Problems interfacing MMA3204EG dual axis accelerometer with 8 bit microcontroller

[apples1234]

Hi. I am a newbie at this. I am trying to interface MMA3204EG dual axis accelerometer with an 8-bit PIC microcontroller. My question is mainly regarding the self-test pin provided in the accelerometer. The datasheet simply states the following: 

When the user applies a logic high input to the selftest pin, a calibrated potential is applied across the self-test plate and the moveable plate. The resulting electrostatic force (Fe = 1/2 AV2/d2) causes the center plate to deflect. The resultant deflection is measured by the accelerometer's control ASIC and a proportional output voltage results.

My question is, if there is a way to find out a value of this output voltage. When I connect the accelerometer to the microcontroller (as per the specifications mentioned in the datasheet) the values simply fluctuate between 0V and 1V. I do not get any other values. Please help me. Thank you

[bigmac]

Hello, and welcome to the forum.

My interpretation of the datasheet, with respect to the MMA3204 self test operation, is that a test force equivalent to 12g +/-20 percent is applied to each g-cell.  Assuming that Vdd is 5.0 volt, the X-output should step by nominally 0.24 volt, and the Y-output by nominally 0.80 volts, during self test mode.  For different Vdd, the output step will be proportional to the Vdd level.

You might check this out, initially using a DVM, prior to attempting analog-to-digital conversion within the MCU.

Regards,

Mac

[apples1234]

Thanks a lot for your suggestion. Could you please tell me what a DVM is?

Thanks

[bigmac]

Hello,

DVM = digital voltmeter.

Regards,

Mac

[apples1234]

Thanks once again for your suggestion. When I connected the accelerometer to an oscilloscope or a DVM I did get some readings. They were not 2.5V at 0g but they were again in the range of 0V to 1V. This is when I kept the sensor circuit on the board. When I tap it slightly I do get some changes in the voltage upto 3V for both the axes. I guess my question is do you get exact 2.5V values? I have rechecked my circuit and it is according to the scematic given in the data sheet. So I do not know where I am going wrong. Any suggestion would be helpful. Thanks a lot.