Multiple Accelerometers (24)

Our weekly LIVE video chat. Every Wednesday at 8pm ET!

Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.
Locked
User avatar
Allar
 
Posts: 5
Joined: Fri Jan 16, 2015 12:04 pm

Multiple Accelerometers (24)

Post by Allar »

Hello techies,

I'm in need of connecting up 24(twenty-four) 3-axis Accelerometers, which means a hell of a lot of wires and pins needed. Though I need the lowest amount of wires

Here's the Idea
Using a Teensy 3.1 as the main board. Connect 24 Accelerometers in one of the three methods.

Using I2C?
In need of a 3-axis Accelerometer that the 4 LSB of the I2C address can be selected ,giving 32 I2C addresses. I have found the FXLS8471Q 3-Axis, Linear Accelerometer.
Which allows one to select the 2 LSB of the I2C Address for a possible 4 addresses. However this wont do because I would need 32 unique addresses. issue: finding a accelerometer capable, refresh rate of data is slow

Using Analog Accelerometers?
Im not too sure how this would work but possible having all 24 - 3 analog outputs go into a ADC and then that into SPI? issues: TONS OF WIRES

Using SPI Accelerometers?
Possibly having a all 24 Accelerometers connected through SPI and select each one using an shift register? issues:quite a few of wires

Which way or is there another way to best accomplish this project? and how should I begin to start.

User avatar
adafruit_support_bill
 
Posts: 88093
Joined: Sat Feb 07, 2009 10:11 am

Re: Multiple Accelerometers (24)

Post by adafruit_support_bill »

Another option to consider is an I2C multiplexer. There are i2c multiplexer chips from several manufacturers.

User avatar
Allar
 
Posts: 5
Joined: Fri Jan 16, 2015 12:04 pm

Re: Multiple Accelerometers (24)

Post by Allar »

So Im not sure how I2C multiplexers work however i found an 8-Channel I2C Switch would this mean instead of have a I2C accelerometer with 4LSB addressable I would need one with 3 that way I can set 8 unique addresses 1 per channel of the switch?
Or does it work that I could have 8(eight) - I2C (0xC3) addressed accelerometers all having the same address and it would work ?

Which then I can have 3 I2C multiplexers with each having a different address thus having the ability to have 24 accelerometers?

User avatar
adafruit_support_bill
 
Posts: 88093
Joined: Sat Feb 07, 2009 10:11 am

Re: Multiple Accelerometers (24)

Post by adafruit_support_bill »

The PCA9548A can switch between 8 separate i2c buses. So it can address multiple devices on each of those buses.

But the PCA9548A is also addressable, so you can have up to 8 PCA9548A chips connected to the Teensy. If you had 3 PCA9548A chips, you would have 24 separate i2c buses and could use non-addressable accelerometers also.

User avatar
paulstoffregen
 
Posts: 444
Joined: Sun Oct 11, 2009 11:23 am

Re: Multiple Accelerometers (24)

Post by paulstoffregen »

SPI to 24 chips is not wise. The signals are very fast, which makes long wires challenging.

I2C is much slower and uses slew rate limited signals that allow for wire runs of several feet (a couple meters).

User avatar
sembazuru
 
Posts: 141
Joined: Sat Mar 30, 2013 2:50 am

Re: Multiple Accelerometers (24)

Post by sembazuru »

I have a project that I'm working on that uses shift buffer outputs as the CS signals for up 8 different SPI devices, and loads the shift buffers from the SPI bus. Because it is using shift buffers the modules are daisy chainable. 24 SPI based accelerometers would take 3 boards daisy chained.

I have built up 2 boards so far, but haven't had a chance to test them out to make sure timing issues don't kill the project.

If I remember, I'll try to put the project up on GitHub tonight as I'm already planning on releasing it as OSHW. (Boards designed in KiCAD, schematic also provided as a PDF.)

User avatar
sembazuru
 
Posts: 141
Joined: Sat Mar 30, 2013 2:50 am

Re: Multiple Accelerometers (24)

Post by sembazuru »

Silly me. It was already on GitHub. See this repository https://github.com/Sembazuru/SPI-shift-register-CS to see the schematic for ideas for your own project.

If you want boards made, my Gerbers are provided. I had my first run of boards made at Bay Area Circuits. But, I don't see why most other board houses would have any issue with the gerbers as provided.

Locked
Please be positive and constructive with your questions and comments.

Return to “Ask an Engineer! VIDEO CHAT (closed)”