connecting 4 GPIO pins to DAC using SPI interface

I think that one mistake I made was that I created an SPI slave rather than SPI master to write out signals. I've changed the SPI from slave to master, but still it doesn't work. I have the following code that basically runs if an button is pressed:

if (print_cnt == 1)

printf("LED Pattern 1\n");

alt_u8 tx = 10020;

alt_u8 rx = 0;

return_code = alt_avalon_spi_command(SPI_0_BASE,0 ,

1, tx,

0, rx,

0);

For some reason, this causes the FPGA to get stuck and stop working. Any thoughts why?