I have a sensor which gives its output in three bytes. I read it like this:
unsigned char byte0,byte1,byte2;
byte0=readRegister(0x25);
byte1=readRegister(0x26);
byte2=readRegister(0x27);
Now I want these three bytes merged into one number:
int value;
value=byte0 + (byte1 << 8) + (byte2 << 16);
it gives me values from 0 to 16,777,215 but I'm expecting values from -8,388,608 to 8,388,607. I though that int was already signed by its implementation. Even if I try define it like signed int value; it still gives me only positive numbers. So I guess my question is how to convert int to its two's complement?
Thanks!
What you need to perform is called sign extension. You have 24 significant bits but want 32 significant bits (note that you assume int to be 32-bit wide, which is not always true; you'd better use type int32_t defined in stdint.h). Missing 8 top bits should be either all zeroes for positive values or all ones for negative. It is defined by the most significant bit of the 24 bit value.
int32_t value;
uint8_t extension = byte2 & 0x80 ? 0xff:00; /* checks bit 7 */
value = (int32_t)byte0 | ((int32_t)byte1 << 8) | ((int32_t)byte2 << 16) | ((int32_t)extension << 24);
EDIT: Note that you cannot shift an 8 bit value by 8 or more bits, it is undefined behavior. You'll have to cast it to a wider type first.