From c3cd35f1979290d072ab77a6805dc4419f2ef6fb Mon Sep 17 00:00:00 2001 From: Matthijs Kooijman Date: Wed, 18 Dec 2013 23:21:45 +0100 Subject: In HardwareSerial::write, bypass the queue when it's empty 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). --- cores/arduino/HardwareSerial.cpp | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/cores/arduino/HardwareSerial.cpp b/cores/arduino/HardwareSerial.cpp index 5d62273..9f0a2ec 100644 --- a/cores/arduino/HardwareSerial.cpp +++ b/cores/arduino/HardwareSerial.cpp @@ -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 -- cgit v1.2.3-18-g5258