RpiSoft-UART

This driver will create a software-based serial port/UART using any GPIO pin, similiar to /dev/tty* devices, with some unique features.

Features

Works with Minicom
Works with cat and echo commands
256 bytes RX buffer (read the data whenever you want, no data lost!)
256 bytes TX buffer (no need to send each byte separately, send the whole array once!)
RX buffer cleared automatically after a read operation (Ex: cat)
TX buffer cleared automatically after a write operation (Ex: echo)
No pooling needed
Loopback mode
Runtime changeable parameters (GPIO pins, baudrate and loopback mode)

Compiling

Export the path location of your toolchain/GCC, Ex:

export PATH=$PATH:/root/raspberry/tools/arm-bcm2708/arm-bcm2708-linux-gnueabi/bin

Edit Makefile to match your kernel source directory and GCC prefix:

KERNEL_SRC_DIR := /root/raspberry/linux
GCC_PREFIX := arm-bcm2708-linux-gnueabi-

Now type make and you have the softuart.ko driver.

Structure

The driver will be created at /sys/class/softuart/softuart/:

root@raspberrypi:/home/pi# cat /sys/class/softuart/softuart/
baudrate   data       gpio_rx    gpio_tx    loopback   power/     subsystem/ uevent

Where:

baudrate - Selected baudrate
data - Used to send and receive data, is the /dev/tty* like file
gpio_rx - Selected GPIO pin to receive data
gpio_tx - Selected GPIO pin to transmit data
loopback - Loopback mode, 1 - on / 0 - off (default)

Usage

Driver parameters:

root@raspberrypi:/home/pi# modinfo softuart.ko 
...
parm:           BAUDRATE: Baudrate value [default = 4800] (min=1200, max=19200) (int)
parm:           GPIO_TX: GPIO TX pin [default = 4] (int)
parm:           GPIO_RX: GPIO RX pin [default = 2] (int)

Loading module with default parameters:

insmod softuart.ko

Loading module specifying all possible parameters:

insmod softuart.ko GPIO_TX=4 GPIO_RX=2 BAUDRATE=4800

Examples

Minicom:

minicom -b 4800 -D /sys/class/softuart/softuart/data

Send data with echo:

echo abc > /sys/class/softuart/softuart/data

Receive data with cat:

cat /sys/class/softuart/softuart/data

Loopback mode:

echo 1 > /sys/class/softuart/softuart/loopback
echo abc > /sys/class/softuart/softuart/data
cat /sys/class/softuart/softuart/data
abc

Change baudrate to 9600 kbps:

echo 9600 > /sys/class/softuart/softuart/baudrate

Advanced

Just don't send more than 256 bytes at once and all must be ok, 256 is enough for most applications, but if you need more just change in the source code.

Understanding the RX buffer

The driver have a 256 bytes RX buffer. This mean you can receive up to 256 bytes at once and they will be stored, without loss, if more bytes are received the buffer will be cleared and start store again. Any read operation at 'data' entry, will clear the buffer (Ex: cat). The buffer have only effect without the use of a software like minicom (because have it's own buffer, and due the pooling, keep the buffer clear).

Understanding the TX buffer

The driver have a 256 bytes TX buffer. This mean you can send up to 256 bytes at once and they will be stored and transmitted until the last byte, clearing the buffer automatically. If more bytes than the buffer capacity is written, the buffer will clear and start store again. Any write operation at 'data' entry, will start the transmission (Ex: echo). The buffer have only effect without the use of a software like minicom (because transmit each byte separately, using just one byte of the TX buffer at time).

Remember, this is a software-based UART, subject to preemption, not a piece of dedicated hardware. Details, limitations and a user-space version can be found HERE.

License

GPLv2 License, details HERE.

Updated (june 2016)

Tag 1.0 released and a pull request was accept on the Master branch, seems to be that the new raspberry pi toolchain can't understand some of the old symbols, so they were hardcoded

sigwin/RpiSoft-UART