// Compile:  gcc mcp4822.c -std=c99 -o mcp4822
//     Use: ./mcp4822 1.23 0.12 

#include <stdint.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/types.h>
#include <linux/spi/spidev.h>

#define SPI_SPEED 2100000                       	// SPI frequency clock
#define VREF 2.048									// MCP4822 voltage reference [V]
static const char *device = "/dev/spidev32766.0";	// SPI dev
static uint8_t mode = SPI_MODE_3;					 

// Exit and print error code

static void exit_on_error (const char *s)			
{ 	
	perror(s);
  	abort();
} 

int main(int argc, char *argv[])
{
	int fd;
	int dataA, dataB;
	double voltA, voltB;
	
	uint16_t tx;    	// RX buffer (16 bit unsigned integer)
							
	struct spi_ioc_transfer tr = 
	{	.tx_buf = (unsigned long)&tx,         
		.rx_buf = (unsigned long)NULL,        
		.len = 2,             
		.delay_usecs = 0,
		.speed_hz = SPI_SPEED,   
		.bits_per_word = 8,
		.cs_change = 0,
	};

	if (argc != 3) 
	{ 
		printf ( "Usage: \n ./mcp4822 VA VB\n Vx is a float number from 0 to 2.048\n\n");
		exit(-1);        
	}
         
	voltA = atof(argv[1]);
	voltB = atof(argv[2]);
        
	if ((voltA > VREF) || (voltA < 0))
	{ 
		printf ( "VA out of range (0 -> %f)\n\n", VREF);
		exit(-2);        
	}
                                                      
	if ((voltB > VREF) || (voltB < 0))
	{
		printf ( "VB out of range (0 -> %f)\n\n", VREF);
		exit(-3);
	}
        
	// Open SPI device
	if ((fd = open(device, O_RDWR)) < 0) exit_on_error ("Can't open SPI device");

	if (ioctl(fd, SPI_IOC_WR_MODE, &mode) == -1) exit_on_error ("Can't set SPI mode");

	dataB = voltB / VREF * 4096; 	// Create data to send (see textand data sheet)
	dataB = dataB | 0xF000;	
        
	tx = (dataB << 8) | (dataB >> 8);	// Adjust bits
      

	// Write data
	if (ioctl(fd, SPI_IOC_MESSAGE(1), &tr) < 1) 
		exit_on_error ("Can't send SPI message");

	dataA = voltA / VREF * 4096;
	dataA = (dataA & 0x0FFF) | 0x3000;
        
	tx = (dataA << 8) | (dataA >> 8);                     
                            
	// Write data
	if (ioctl(fd, SPI_IOC_MESSAGE(1), &tr) < 1) exit_on_error ("Can't send SPI message");

	printf("Sending data to MCP4822 on %s\n\n", device);
	
	printf("Voltage A: %f V - Hex value: 0x%X \n\n", voltA, dataA );
	printf("Voltage B: %f V - Hex value: 0x%X \n\n", voltB, dataB );
                                  
	close(fd);
	return (0);
}