte cmdWritePot2 = B00010010; // write data, to pot 2

char spi_transfer(volatile char data)
{
SPDR = data; // Start the transmission
while (!(SPSR & (1<<SPIF))) // Wait the end of the transmission
{
};
return SPDR; // return the received byte
}

// Setup Function Begins Here

void setup()
{
Serial.begin(9600);
byte i;
byte clr;
pinMode(DATAOUT,OUTPUT);
pinMode(SPICLOCK,OUTPUT);
pinMode(SLAVESELECT,OUTPUT);
digitalWrite(SLAVESELECT,HIGH); //disable device

SPCR = B01111000;
SPCR = (1<<SPE)|(1<<MSTR);
clr=SPSR;
clr=SPDR;
delay(10);
}

byte write_pots(int potValue11, int potValue12)
{
// spi pot chip 1
digitalWrite(SLAVESELECT,LOW); // enable chip 1
// set value for pot A
spi_transfer(cmdWritePot1); // command byte out to select pot and init write
spi_transfer(potValue11); // 8 bit pot value
// set value for pot B
spi_transfer(cmdWritePot2); // command byte out to select pot and init write
spi_transfer(potValue12); // 8 bit pot value
digitalWrite(SLAVESELECT,HIGH); // disable spi pot chip 1

}

// Main Loop Begins Here

void loop()
{

write_pots(255,0);
delay(100);

}

Here is a pic of my breadboard:

2

Okay, so now i think i know what i did wrong - the four SPI pins have to be 10,11,12,13 have to be CS, MOSI, MISO and SCK. So i set the pins correctly, but when i set the pot at a given value or a range of values from 0-255, it gives very strange readings - they are non-linear values that jump around but they do consistently change when i send a different value to the pot. Any ideas what could be causing this sort of problem would be appreciated.

Cheers,

3

hi

do you have the right hand (in the picture) power bus connected to VCC? A .1Uf bypass capacitor on the power is a good idea too.

D

4

Hi little-scale
I looked at the datasheet and I think you have to raise the CS signal after you wrote the command byte and the value for each pot.

Basic operation in write_pots should be

byte write_pots(int potValue11, int potValue12)  { 
   // spi pot chip 1  
   digitalWrite(SLAVESELECT,LOW); // enable chip 1 
   // set value for pot A 
   spi_transfer(cmdWritePot1); // command byte out to select pot and init write 
   spi_transfer(potValue11); // 8 bit pot value 
   digitalWrite(SLAVESELECT,HIGH); // load data for pot A into the registers on chip 1 

   //now do the same for pot B
   digitalWrite(SLAVESELECT,LOW); // enable chip 1 
   // set value for pot B 
   spi_transfer(cmdWritePot2); // command byte out to select pot and init write 
   spi_transfer(potValue12); // 8 bit pot value 
   digitalWrite(SLAVESELECT,HIGH); // load data for pot B into the registers on chip 1  
  
 }

Eberhard

5

Thank you both for your help. i got it to work now. i added a 0.1uF cap to filter the power supply to the digital pot. i changed the spi control register to B01010000. i made_sure that i was only sending single bytes and not ints in place of the data byt for the digital pot. i also got rid of some useless things in the code. you can see the updated and working version of the sketch in the link here: milkcrate.com.au. (its an RTF file called "spi_pot_working).

Thanks again for your time, i appreciate it.

Seb (from adelaide, australia)