Software Serial

A software approach to serial (UART with just TX/RX) communication using Linux (Raspbian kernel module w/ Device Tree). The device driver utilizes interrupts and hrtimers for the lower half. The lower half push/pop bytes from the buffer and bitbang using the timer to transmit and receive bits. There are 1 start bit, 8 data bits, and 1 stop bit.

This module is built on the assumption that there is only 1 device. Therefore, the bottom half of the device driver does not do locking when extracting a byte to transmit. In the upper half, the device driver assumes that there is only 1 reader although there can be multiple writers.

Requirements

• kernel headers : the kernel headers are needed to compile this kernel module. The version to download will depend on your (target) kernel version which can be found running uname -r.
• dtc : RPi uses device tree for hardware enumeration. The dtc command will be used to compile the overlay and it may already be installed by default. The compiled overlay can be placed in the overlays directory and configured to be applied on boot.

Install

Clone the repository, run make to compile the device tree overlay and the kernel module.

Device Tree Overlay

This kernel module uses the GPIO pins specified in overlay.dts. Therefore, the overlay must be compiled, and put into /boot/firmware/overlays/ for it to be accessible. Specify the overlay in /boot/config.txt to apply the overlay. For example,

# Compile the overlay
dtc -@ -I dts -O dtb -o $(DT_OVERLAY).dtbo $(DT_OVERLAY).dts

# Place the overlay in the overlays dir
cp $(DT_OVERLAY).dtbo /boot/firmware/overlays/.

# Edit config.txt to include the overlay
echo "dtoverlay=$(DT_OVERLAY)" >> /boot/config.txt

After rebooting, check that the overlay has been properly applied to the device tree by running

dtc -I fs /sys/firmware/devicetree/base | less

There should be the module name in the device tree.

Kernel Module

To install the kernel module, run :

insmod soft_serial.ko

sudo may be needed due to permission. modprobe may be used instead of insmod but there are no other dependencies for this module.

The baudrate (default 38400) for the serial communication can be specified during module installation:

insmod soft_serial.ko baudrate=19200

To ensure that the kernel module has been installed, check the messages from the kernel by running :

dmesg -wH

To remove (uninstall) the kernel module, run :

rmmod soft_serial

Usage

As it is a kernel module, you can use applications like cat and echo to read and write data to the device. For example:

# Read
cat /dev/soft_serial

# Write
echo -ne "Hello World\r\n" >> /dev/soft_serial

License

GPL