Why is not my SPI communication working? (Atmega644) - c

Question

I'm building a drum machine, and I have stored a sample header file with a kick sound which takes values between 0 and 170. I want to send it over SPI to a 10-bit MCP4811 DAC which then outputs it to a 3.5mm audio jack.
I have my MISO,MOSI,SCK and RESET pins connected to my USB programmer as well as the DAC.
Here is a snippet of the audio file stored in "samples.h"
unsigned const char sample_1[2221] PROGMEM = {0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6, ...}
unsigned int sample_len[1] = {2221}
So it is a sample of 2221 bits. This I want to send to the DAC using SPI with freq = 22 kHz.
I'm using a 16 MHz Crystal, so I set the fuses accordingly to use it.
I am using a Timer which overflows 22 kHz.
volatile unsigned int sample_count[1] = {0};
volatile unsigned int audio_out = 0;
volatile unsigned char spi_junk;
int main (void)
sei();
DDRB = 0b10110000; //Set MOSI, SCK and SS as output.
PORTB = (1 << PINB4) //active low on SS.
TIMSK1 = (1<<OCIE1A); //Enable interrupt
TCCR1B = (1<<WGM12) | (1<<CS11); // set CTC mode and divide clk by 8
OCR1A = 91; //16 MHz/(8*91) ~ 22068 Hz
//SPI Init
SPCR = (1<<SPE) | (1<<MSTR); //master, 8 MHz
SPSR = (1<<SPI2X);
ISR (TIMER1_COMPA_vect) {
audio_out = 0;
//If play_track == 1, then the sound should be played back.
if (play_track && sample_count[0] < sample_len[0]){
audio_out += (pgm_read_byte(&(sample_1[sample_count[0]++)));
// send audio_out to 10-bit DAC on SPI
PORTB &= ~(1<<PINB4); // B.4 (DAC /CS)
SPDR = (char) ((audio_out >> 6) & 0x000f); //byte 1 0 0 0 0 b9 b8 b7 b6
while (!(SPSR & (1<<SPIF)));
spi_junk = SPDR;
SPDR = (char) ((audio_out & 0x003f) << 2); //byte 2 b5 b4 b3 b2 b1 b0 0 0
while (!(SPSR & (1<<SPIF)));
spi_junk = SPDR;
PORTB |= (1<<PINB4);
}
My PIN set up is.
Atmega644 -> DAC
MOSI -> SDI
SCK -> SCK
SS -> /CS
On MCP4811
Vdd -> 5V
Vss -> GND
V_out -> Audio jack.
The rest of the pins on the MCP4811 are not connected to anything.
I have seen that the audio_out is working as expected, by displaying the audio_out value on a LCD screen. But nothing is being output to the DAC. Anyone see what could be wrong?
EDIT: Added the SPI init which I had missed to add.

Answer 1

Your line here
SPDR = (char) ((audio_out >> 6) & 0x000f); //byte 1 0 0 0 0 b9 b8 b7 b6
Will set ¬SHDN to 0 which will shutdown the DAC
0 = Shutdown the device. Analog output is not available. VOUT pin is connected to 500 kohm typical)
Set bit 12 to 1 instead
SPDR = (char) ((audio_out >> 6) & 0x0f)|0x10; //byte 1 0 0 0 1 b9 b8 b7 b6
from datasheet
1 = Active mode operation. VOUT is available.

Answer 2

There is no SPI initialization in your code.
add into main()
SPSR = (1 << SPI2X); // double speed (to get maximum of 8MHz output)
SPCR = (1 << SPE) | (1 << MSTR); // 1:1 prescaler, master mode, SPI mode 0, SPI enable
Also several remarks to your code:
use sei() only after all initializations are done, to avoid interrupts happened on uninitialized peripheral.
better first set PB4 high then setup it as output to avoid PB output low between two commands:
PORTB = (1 << PINB4) //active low on SS.
DDRB = 0b10110000; //Set MOSI, SCK and SS as output.