Software emulated I2C for Raspberry Pi

This is a basic software implementation of the I2C protocol. This library uses WiringPi as a backend, but it can be easily adapted to others as long as you provide suitable alternatives for pinMode, digitalWrite and delayMicroseconds functions.

Note: If you are looking for an I2C LCD library that uses this bit-banged protocol, check wPi_soft_lcd: Library to use PCF8574 based LCD via software I2C bus.

Getting Started

For a quick start guide, please follow these steps:

If you are willing to use pins 8 and 9, please notice these are hardware enabled I2C pins. So make sure you unloaded i2c kernel modules before using these routines.

rmmod i2c_dev 
rmmod i2c_bcm2708


These are some files that you can use as a quick start guide.

If you want to compile and run them, make sure you modify i2c_init line to match your pin numbers.


This is a I2C scanner. It just tries every I2C address from 0 to 128 and test the acknowledge.

Just modify the i2c_init line to set up your ports. Remember to use WiringPi numbers. Run the scan and…

$ ./scan
I2C scan for soft_i2c project
 * Device found at 50h


This is a small software to read PCF8591 ADC/DAC. It setup the device to read channel 1 and reads it in continuous mode until the user stop it.


This example writes a string to a 24ls128 chip and reads it back. It uses 16 bit addressing.


This is a not so simple data logger for humidity and temperature sensor HTU21D.


This is a little more complicated example. A full command line interface for I2C. Simple but raw. Latest version has readline and history capabilities as well as some basic checks. You will need to install libreadline-dev package in order to compile it.

You can use i2cli as a quick approach to a new device. Then save your commands to a script file like htu21d.i2c.

Or you can send commands one by one by hand to test or to study a part. For example, to write and then read 24lc128 EEPROM, like in the last example, you will use this sequence:

$ ./i2cli 9 8
I2C Command Line Interface
I2C ready. SCL: 9, SDA: 8

s: start   p: stop   a: ack   n: nak   wHH: write byte HH   r: read byte   q: quit   C: scan
i2cli> C
 * Device found at 50h  (R: a1, W: a0)
i2cli> s      ; start
i2cli> wa0    ; 50h write
a0 -> ACK
i2cli> w00    ; address MSB 00h
00 -> ACK
i2cli> w00    ; address LSB 01h
00 -> ACK
i2cli> w5b    ; write 0001: 5b
5b -> ACK
i2cli> w5c    ; write 0002: 5c
5c -> ACK
i2cli> p      ; stop
i2cli> s      ; start
i2cli> wa0    ; 50h write
a0 -> ACK
i2cli> w00    ; address MSB 00h
00 -> ACK
i2cli> w02    ; address LSB 02h
02 -> ACK
i2cli> s      ; restart
i2cli> wa1    ; 50h read
a1 -> ACK
i2cli> r      ; read byte
5c            ; byte in 0002: 5c 
i2cli> p      ; ack
i2cli> q      ; quit
i2cli> Bye!


Include headers file in your main program:

#include "soft_i2c.h"

Since this is not a library, you need to add the source file to gcc compile line.

gcc -lwiringPi soft_i2c.c pcf8591.c -o pcf8591


i2c_t i2c_init(int scl, int sda);

Initializes a new i2c bus. The parameters SCL and SDA are SCL and SDA pin numbers. Please use WiringPi numeration. Returns a new i2c_t structure and set the indicated pins as inputs with pull-up enabled.

You can have as many busses defined as you need.

If i2c_init detects the bus is busy, it will wait a moment, then send a clock pulse in order to make the device release SDA so it can create a stop condition.

i2c_t my_bus1 = i2c_init(9, 8);

void i2c_start(i2c_t bus);

First, wait until both lines goes up, this means the bus is not busy. Then it sends a I2C start signal to the bus passed as parameter. As a result, it drives both lines low. It does not return anything.


void i2c_stop(i2c_t bus);

Sends a I2C start signal to the bus passed as parameter. After that, both lines are released. It does not return anything.


void i2c_reset(i2c_t bus);

Issues a bus recovery sequence. Tries to raise SDA. If it fails, reads 9 bit until we get SDA high. This function is called automatically by init, start and stop functions. So you seldom need to use this.


void i2c_send_bit(i2c_t bus, int bit);

Sends one bit through the bus. If second argument is true, it sends one. Zero in other case. Can be used to send an ack from the master. It does not return anything.

This routine is clock stretching aware, it take into account that SCL line should go up before sending more data.

i2c_send_bit(my_bus, 1);

You can also use the pre-defined constants:

i2c_send_bit(my_bus, I2C_ACK);


i2c_send_bit(my_bus, I2C_NACK);

int i2c_read_bit(i2c_t bus);

Reads one bit and returns it. It is not much useful on its own but used by i2c_read_byte to receive the acknowledge bit.

status = i2c_read_bit(my_bus);

int i2c_send_byte(i2c_t bus, uint8_t byte);

Sends 8 bits (1 byte) through the bus passed as argument. Returns I2C_ACK if the slave ack’ed the byte, and I2C_NACK in case of error. Remember I2C bytes are sent or received MSB first.

ack = i2c_send_byte(my_bus, address_byte);

For example, to test if there is a device with address 0x48 you can do this:

i2c_t my_bus1 = i2c_init(9, 8);


if (i2c_send_byte(my_bus, 0x48 << 1 | I2C_READ) == I2C_ACK)
	puts("Device found!");


Note the use of I2C_READ and I2C_WRITE to indicate the bus operation.

uint8_t i2c_read_byte(i2c_t bus);

Reads a byte from the bus and returns it. But NOT ack the byte. You must ack the byte using i2c_send_bit as you wish. Remember I2C bytes are sent or received MSB first.

int byte = i2c_read_byte(i2c);

if (we_liked(byte)) {
	i2c_send_bit(i2c, I2C_ACK); // 0 is a positive ack
	printf("We read: %d\n", byte);
else {
	i2c_send_bit(i2c, I2C_NACK); // 1 is NOT acknowledge
	printf("We didn't like the byte.\n");

Known bugs and limitations

See also

El bus I2C a bajo nivel (Spanish). Extensive description of this code.


See also the list of contributors who participated in this project.


This is free and unencumbered software released into the public domain. See the LICENSE file for details.