[SOLVED] two complement problem

[D_A_V_E]

Dear all,

I have a small problem here. I have a accelerometer, which read through I2C. the sensor data is 16bit two complement format and is store in 2 seperate registers(L and H). I am reading the data correctly and I would like to store the sensor data in an integer. So I am using the following routing.

acc_x_raw = (data.accelsH[1]) <<8) | data.accelsL[0];

But the result is not what I had expected. I think I am skipping a vital part here.....

Could anyone please tell me what I am doing wrong!!!

Thanks in advance

D

 

[millwood]

But the result is not what I had expected.

what is the result that you are getting and what is the result that you are expecting?

Could anyone please tell me what I am doing wrong!!!

articulate your problem concisely so others can better help you.

 

[D_A_V_E]

first thanks for your reply,

I am expecting values from about -32768 to +32768. then I would like to scal these values down to mG. But I am getting always negative values e.g. -65536.

 

[millwood]

acc_x_raw = (data.accelsH[1]) << | data.accelsL[0];

now, change it to the following and tell us what you get:

data.accelsH[1]=0x0f; data.accelsL[0]=0x55;
acc_x_raw = (data.accelsH[1]) << | data.accelsL[0];

 

[D_A_V_E]

doing:

acc_x_raw = ((data.accelsH[1]) << 8 ) | data.accelsL[0];

the value i am getting from this is 3925

 

[millwood]

great.

what that tells you is that the conversion from two bytes to a word is done correctly.

your problems are the result of data.accelsH[1] and data.accelsL[0] having the wrong values in them BEFORE the conversion to acc_x_raw took place.

 

[D_A_V_E]

Dear millwood,

thank you for your help.

Ok I am going to do this:

read from the sensor the High and low byte

acc_x_raw = ((highbyte) << 8 ) | lowbyte;

and then convert acc_x_raw to mG

yours,

D