Store float value in uint32_t pointer

Question

I want to store some float data in the FRAM register of my TI MSP430 microcontroller and have some prolems.

I don't know how I can do this.

With normal integer variables it is no problem.

Normal integer variables:

void main()
{
    uint32_t value = 25;
    uint32_t framPtr = 0xD000;

    FRAMC_write_uint32_t(value, (uint32_t*)framPtr);
}

void FRAMC_write_uint32_t(uint32_t value,
                          uint32_t *framPtr)
{
    *framPtr = value;
}

But with float values it doesn't work. I tried to change the value inside the function to float, but no result.

This is my float data:

    float value = 1.25;
    uint32_t framPtr = 0xD000;

With this function it doesn't work:

void FRAM_write_float(float value,
                      uint32_t *framPtr)
{
    *framPtr++ = (float)value;
}

It saved the data 1.40129846e-45 (DEN) (HEX: 0x00000001) in my memory bank.

I hope somebody can help me with my problem. Thanks!

Answer 1

The simplest approach to reinterpret the bits would be to use memcpy, if you know that sizeof(float) == sizeof(uint32_t) ¹

float f = /* some_val */;
uint32_t fbits = 0;
memcpy(&fbits, &f, sizeof fbits);

Should be safe enough, since unsigned integers don't usually have trap values.

If your compiler supports C99 and onward. You can also do type punning via a union.

union {
  float    from;
  uint32_t to;
} pun = { .from = /*some val*/ };

// use pun.to

The above doesn't actually copy anything, so may be marginally faster in a tight loop. (And as Olaf points out, this is the standard compliant way for modern C).

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¹: If your compiler supports it, you should probably _Static_assert on it.