0

How can I hook up four (adafruit) MMA8451 accelerometers (using I2C protocol) using one Arduino Mega? What I'm confused about is I only have one SDA and SCL on the Arduino Mega obviously. So how can I connect these all? Multiplexer? I need four because I have one on each leg of a robot. Or is perhaps this impractical to put an accelerometer on each leg of robot? I'm a little new to electronics so please explain in depth if you could. Thanks.

Note to moderators: I just would really love an answer to this even if this doesn't meet the super strict question guidelines. It seems every time I post here there's always that one person who takes it down for any number of trivial reasons. Please let me get an answer at least. Thank you!

  • 1
    Looking at the datasheet for the MMA8451Q, factory default it has two distinct I2C addresses 0x1C and 0x1D, selectable with the SA0 pin, but if you want more to share a bus, they can factory program other IDs, so you could share 4 on one bus if you could buy a couple non-default chips. Otherwise you need two busses to differentiate 4 chips. – Dave X Apr 18 at 20:35
2

Talking about I2C protocol. The slave Address is of 7 Bits that means one I2c can handle 127 Slave I2C devices of different address. So you can connect 127 devices with Single I2C Port just make sure there slave address is different from each other.

Some I2C devices are provided with the option to select salve address by setting some pins high and low. If you are using some identical sensors you are set their slave address different.

2

Brief explanation: When you talk to the slave device through I2C you start with (slave) device address. Then you send the message. It is like calling your colleague in an office with many other people. You say: "John", and when he is aware you continue with message, like "let's go for coffee".

This arrangement allows for adding up to 127 devices on one I2C bus. Each device must have different address (sorry, you cannot have two John's in the office, it will cause conflict). Generally two different type of devices will have different I2C addresses, but it may happen you need to use two (or more) of the same (like in your example). In this case, see below.

Many devices with I2C interface allows for address change. Using this facility you are able to communicate with many of the same devices through the same pair of cables.
You can check if your chosen accelerometer has possibility for address change: On picture below there is one with Address tab. When jumper is not fitted, you will get one address, when is fitted, the address of this device will be different, allowing of two of this accelerometers on one I2C bus. Unfortunately in this example there will be possibility of maximum of two of the same devices.
Accel

Of course there are devices with more than two possible addresses, you need to search for them.
And, yes, it is fine to use one Arduino to communicate with many I2C devices.

EDIT:
Looking after some of Adafruit accelerometers, I have found this page: https://learn.adafruit.com/i2c-addresses

And small off topic note: If you struggling on finding I2C with more than 2 addresses, check if it is easier to use SPI protocol. In this case you need (edited to correct bus lines names after user CrossRoads comment) MISO, MOSI, SCK and CS (chip select) wires. Chip select is individually connected to each slave device. This allows selecting particular device without addressing issue.

  • SPI would also be faster to query all the devices. SPI uses SCK, MISO, MOSI and a unique chip select per device. Communication with a Mega can be at up 8 MHz clock rate, 20 times faster than the 400 KHz of I2C. – CrossRoads Apr 18 at 13:17
1

The general technique for addressing multiple I2C devices is:

  1. If the device(s) have only one address: use an I2C bus multiplexer.
  2. If the device(s) have two addresses controlled by an addressing pin: use digital output pins from the Arduino to select which device to address, e.g., set the control pin for the selected device low and others high and use the corresponding address to address the device.

With MMA8451Q, factory default it has two distinct I2C addresses 0x1C and 0x1D, controlled by the SA0 pin, use digital output pins on the Arduino Mega to address the selected device (address 0x1C) by setting the corresponding output pin low for the selected device and the control pin for the others to high (address 0x1D). Use address 0x1C to interact with the selected device.

Cheers!

0

Simple just connect all SCK and SDL accelerometer lines together and then u just need to get a specific code address in your use to get a data from any of the accelerometers

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.