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).
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@ -197,6 +197,15 @@ void HardwareSerial::flush()
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size_t HardwareSerial::write(uint8_t c)
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{
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// If the buffer and the data register is empty, just write the byte
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// to the data register and be done. This shortcut helps
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// significantly improve the effective datarate at high (>
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// 500kbit/s) bitrates, where interrupt overhead becomes a slowdown.
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if (_tx_buffer_head == _tx_buffer_tail && bit_is_set(*_ucsra, UDRE0)) {
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*_udr = c;
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sbi(*_ucsra, TXC0);
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return 1;
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}
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int i = (_tx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
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// If the output buffer is full, there's nothing for it other than to
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