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Adding improved baud rate calculation, including use (or not) of the U2X bit. (Code from gabebear).
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@ -128,11 +128,38 @@ HardwareSerial::HardwareSerial(ring_buffer *rx_buffer,
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// Public Methods //////////////////////////////////////////////////////////////
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void HardwareSerial::begin(long speed)
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void HardwareSerial::begin(long baud)
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{
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*_ubrrh = ((F_CPU / 16 + speed / 2) / speed - 1) >> 8;
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*_ubrrl = ((F_CPU / 16 + speed / 2) / speed - 1);
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cbi(*_ucsra, _u2x);
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uint16_t baud_setting;
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bool use_u2x;
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// U2X mode is needed for baud rates higher than (CPU Hz / 16)
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if (baud > F_CPU / 16) {
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use_u2x = true;
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} else {
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// figure out if U2X mode would allow for a better connection
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// calculate the percent difference between the baud-rate specified and
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// the real baud rate for both U2X and non-U2X mode (0-255 error percent)
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uint8_t nonu2x_baud_error = abs((int)(255-((F_CPU/(16*(((F_CPU/8/baud-1)/2)+1))*255)/baud)));
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uint8_t u2x_baud_error = abs((int)(255-((F_CPU/(8*(((F_CPU/4/baud-1)/2)+1))*255)/baud)));
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// prefer non-U2X mode because it handles clock skew better
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use_u2x = (nonu2x_baud_error > u2x_baud_error);
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}
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if (use_u2x) {
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*_ucsra = 1 << _u2x;
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baud_setting = (F_CPU / 4 / baud - 1) / 2;
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} else {
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*_ucsra = 0;
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baud_setting = (F_CPU / 8 / baud - 1) / 2;
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}
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// assign the baud_setting, a.k.a. ubbr (USART Baud Rate Register)
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*_ubrrh = baud_setting >> 8;
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*_ubrrl = baud_setting;
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sbi(*_ucsrb, _rxen);
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sbi(*_ucsrb, _txen);
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sbi(*_ucsrb, _rxcie);
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