Modern Computer: Embedded Systems

ECE 320 · Week 8 · Lecture

A driver in userspace

• The kernel's generic spidev driver gives a device file, /dev/spidevB.C for bus B, chip select C.
• Your library turns "read channel n" into bytes on the wire, from the part's datasheet. A kernel module is not accepted.
• Reach the bus only through one function pointer, so a test can swap in a fake bus:

struct bus {
	int fd;   /* open("/dev/spidevB.C", O_RDWR) */
	int (*xfer)(struct bus *b, const unsigned char *tx,
		    unsigned char *rx, unsigned len);
};

ECE 320 · Week 8 · Lecture

The real bus: one spidev transfer

/* needs string.h, sys/ioctl.h, linux/spi/spidev.h */
int spidev_xfer(struct bus *b, const unsigned char *tx,
		unsigned char *rx, unsigned len)
{
	struct spi_ioc_transfer t;
	memset(&t, 0, sizeof t);
	t.tx_buf = (unsigned long)tx;
	t.rx_buf = (unsigned long)rx;
	t.len = len;
	if (ioctl(b->fd, SPI_IOC_MESSAGE(1), &t) < 0)
		return -1;
	return 0;
}

ECE 320 · Week 8 · Lecture

The mocked bus

struct mock {
	struct bus bus;            /* first, so the casts work */
	unsigned char reply[3];    /* what the "chip" sends back */
	unsigned char sent[3];     /* what the library sent */
};
int mock_xfer(struct bus *b, const unsigned char *tx,
	      unsigned char *rx, unsigned len)
{
	struct mock *m = (struct mock *)b;
	if (len > sizeof m->reply) return -1;
	memcpy(m->sent, tx, len);
	memcpy(rx, m->reply, len);
	return 0;
}

ECE 320 · Week 8 · Lecture

A UART byte, and a baud mismatch

• There is no clock wire. The receiver times every bit from the start edge at its own baud rate.
• When the two ends disagree, the receiver samples in the wrong places.
• stty -F /dev/ttyUSB0 9600 raw -echo sets the bench PC's side.