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Topic: wrong readings from ADXL345 (Read 2 times) previous topic - next topic

Stan09

When I am reading accelerometer data (with my settings) from MEGA it returns just zeroes and sometimes '-1'.
This is normal, because I set it up with lowest sensitivity (16g) and accelerometer just sits on the desk.

But when I run the same code on DUE instead of '-1' I see 65535.

I wonder, does it have anything to do with different int type for DUE?
I believe this is all about that line

Code: [Select]
x = (((int)byte_buff[1]) << 8) | byte_buff[0];

Probably there is something wrong with signed integer representation. As I understand, when INT is stored in 2 bytes, then '-1' in binary would be just 16 '1'-s, but  when this number is interpreted as 4 bytes, it is equal to 65535. I am not quite good in understanding details of signed integers representation, so I might be wrong.

Any ideas?



pico

#1
Nov 18, 2012, 06:10 pm Last Edit: Nov 18, 2012, 06:12 pm by pico Reason: 1
No, you've already figured it out. All 16 bits set in a two byte signed integer represents -1; in a 4 byte signed integer, the lower 16 bits set represents 65535 (-1 would be represented by all 32 bits set). The highest bit in a signed integer holds the sign of the number, + or -ve; the rest of the bits hold the actual value. To change the sign of an integer, the rule is to change all the zero bits to ones, and all the one bits to zero. That produces what is called the "one's complement". And then add one to that (using unsigned overflow arithmetic). That produces the "two's complement". If you are curious, you can read why it is represented this way here:

http://en.wikipedia.org/wiki/Two%27s_complement

More practically, for your problem, a quick fix would be to declare the offending 32 bit int as int16_t in the Due code. Then you'll be working with a 16 bit signed int on both platforms.
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Stan09

Hurray, we did it!
It works!
But I wonder, am I the only one who has noticed that?
Does not this mean that f.e ADXL345 library (and probably, many others?) must be corrected for DUE?

pico


Does not this mean that f.e ADXL345 library (and probably, many others?) must be corrected for DUE?


It certainly does.
WiFi shields/Yun too expensive? Embeddedcoolness.com is now selling the RFXduino nRF24L01+ <-> TCP/IP Linux gateway: Simpler, more affordable, and even more powerful wireless Internet connectivity for *all* your Arduino projects! (nRF24L01+ shield and dev board kits available too.)

Markus_L811


Hurray, we did it!
It works!
But I wonder, am I the only one who has noticed that?
Does not this mean that f.e ADXL345 library (and probably, many others?) must be corrected for DUE?



Hey Stan,

would you post your complete sketch, so we can see what you have done.

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