F_CPU define not picked up correctly in Arduino IDE

[TMRh20]

Describe the bug
Attempting to use F_CPU for device detection doesn't work. It seems that F_CPU is actually defined later in the build process, so cannot be picked up properly using defines.

To Reproduce

#if defined F_CPU
  #if F_CPU >= 45000000
    #define bufferSize 1000
  #else
    #define bufferSize 100
  #endif

#else
  #define bufferSize 500
#endif

void setup() {
    Serial.begin(115200);
}

void loop() {
  Serial.print("Buffer Size: ");
  Serial.println(bufferSize);
  Serial.print("F_CPU: ");
  Serial.println(F_CPU);
  delay(1000);
}

Steps to reproduce the behavior:

1. Run the included example code
2. See F_CPU is displayed as 96MHz, although the selected buffer size is the smallest (detected at <45Mhz)

Expected behavior
Expected F_CPU to actually detect the configured clock speed, and allow configuration of buffer sizes based on CPU speed.

Desktop (please complete the following information):

• OS: Windows 11
• Arduino IDE version: 2.3.10
• STM32 core version: 3.0.0
• Tools menu settings if not the default: Any
• Upload method: DFU

Board (please complete the following information):

• Name: Black Pill STM32F411CEU6
• Hardware Revision: n/a
• Extra hardware used if any: none required

Additional context
See nRF24/RF24Network#278
nRF24/RF24Network#277

[fpistm]

Hi @TMRh20,
This is not a constant.
See https://stm32duino.github.io/latest/customization/Custom-definitions/#f_cpu

[TMRh20]

Thanks for the quick response!

I guess I'm mainly wondering about why this behaviour differs from all other Arduino cores we've come across? We were trying to define buffer sizes based on F_CPU, and it appears to work on all other cores we've encountered. In this case we have worked around the problem by using #if defined ARDUINO_ARCH_STM32 instead.

As a real world example, it ended up causing a buffer overflow in the RF24Network library, because the behaviour is different.

I'm not sure if this is important to other users so feel free to close the discussion if changing the behaviour is not desired.

[fpistm]

You çan define it as a constant but default behaviour is to define it as the system clock config to be in-line with the latest core clock config.