SPI1.c
#include"SPI1.h"

// send a byte via spi and return the response
unsigned char SPI1_IO(unsigned char ch) {
    SPI1BUF = ch;
    while(!SPI1STATbits.SPIRBF) {;} // wait to receive the byte
    return SPI1BUF;
}

// initialize SPI1
void SPI1_init() {
    // Master - SPI1, pins are: SDI1(B8), SDO1(A1), SCK1(B14).  
    // we manually control SS1 as a digital output (A0)
    RPA1Rbits.RPA1R = 0b0011; // Set SDO1 to A1
    SDI1Rbits.SDI1R = 0b0100; // Set SDI1 to B8, actually not connected to DAC
    SPI1CON = 0;              // turn off the SPI module and reset it
    SPI1BUF;                  // clear the rx buffer by reading from it
    SPI1BRG = 0x1;            // SPI1BRG = (80MHz/(2*20MHz))-1 = 1
    SPI1STATbits.SPIROV = 0;  // clear the overflow bit
    SPI1CONbits.CKE = 1;      // data changes when clock goes from HIGH to LOW (since CKP is 0)
    SPI1CONbits.MSTEN = 1;    // master operation
    SPI1CONbits.ON = 1;       // turn on SPI1
    TRISAbits.TRISA0 = 0;
}
//where channel is 0 or 1 (for VoutA and VoutB), and voltage is the 8-bit output level.
void setVoltage(char channel, unsigned char voltage){
    unsigned char LSB,MSB;
    
    LSB = voltage << 4;               
    MSB = channel << 7;
    MSB |= 0b01110000;
    MSB |= voltage >> 4;  
    CS = 0;                        // enable SPI1 by set chip select line low
    SPI1_IO(MSB);
    SPI1_IO(LSB);      
    CS = 1;                        // raise the chip select line, ending communication
}