Is it possible to store the float type results obtained from the code calculation in FLASH memory?

Question

I am using Attiny1616 with Microchip Studio.

I want to store the float type results obtained from the calculation of the code in FLASH memory.

I have looked into PROGMEM, but PROGMEM does not support the float data type.

So I tried to convert the float type result to char type data using snprintf or dtostrf and save it, but it did not work.

This is my C code.

#include <avr/io.h>
#include <avr/pgmspace.h>
#include <stdio.h>
#include <stdlib.h>

int main(void) {

    const char test[] PROGMEM = "Hello, world!";

    char buf[5] = "";
    float cal_result = 10.83;
    
    //dtostrf(cal_result, 5, 2, buf);
    snprintf(buf, 6, "%f", cal_result);

    const char out[5] PROGMEM = {buf[0],buf[1],buf[2],buf[3],buf[4]};
    
        while(1){   
    }

}

Thank you in advance.

You can write the byte representation of the float to flash. You are creating a string representation of the float which is not what you want. — Fredrik
– Fredrik, Commented Jan 10, 2024 at 11:34
@Takeru Tsuji, So const float PROGMEM cal_result = 10.83f; fails for you? — chux
– chux, Commented Jan 10, 2024 at 11:46
@chux-ReinstateMonica he wants to write a value from a calculation during runtime, PROGMEM wont help. He will need to erase a flash sector then write the float to flash. — Fredrik
– Fredrik, Commented Jan 10, 2024 at 11:48
Do you really need to use float? It will be very inefficient to do calculations with floating point types on low end 8-bit MCU. Most calculations can be done in pure integer types with little bit of adjustment. — user694733
– user694733, Commented Jan 10, 2024 at 13:50
Your MCU does not have a FPU so all floating point will get calculated by software floating point libraries. That's super-inefficient, similar to using *printf family of functions. This little code you posted has likely eaten up some 90% of your AVR's resources. Don't use PC stuff when programming ancient microcontrollers. Or alternatively, pick a modern microcontroller instead. — Lundin
– Lundin, Commented Jan 10, 2024 at 13:51

Mathieu · Accepted Answer · 2024-01-10 13:25:59Z

2

What you want to do is not possible with PROGMEM.

PROGMEM is a compiler directive that prevent to use RAM to store a constant value. Instead the program will read it directly from the flash each time it need it (I ignore optimizations here)

See 6.2 memory map on datasheet

A variable will be stored in 0x3800 – 0x3FFF range
A PROGMEM constant value will stored in 0x8000 – 0xBFFF range

If you want to store data computed during the program is running, you will have to use:

eeprom (low space, quite easy to use), see avr/eeprom.h
flash (more space, difficult to use (can only be written by page, has a limited number of writing), dangerous to use (you can wipe out your program accidentally)

answered Jan 10, 2024 at 13:25

Mathieu

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3 Comments

Takeru Tsuji Over a year ago

Is there any other way to write to Flash memory other than PROGMEM? If there is a way, can you provide the URL or an example of the code?

Fredrik Over a year ago

@TakeruTsuji you will have to do some reading, but that is common when dealing with embedded development ww1.microchip.com/downloads/aemDocuments/documents/OTH/…

Takeru Tsuji Over a year ago

I read the datasheet, but I couldn't understand it very well. I tried writing some code, but it doesn't seem to be able to write data to FLASH. ''' CCP = 0x9d; NVMCTRL.CTRLA = 0x06; NVMCTRL_ADDR = 0x8E00; NVMCTRL.DATA = 0xABCD; CCP = 0x9d; NVMCTRL.CTRLA = 0x04; '''

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