Manually cast signed char

Question

I'm working with some embedded hardware, a Rabbit SBC, which uses Dynamic C 9.

I'm using the microcontroller to read information from a digital compass sensor using one of its serial ports.

The sensor sends values to the microcontroller using a single signed byte. (-85 to 85)

When I receive this data, I am putting it into a char variable

This works fine for positive values, but when the sensor starts to send negative values, the reading jumps to 255, then works its way back down to 0. I presume this is because the last bit is being used to determine the negative/positive, and is skewing the real values.

My inital thought was to change my data type to a signed char.

However, the problem I have is that the version of Dynamic C on the Microcontroller I am using does not natively support signed char values, only unsigned.

I am wondering if there is a way to manually cast the data I receive into a signed value?

Answer 1

You just need to pull out your reference book and read how negative numbers are represented by your controller. The rest is just typing.

For example, two's complement is represented by taking the value mod 256, so you just need to adjust by the modulus.

int signed_from_unsignedchar(unsigned char c)
{
    int result = c;
    if (result >= 128) result -= 256;
    return result;
}

One's complement is much simpler: You just flip the bits.

int signed_from_unsignedchar(unsigned char c)
{
    int result = c;
    if (result >= 128) result = -(int)(unsigned char)~c;
    return result;
}

Sign-magnitude represents negative numbers by setting the high bit, so you just need to clear the bit and negate:

int signed_from_unsignedchar(unsigned char c)
{
    int result = c;
    if (result >= 128) result = -(result & 0x7F);
    return result;
}

Answer 2

I think this is what you're after (assumes a 32-bit int and an 8-bit char):

unsigned char c = 255;
int i = ((int)(((unsigned int)c) << 24)) >> 24;

of course I'm assuming here that your platform does support signed integers, which may not be the case.