The Serial.flush()/HardwareSerial::flush() sends all data in the serial transmit buffer.
Souce Code
The HardwareSerial::flush() is defined in hardware/arduino/avr/cores/arduino/HardwareSerial.cpp as below.
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voidHardwareSerial::flush(){// If we have never written a byte, no need to flush. This special
// case is needed since there is no way to force the TXC (transmit
// complete) bit to 1 during initialization
if(!_written)return;while(bit_is_set(*_ucsrb,UDRIE0)||bit_is_clear(*_ucsra,TXC0)){if(bit_is_clear(SREG,SREG_I)&&bit_is_set(*_ucsrb,UDRIE0))// Interrupts are globally disabled, but the DR empty
// interrupt should be enabled, so poll the DR empty flag to
// prevent deadlock
if(bit_is_set(*_ucsra,UDRE0))_tx_udr_empty_irq();}// If we get here, nothing is queued anymore (DRIE is disabled) and
// the hardware finished transmission (TXC is set).
}
No inputs, no outputs.
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voidHardwareSerial::flush(){
If the _written is 0, it means no HardwareSerial::write() is used, does nothing and returns.
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// If we have never written a byte, no need to flush. This special
// case is needed since there is no way to force the TXC (transmit
// complete) bit to 1 during initialization
if(!_written)return;
If UDRIE0 bit of _ucsrb(UCSR0B) is 1 or TXC0 bit of _ucsra(UCSR0A) is 0, the transmit buffer is not empty. Wait for the buffer to become empty.
while(bit_is_set(*_ucsrb,UDRIE0)||bit_is_clear(*_ucsra,TXC0)){if(bit_is_clear(SREG,SREG_I)&&bit_is_set(*_ucsrb,UDRIE0))// Interrupts are globally disabled, but the DR empty
// interrupt should be enabled, so poll the DR empty flag to
// prevent deadlock
if(bit_is_set(*_ucsra,UDRE0))_tx_udr_empty_irq();}// If we get here, nothing is queued anymore (DRIE is disabled) and
// the hardware finished transmission (TXC is set).
}
If the SREG_I bit of SREG is 0 and the UDRIE0 bit of _ucsrb(UCSR0B)
is 1, it means interrupts are globally disabled. In case of UDRE0 bit of
_ucsra(UCSR0A) is 1, the transmit buffer is not full, calls HardwareSerial::_tx_udr_empty_irq() to send data.