I am working on Raspberry Pi Model B.

I am trying to interface this device over the I2C Bus on the Rpi.

My hardware Connections are:

RPi                    Accelerometer
Vcc P1_1 ------------------Vcc
Gnd P1_9 ------------------Gnd
SDA P1_3 ------------------SDA
SCL P1_5 ------------------SCL

I am using the wiring Pi Library.

I compile the code by the command:

pi@raspberrypi ~/acc $gcc main.c acc.c -lwiringPi

and run it with the command:

pi@raspberrypi ~/acc $ sudo  ./a.out

I do the following before executing my code:

pi@raspberrypi ~/acc $ gpio load i2c
pi@raspberrypi ~/acc $ i2cdetect -y 1

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- 1d -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Here I am getting the correct slave address, but when I try to read anything from the device I always a 00 from the device.

Here is my code.

Anything that I am missing out.

I would appreciate the help


pi@raspberrypi ~/acc $ i2cget 1 0x1D 0x0D

WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will read from device file /dev/i2c-1, chip address 0x1d, data address
0x0d, using read byte data.
Continue? [Y/n] Y


pi@raspberrypi ~/acc $ sudo strace -e open 2>&1 | grep dev

pi@raspberrypi ~/acc $sudo ./a.out

Return Value is 0

Success setting up i2c device with the fd = 4

The value of the WHO_AM_I register is 0

Coudnot connect to the device

X : Y : Z : 0, 0, 0
X : Y : Z : 0, 0, 0
X : Y : Z : 0, 0, 0
  • Can you check that the device is working correctly when using i2cget command? Also, could you check if proper /dev/i2c-X device is being opened on your device by running sudo strace -e open 2>&1 | grep dev ? Apr 22, 2013 at 7:03
  • @KrzysztofAdamski--- I tried doing i2cget 0x1D 0x0D, where 0x1D is the slave address and 0x0D is a internal register in the slave, it returned 0x00, but it should have returned 0x2A I will check if the device is getting opened or not and update you
    – Jay K
    Apr 22, 2013 at 7:11
  • Note that first argument to i2cget should be the i2c bus number which should be 1 for you. Please check i2cget 1 0x1D 0x0D. Please also check the strace command I gave you and update your question with the results. Apr 22, 2013 at 7:42
  • @KrzysztofAdamski--I have added the result to the question. Doing the sudo strace -e open 2>&1 | grep dev did not print anything on the terminal.
    – Jay K
    Apr 22, 2013 at 8:49
  • It should be strace -e open ./a.out 2>&1 | grep dev, actually. The most important information is that i2cget gives you 0x00 so it's not your program that is wrong. Unfortunately I don't know this device and I don't have enough time to read the datasheet so I can't help you any more. You have to double check all your wiring and read datasheet carefully. Make i2cget/i2cset working before you start debugging the C program. Apr 22, 2013 at 8:49

2 Answers 2


From your module datasheet (5.10.1):

The MMA8452Q expects repeated STARTs to be used to randomly read from specific registers.

Unfortunately it seems that BCM2835 (RaspberryPi's SoC) does not support repeated START sequence. This means that your module is not compatible with the I²C bus on RaspberryPi.

There are two workarounds, none of them is easy to do, however.

Bitbanged I²C

You can use software (bitbanged) I²C bus on your RaspberryPi. I can't test this but reading the driver code it seems that repeated start sequence is supported by i2c-gpio driver. My tests with logic analyzer confirm (although I only tested this with pfc8574 which is very stupid device) that i2c_bcm2708 sends stop+start bit after data address while i2c_gpio only sends start bit.

It so happens that I recently wrote a Linux kernel module that let you create such bus without manually changing Kernel code. It's not (yet?) a part of the RaspberryPi kernel so you would have to compile this yourself. Here's the code and some instructions (but you need to know how to compile your own kernel to use it).

Since WiringPi uses i2c_smbus_read_byte_data function, it should "just work".

HiPi commandline utility

You could use HiPi utility (here are installation instructions) and check that it works from the provided commandline utilities. If it does, you could use them instead of WiringPi to do the communication. I know that it is not pretty solution but it may work.

  • Alternatively, I could make my own precompiled kernel version with my module installed available if you can't follow my instructions on github and want to test this. This is not a long-run solution since it will prevent you from updating your kernel. If you test that it works, I might be able to integrate this to the official kernel, though. Apr 22, 2013 at 10:54
  • @KrzysztofAdamkski Yes, I can test if that works.
    – Jay K
    Apr 22, 2013 at 20:24
  • @JayK: Here's precompiled kernel, if you didn't compile one yourself yet. Apr 22, 2013 at 21:53
  • ---Meanwhile I started working on HiPi, and I was able to read the WHO_AM_I register. You were correct with the Repeated Start Problem. Thanks..I will test the kernel too..
    – Jay K
    Apr 23, 2013 at 0:43
  • @KrzysztofAdamski--I need some more pointers on using the modules.tar.gz file in your kernel link. Could you add a little more info. I am a newbie to Linux
    – Jay K
    Apr 25, 2013 at 5:43

The Raspberry PI I2C Kernel Driver did not support repeated starts for a specific time. However, the I2C Kernel Driver has been updated and now supports the repeated start, though this functionality must be activated explicitly.

To set combined transfers 'on'

sudo sh -c '/bin/echo Y > /sys/module/i2c_bcm2708/parameters/combined'

To set combined transfers 'off'

sudo sh -c '/bin/echo N > /sys/module/i2c_bcm2708/parameters/combined'

Information found here: http://raspberrypi.znix.com/hipidocs/topic_i2c_rs_and_cs.htm

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.