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I have two version of the boards , where a particular portion of the code gets executed based on the value of the #define BIAS_VOLTAGE 

if(vol < BIAS_VOLTAGE)
{

    //Execute this

}
else if (vola >= BIAS_VOLTAGE)
{
 //Execute This

}

Now BIAS_VOLTAGE is #defined in a header file (#define BIAS_VOLTAGE 3) but for a different board it is 5(5V).

The only way to detect this is by polling one GPIO status (which remains High or low after GPIO initialization in main .This is done just once).

If it is high , BIAS_VOLTAGE 3 other wise 5 Since #BIAS_VOLTAGE is used at many places , how can i know the status of GPIO at the startup ( even before main) and fix the #define value.

I know #define gets fixed at the compile time , but I need to fix this constant value at the runtime.(and this will remain forever) Update : tool chain is IAR and microcontroller is STM32F4

Raulp
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    Why not make `BIAS_VOLTAGE` a *variable* that is set in the `main` function once the initialization is done and you actually can read the GPIO pin status. – Some programmer dude Sep 13 '19 at 08:14
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    You can't change a define value at run-time, so the only alternative to spdude's suggestion that I can see is to have two versions of your app and the use a small startup app to detect the actual voltage level and invoke the appropriate version of the main app based on that.. – 500 - Internal Server Error Sep 13 '19 at 08:18
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    Defines are handled by the pre-processor (before compilation), can't be modified at run time. – danrodlor Sep 13 '19 at 08:20
  • If you can’t auto detect it, you have to hard code it in some form (either platform data in the driver, or platform description like DT blob in some OSes). Do you use any OS (like Zephyr) by the way? – 0andriy Sep 13 '19 at 08:24
  • You may get more specific advice if you specify your toolchain and target device. – Clifford Sep 13 '19 at 11:46
  • Its iar and stm32 cortex m4 – Raulp Sep 13 '19 at 11:47
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    Why does this need to be before `main()`? Simply needs to be before it is used surely? – Clifford Sep 13 '19 at 11:47
  • @Raulp : The information should be added to the question, not a comment. – Clifford Sep 13 '19 at 11:48

2 Answers2

2

You may be making a simple thing complicated:

Given:

#define BIAS_3V0 3
#define BIAS_5V0 5

int getBiasVoltage()
{
    static int bias_voltage = 0 ;

    // Initialise on first use...
    if( bias_voltage == 0 )
    {
        bias_voltage = boardId() ? BIAS_3V0 : BIAS_5V0 ;
    }

    return bias_voltage ;
}

where boardId() is the GPIO read to identify the variant (replace with your own code as required), then your code becomes:

if(vol < getBiasVoltage() )
{

    //Execute this

}
else if (vola >= getBiasVoltage() )
{
 //Execute This

}

The board identity is checked once on first use and thereafter the previously determined value will be returned. Because the variable is hidden inside the function, it is read-only too.

Clifford
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1

Smells like something that should be done very early on. If so, you have to either tweak your current "CRT" (library start-up code) or write it all yourself, which is a job I would only recommend to veteran embedded devs.

Most tool chains do unfortunately come with a CRT which is useless beyond hobbyist projects. Some advice for how it should be written properly can be found here: https://stackoverflow.com/a/47940277/584518. If your current CRT doesn't resemble the advise given there - particularly if it lets the CRT run the whole of .data/.bss setup using the default internal RC oscillator, and with no wdog enabled, you should ask your tool vendor why they let incompetent quacks write the CRT.

Lundin
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