STM32F405 and UART4 compiling problem

[luca-stm32]

Core 2.1.0
Arduino IDE 1.8.16
Linux 64 bit

I'm developing a board with a STM32F405ZGT6 processor. I'm using the variant "GenericF407ZGTx" because it has the pheripherals that I need.

I need Serial1, Serial4 and Serial5, so I wrote the declaration code in this way:

// RX TX
HardwareSerial Serial1(PA10, PA9);

// RX TX
HardwareSerial Serial5(PD2, PC12);

// RX TX
HardwareSerial Serial4(PC11, PC10);

But, if I compile the code, the copiler tells me that Serial4 is already declared in HardwareSerial.cpp.

So I opened te file variant_generic.h (under hardware\stm32\2.1.0\variants\STM32F4xx\F407Z(E-G)T and changed the line:
// UART Definitions
#ifndef SERIAL_UART_INSTANCE
#define SERIAL_UART_INSTANCE 4
#endif

in:

// UART Definitions
#ifndef SERIAL_UART_INSTANCE
#define SERIAL_UART_INSTANCE 6
#endif

In this way, sketch compiles and serial is running.

What is the purpose of this define?
Is there something that I need to do to make serial4 work without modify sources?

Best regards.
Luca

[fpistm]

Hi @luca-stm32
in fact the generic define the SERIAL_UART_INSTANCE to be the default Serial instance.
In that case Serial4 is instantiate by default and mapped to Serial4 and using default pins: PA1/PA0.
So you don't need to instantiate Serial4 but as you need PC11/PC10 simply change it using the setRx() and setTx() before the begin():

    Serial.setRx(PC11);
    Serial.setTx(PC10);
    Serial.begin(9600);

ref: https://github.com/stm32duino/wiki/wiki/API#hardwareserial