Modern Computer: Von Neumann

ECEC 201 · Week 9 · Lecture

Every signal pin is also a GPIO line

ECEC 201 · Week 9 · Lecture

A GPIO line is requested, then used

• Each GPIO controller is a file, /dev/gpiochipN; a line is an offset on that chip.
• gpiodetect lists the chips and gpioinfo lists their lines and who holds them; both are on the board's image.
• A program asks the chip for the line as an input or an output and gets back a new file descriptor for it.
• The line stays yours until that descriptor is closed or the program exits.

ECEC 201 · Week 9 · Lecture

Requesting one input line

/* needs linux/gpio.h, string.h, sys/ioctl.h */
int request_input(int chip_fd, unsigned offset)  /* -1: error */
{
	struct gpio_v2_line_request req;
	memset(&req, 0, sizeof req);
	req.offsets[0] = offset;
	req.num_lines = 1;
	req.config.flags = GPIO_V2_LINE_FLAG_INPUT;
	strcpy(req.consumer, "button");
	if (ioctl(chip_fd, GPIO_V2_GET_LINE_IOCTL, &req) < 0)
		return -1;
	return req.fd;
}

ECEC 201 · Week 9 · Lecture

Reading the line, and why buttons bounce

struct gpio_v2_line_values v = { .mask = 1 };

if (ioctl(line_fd, GPIO_V2_LINE_GET_VALUES_IOCTL, &v) == 0)
	pressed = v.bits & 1;

• A mechanical contact chatters for a few milliseconds when pressed or released.
• Read fast enough and one press looks like several.
• Debounce: count a change only after the value has stayed the same for an interval you choose and justify.

ECEC 201 · Week 9 · Lecture

make rebuilds only what changed

CFLAGS = -Wall -Wextra -g

button: main.o gpio.o
	$(CC) $(CFLAGS) -o $@ $^

• A rule is a target, a colon, its prerequisites, then recipe lines.
• Each recipe line starts with a tab, not spaces.
• $@ is the target; $^ is every prerequisite.