HardwareSerial: use correct UART HW for TX

[danielkucera]

Fixes #159

[maxint-rd]

Tried to compile for CH32X033, but failed. Simple errors though:

• HardwareSerial.cpp - HardwareSerial::write - len should be size
• uart.c - uart_putc: USART_SendData returns void, so ending should probably be

  USART_SendData(uart_handlers[obj->index]->Instance, c);
  return 0; // 0 means no error

After making these changes it compiled.
(Currently still figuring out how to properly use UART2 as Serial and get PA3 working as RX on CH32X033F8P6)

[danielkucera]

@maxint-rd , thanks for the notes, I have no idea why my compiler just took that.

What is the issue you are still having? I ended up implementing the RX as interrupt for concurrency reasons.

  NVIC_EnableIRQ( USART1_IRQn );
  USART_ITConfig(USART1, USART_IT_RXNE, ENABLE);
...
 void USART1_IRQHandler(void) __attribute__((interrupt("WCH-Interrupt-fast")));
  void USART1_IRQHandler(void){
    __disable_irq();

    if(USART1->STATR & USART_FLAG_RXNE){
      buf[p] = USART1->DATAR & (uint16_t)0x01FF;
...

[maxint-rd]

Hi @danielkucera - Thanks for your reply. I have things working now.

In PR #145 I implemented interrupt driven RX for the CH32V003. That PR only supported Serial1. The CH32X033 has no free pins for regular RX/TX on UART1 (I also want to use SCL/SDA), so I wanted to use UART2 for serial. (RX2/TX2 on PA4/PA3). For that I 've just successfully tested adding a UART2 interrupt handler.
(My interrupt handler code is comparable to yours, but slightly different, since I've reused some old core code that had a cyclic buffer implemented with head and tail pointers. Those are used to properly implement available() and peek().)

In PR #171 I published my addition of board files for the CH32X033. In variant_CH32X033F8P.h I now made SERIAL_UART_INSTANCE 2 the preferred serial port and got it working for both TX and interrupt driven RX. (Hooray!)

BTW. Since my X033's are those with a 0 in their number they don't have hardware I2C (or VREF). In my tests I luckily could use the SoftwareI2C library, so the chips are getting more useful by the hour :-)
I still would like to be able to use USB for serial and also use multiple serial ports simultaneously (see issue #159), but I guess that's something for future fun...