Communication between PIC18F26K80 and DAC7612

Question

I bought the DAC7612 and would like to make it communicate with a PIC18F26K80 microcontroller. It is specified on the Farnell website that you can use SPI communication, but there is nothing about SPI in the DAC7612 datasheet.

Indeed, the microcontroller must send 14 bits to the DAC (2 to choose the output of the DAC and 12 data bits). The SPI communication allows to send 8 bits at a time. It is therefore not possible to use it I guess.

So I decided to use bit banging but I don't get any voltage at the output of the DAC7612. However, I can observe with the oscilloscope the signals sent by the microcontroller (CLK, SDI, CS, LOADDACs). Here is the code:

void loadDAC(uint16_t d)
{
uint16_t dac;
uint8_t i;
 
dac = d | 0x2000;           // DAC Port A
LOADDACS_SetHigh();
CS_DAC_SetLow();            // CS_DAC low level
__delay_us(0.03); 
 
for(i = 0; i < 14u; i++)
 {
 if(dac & 0x2000)
      SDO_DAC_SetHigh();        // SDO high level
 else
      SDO_DAC_SetLow();         // SDO low level
 
 CLK_DAC_SetHigh();             // SCK high level
 __delay_us(0.1);
 CLK_DAC_SetLow();              // SCK low level 

 dac <<= 1;  
 }
__delay_us(0.03); 
CS_DAC_SetHigh();           // CS_DAC high level
LOADDACS_SetLow();
__delay_us(0.03);
LOADDACS_SetHigh();
}

Thank you for reading.

---

Code for SPI Communication but still nothing:

        dac_value =0xFFF;   // 12 bits resolution
        
        LOADDACS_SetHigh();
        SPI_Open(SPI_DEFAULT);
        CS_DAC_SetLow();
        SSPBUF = (uint8_t)(dac_value >> 6)| 0x80; // 0x80 : select DAC Port A and send 4 MSB
        while(!SSPSTATbits.BF);
        SSPBUF = (uint8_t)(dac_value << 2 & 0x00FF);    // Send 8 LSB
        while(!SSPSTATbits.BF);
        CS_DAC_SetHigh();
        SPI_Close();
        LOADDACS_SetLow();
        __delay_us(0.03);
        LOADDACS_SetHigh();

Answer 1

In the bit-banged code you seem to be missing a delay between setting the output and setting the clock high (\$t_{DS}\$ in the datasheet and also \$t_{CL}\$). Not sure if this is significant though.

Your 0.03 us delay (but how accurate is that delay function and does it actually take a float argument?) is also exactly the minimum value and should be increased a bit. Maybe start with 1 us delays everywhere -- or even 10 us, then fine-tune.

You may also be missing a first high-to-low clock transition before the first iteration of the loop (clock should idle high), which would offset everything by one bit.

---

In your SPI code, you've left-aligned the 14 bits instead of right-aligning them, so everything is off by two bits and the device would use the 2 MSB of the value for output selection instead of A1 and A0 (which get shifted out). The first word for output A should be (dac >> 8) | 0x20 and the second word dac & 0xFF.

But, your SPI clock phase and polarity settings also look wrong -- I think it should be SPI_CPHA1 | SPI_CPOL1 instead of SPI_DEFAULT. As you've set it, the DAC will be off by one bit in the other direction.

So the net effect of the two mistakes is that everthing is shifted by one bit, just like for the bit-banged code! It will read A0 for A1 and the MSB of the value for A0. This produces the behaviour you describe: your A1 input doesn't matter, A0 = 1 (P odd) combines with the MSB of 0xFFF to give you register B (H-H in the truth table), and A0 = 0 (P even) combines with the MSB of 0xFFF to give you register A and B (L-X in the truth table).