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How do I store floats in flash and how do I read them from there? (I need a look-up table for pairs of float values.)

The storing part doesn't seem to be so difficult as this compiles:

PROGMEM float value1 = 1.23456;

But how do I read them? Do I need to copy them byte by byte to a float variable, or is there a better way? (I am using an Arduino ATmega 328p with 4-byte floats.)

fuenfundachtzig
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2 Answers2

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There is a helper macro in avr-libc that is designed just for this purpose:

#define pgm_read_float_near(address_short) __LPM_float((uint16_t)(address_short))

Read a float from the program space with a 16-bit (near) address.

Note
    The address is a byte address. The address is in the program space.

Use it like:

PROGMEM float pi=3.141592653;  // constant value in flash
float diameter = 24.332154;    // variable user value

float circumference = diameter * pgm_read_float_near(&pi);
Majenko
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2

I realize that the OP asked about a single value, but I thought I'd add info on retrieving from an array.

const float pmdata[] PROGMEM = {1.0, 2.0, 3.0};

then retrieve thusly:

float val = pgm_read_float(&pmdata[idx]);
dlchambers
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