Modern Computer: Full Stack

ECE 440 · Week 2 · Lecture

Each pin has a role on this board

ECE 440 · Week 2 · Lecture

Pad, pin, function

• A pad is a physical connection on the chip; a header pin is where a pad reaches the board's edge.
• Each pad can carry one of several functions: GPIO, a UART line, an I2C line, and so on.
• The pin controller picks one function per pad. Two drivers cannot both own a pad.
• Claiming a function means some other function on that pad is now off.

ECE 440 · Week 2 · Lecture

How the device tree claims a pad

&mydev {
	pinctrl-names = "default";
	pinctrl-0 = <&mydev_pins>;
	status = "okay";
};

• pinctrl-0 points at a pin group node the pin controller defines.
• When the driver probes, the kernel applies the "default" state before probe() runs.

ECE 440 · Week 2 · Lecture

Reading what is claimed now

# the tree the kernel booted with
dtc -I fs -O dts /proc/device-tree > live.dts

# which pads are claimed, and by whom (root, debugfs)
ls /sys/kernel/debug/pinctrl/
cat /sys/kernel/debug/pinctrl/*/pinmux-pins

Each row of pinmux-pins names a pad and the device that owns it, or says it is unclaimed.

ECE 440 · Week 2 · Lecture

The four columns

ECE 440 · Week 2 · Lecture

Electrical limits are part of the row

• Header I/O runs at the pad's I/O voltage; a higher voltage on an input can damage the chip.
• A pad sources or sinks a limited current: drive a load through a transistor, not straight from the pad.
• Internal pull-ups set in the device tree are weak; an I2C bus relies on proper resistor pull-ups, on the board or the device.