Accelerometer question

Robotics and electronics discussion Electronics
#1

I’d like to setup the MMA7260QT 3-Axis Accelerometer (Pololu Part 766) in a car computer project I’m working on. The accelerometer board will be powered by 5V to the VIN pin. The sensitivity of the accelerometer can be adjusted from 1.5g to 6g through the use of two jumpers. I’m confused however about the following language:

“The sensitivity selection pins GS1 and GS2 are pulled up to the Vcc line, making the default sensitivity 6g; these pins can be pulled low by a microcontroller or through jumpers. For 5 V microcontroller applications, the lines should not be driven high. Instead, the microcontroller I/O pin can emulate an open-drain or open-collector output by alternating between low output and high-impedance (input) states. Put another way, if you are using a 5 V microcontroller, you should make your sensitivity selection I/O lines inputs and rely upon the internal pull-ups on the GS1 and GS2 lines if you want them to be high. It is always safe for you to drive these lines low.”

Since I’m powering with 5V to the VIN pin and not 3.3 to the “3.3V/Vcc” pin, does this mean I should or should not use the jumpers (which limits my adjustability) or does it not matter.

Please forgive a newbie to this.

Thanks, Steve

#2

Hello Steve.

The jumpers are always safe to use since they short the pins to ground when they are on and the pins are internally pulled up to 3.3 V when the jumpers are off. The warning only applies to the situation where one is using an external 5 V controller or voltage source, since applying 5 V to these lines could damage the device. If you want to use a microcontroller running at 5 V to dynamically set the sensitivity, you should make the selection I/O line a high-impedance input to achieve the same effect as having the jumper off or a driving-low output to achieve the same effect as having the jumper on. Does this make sense?

- Ben

#3

Got it! Thanks for the quick reply. I appreciate it.

Steve