fixed_point_to_float.cpp

//#!g++ -O3 -std=c++14 cpp.cpp -o a.out && ./a.out ; ;:

#include <cstdio>
#include <stdint.h>


#include <type_traits>
template <uint8_t int_bits, uint8_t fractional_bits, class T>
inline float fixed_point_to_float(const T fixed) {
	static_assert(std::is_unsigned<T>::value, "argument must be unsigned");
	constexpr uint8_t msb = int_bits + fractional_bits - 1;
	constexpr T mask = (T)~(((T)~(0)) << msb);
	constexpr float deno = 1<<fractional_bits;
	if (fixed & (1<<msb)) {
		// negative
		return -( (float)( (~fixed & mask) + 1) / deno);
	} else {
		// positive
		return (float)fixed / deno;
	}
}

bool is(float got, float expected, const char* desc = NULL) {
	const bool ret = got == expected;
	if (ret) {
		printf("ok\n");
	} else {
		printf("not ok %f != %f # %s\n", got, expected, desc ? desc : "");
	}
	return ret;
}

int main (int argc, char* argv[]) {
	// from ADT7410 datasheet
	is(fixed_point_to_float<9, 4>((uint16_t)0b0000000000001), 0.0625);
	is(fixed_point_to_float<9, 4>((uint16_t)0b0100101100000), 150.0);
	is(fixed_point_to_float<9, 4>((uint16_t)0b0000000000000), 0);
	is(fixed_point_to_float<9, 4>((uint16_t)0b1110010010000), -55.0);
	is(fixed_point_to_float<9, 4>((uint16_t)0b1111111111111), -0.0625);
	is(fixed_point_to_float<9, 4>((uint32_t)0b1111111111111), -0.0625 );
	/** compile error : argument must be unsigned
	printf("%f\n", fixed_point_to_float<9, 4>((int16_t)0b1111111111111));
	*/

	// from MCP3425
	is(fixed_point_to_float<1, 11>((uint16_t)0x001) * (2.048), 1e-3);
	is(fixed_point_to_float<1, 13>((uint16_t)0x001) * (2.048), 250e-6);
	is(fixed_point_to_float<1, 15>((uint16_t)0x001) * (2.048), 62.5e-6);

	// test dynamic variable
	volatile uint16_t x = 0x001;
	is(fixed_point_to_float<1, 11>(x) * (2.048), 1e-3);
}