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In HardwareSerial::write, bypass the queue when it's empty

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This helps improve the effective datarate on high (>500kbit/s) bitrates,
by skipping the interrupt and associated overhead. At 1 Mbit/s the
implementation previously got up to about 600-700 kbit/s, but now it
actually gets up to the 1Mbit/s (values are rough estimates, though).
This commit is contained in:
Matthijs Kooijman 2013-12-18 23:21:45 +01:00 committed by Cristian Maglie
parent 275c0a02b1
commit 9ad14b2f0c
1 changed files with 9 additions and 0 deletions
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9
hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
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@ -197,6 +197,15 @@ void HardwareSerial::flush()
size_t HardwareSerial::write(uint8_t c)
{
// If the buffer and the data register is empty, just write the byte
// to the data register and be done. This shortcut helps
// significantly improve the effective datarate at high (>
// 500kbit/s) bitrates, where interrupt overhead becomes a slowdown.
if (_tx_buffer_head == _tx_buffer_tail && bit_is_set(*_ucsra, UDRE0)) {
*_udr = c;
sbi(*_ucsra, TXC0);
return 1;
}
int i = (_tx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
// If the output buffer is full, there's nothing for it other than to