I²C Clock Stretching

Clock Stretching

Please be aware there are some hardware issues when using the Raspberry Pi with devices that expect to be able to use clock stretching, for more info see “Adventures in I2C: clock stretching on the Raspberry Pi” and “I2C stretch bug. Been fixed or not?".

Clock stretching in I2C allows a slave device to halt the master before a more data is sent. This is often the case when the slave device writes to an EEPROM etc. which takes longer than a usual read or write to a register.

On the Raspberry Pi clock stretching can be configured by using a higher timeout while waiting for a slave to respond. This is something that can not be done by the pi4j, as it requires root privileges.

There are two ways to change the clkt_tout value. This repository path has two files, a i2c1_get_clkt_tout.c and i2c1_set_clkt_tout.c. Build them as follows:


# install gcc to compile the c files
apt install build-essential

mkdir clkt_tout
cd clkt_tout/
wget wget https://raw.githubusercontent.com/raspihats/raspihats/master/clk_stretch/i2c1_get_clkt_tout.c
wget wget https://raw.githubusercontent.com/raspihats/raspihats/master/clk_stretch/i2c1_set_clkt_tout.c


Save as Makefile


.PHONY: all install uninstall clean

all: i2c1_set_clkt_tout i2c1_get_clkt_tout

i2c_get_clkt_tout: i2c1_get_clkt_tout.c
        $(CC) -o i2c1_get_clkt_tout i2c1_get_clkt_tout.c

i2c_set_clkt_tout: i2c1_set_clkt_tout.c
        $(CC) -o i2c1_set_clkt_tout i2c1_set_clkt_tout.c

        cp i2c1_get_clkt_tout /usr/local/bin/i2c1_get_clkt_tout
        cp i2c1_set_clkt_tout /usr/local/bin/i2c1_set_clkt_tout

        rm -f /usr/local/bin/i2c1_get_clkt_tout
        rm -f /usr/local/bin/i2c1_set_clkt_tout

        rm -f i2c1_set_clkt_tout i2c1_get_clkt_tout

Build and install

sudo make install


# read current clkt_tout:
$ sudo i2c1_get_clkt_tout 
i2c1_get_clkt_tout: CLKT.TOUT = 1000

# set timeout to 1 second:
i2c_set_clkt_tout 1000