Hello there,
I am having a problem with understanding how to convert the the MPL3115A2 temperature readings exactly (OUT_T_MSB, OUT_T_LSB). Please consider this function:
inline static float rawTemperature2Float(const uint8_t* const rt)
{
return ((float)((int8_t)rt[0])) + (((rt[1] >> 4) & 0x0F) * 0.0625f);
}
rt[0] = OUT_T_MSB and rt[1] = OUT_T_LSB.
The question is: Does the sign bit in MSB also determinate how should the fractional part be treated, or is it always positive? In the presented function I am always adding the fractional part. But maybe if the sign is negative, I should substract it (or multiply by -1)? Should the function look like this?
inline static float rawTemperature2Float(const uint8_t* const rt)
{
float frac = ((rt[1] >> 4) & 0x0F) * 0.0625f;
if (rt[0] & 0x80)
{
frac *= -1; // variant #1
//frac = (1.0f - frac) * -1; // variant #2
}
return ((float)((int8_t)rt[0])) + frac;
}
If so, which variant in the if?
This approach gives similar results (all bits threaded for twos complement conversion):
inline static float rawTemperature2Float(const uint8_t* const rt)
{
int32_t temp = (((int32_t)rt[0]) << 4) | (rt[1] >> 4);
if (temp & 0x800)
temp = (0x1000 - temp) * -1;
return ((float)(temp)) / 16.0f;
}
I would appreciate all help.
