Modern Computer: Von Neumann

ECEC 201 · Week 8 · Lecture

A register byte is several fields

• This layout is a made-up example, not the station sensor's.
• A mask has ones on the field's bits: MODE is 0xC0, READY 0x08, COUNT 0x07.
• Mask first, then shift the field down to bit 0.

ECEC 201 · Week 8 · Lecture

Masks and shifts in C

#define MODE_MASK   0xC0u
#define MODE_SHIFT  6
#define READY_MASK  0x08u
#define COUNT_MASK  0x07u

unsigned mode  = (reg & MODE_MASK) >> MODE_SHIFT;
int      ready = (reg & READY_MASK) != 0;
unsigned count =  reg & COUNT_MASK;

reg |= READY_MASK;    /* set a bit   */
reg &= ~READY_MASK;   /* clear a bit */

ECEC 201 · Week 8 · Lecture

volatile and byte order

• volatile tells the compiler every access really happens: the value can change without the program writing it.
• It matters when a pointer reads hardware memory directly. A byte that read() copied into your buffer is ordinary memory.
• A 16-bit register sent as two bytes arrives in the order the datasheet states, not the CPU's order.
• Build it explicitly: (b[0] << 8) | b[1] when the high byte comes first.

ECEC 201 · Week 8 · Lecture

One register through the I2C device file

/* needs fcntl.h, unistd.h, sys/ioctl.h, linux/i2c-dev.h */
int read_reg(const char *dev, int addr, unsigned char reg)
{
	unsigned char val;
	int ok = 0, fd = open(dev, O_RDWR);

	if (fd < 0)
		return -1;
	if (ioctl(fd, I2C_SLAVE, addr) == 0 &&
	    write(fd, &reg, 1) == 1)
		ok = read(fd, &val, 1) == 1;
	close(fd);
	return ok ? val : -1;
}