soft_uart

Software-based serial port module for Raspberry Pi.

This module creates a software-based serial port using a configurable pair of GPIO pins. The serial port will appear as /dev/ttySOFT0.

Features

• Works exactly as a hardware-based serial port.
• Works with any application, e.g. cat, echo, minicom.
• Configurable baud rate.
• TX buffer of 256 bytes.
• RX buffer managed by the kernel.

Compiling

Fetch the source:

git clone --depth 1 https://github.com/hydrogeologger/soft_uart

Install the package raspberrypi-kernel-headers:

sudo apt install raspberrypi-kernel-headers

If compilation error of kernel version not found, may need to install 'raspberrypi-kernel' May take roughly up to 30 mins

sudo apt update
sudo apt install raspberrypi-kernel raspberrypi-kernel-headers

Make sure to reboot/restart system after kernel installation for kernel version to be updated.

Run make install, as usual.

cd soft_uart
make
sudo make install

I haven't tried cross-compiling this module, but it should work as well.

Loading

Module parameters:

• gpio_tx: int [default = 16]
• gpio_rx: int [default = 13]

Loading the module with default parameters:

sudo insmod soft_uart.ko

or

make load

Loading module with custom parameters:

sudo insmod soft_uart.ko gpio_tx=10 gpio_rx=11

Unloading

To unload: sudo rmmod soft_uart or make unload

Reload module

make reload

Usage

The device will appear as /dev/ttySOFT0. Use it as any usual TTY device.

You must be included in the group dialout. You can verify in what groups you are included by typing groups. To add an user to the group dialout, type:

sudo usermod -aG dialout <username>

Usage examples:

minicom -b 4800 -D /dev/ttySOFT0
cat /dev/ttySOFT0
echo "hello" > /dev/ttySOFT0

Baud rate

When choosing the baud rate, take into account that:

• The Raspberry Pi is not very fast.
• You will probably not be running a real-time operating system.
• There will be other processes competing for CPU time.

As a result, you can expect communication errors when using fast baud rates. So I would not try to go any faster than 4800 bps.

Load kernel module on boot

To enable auto load of module on rpi bootup. Add soft_uart to a new line in /etc/modules