When to use PA0 or PA_0?

[PerennialNovice]

There exit two different definitions for hardware Pins.
It can be written as PA0 or as PA_0.

The PA_0 notation can be used with hardware serial, but it seems to cause some trouble with I²C or SPI instances. Here the PA0 notation has to be used.

Is there some information on what can be used with which APIs?

[fpistm]

Hi @MasteringTheMess
Arduino API uses pin numbers example on uno connector::

• digital pin number: D0 to D15 or 0 to 15
• analog pin number: A0 to A5
  For convenience with STM32, we define PYn where Y is the GPIO port name and n the GPIO pin number, example PA0.
  This pin number is used with all standard Arduino API.

when you used this pin number (PYn) the core convert it to the type PinName: PY_n
This type is a concatenation, ex: PA_0 = (PortA << 4) + 0x00, which allows to retrieve the HAL GPIO port and pin.
Some extended API allows to use it for convenience or to go faster, ex: digitalWriteFast.
In a general way the pin number is always converted to PinName using macros: digitalPinToPinName.

[PerennialNovice]

Thanks @fpistm for the very abundant explanation :)

so the safe way is to always use the PYn notation.