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Adding improved baud rate calculation, including use (or not) of the U2X bit. (Code from gabebear).

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This commit is contained in:
David A. Mellis 2009-07-12 02:58:59 +00:00
parent 430977c853
commit 3f74888018
1 changed files with 31 additions and 4 deletions
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35
hardware/cores/arduino/HardwareSerial.cpp
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@ -128,11 +128,38 @@ HardwareSerial::HardwareSerial(ring_buffer *rx_buffer,
// Public Methods //////////////////////////////////////////////////////////////
void HardwareSerial::begin(long speed)
void HardwareSerial::begin(long baud)
{
*_ubrrh = ((F_CPU / 16 + speed / 2) / speed - 1) >> 8;
*_ubrrl = ((F_CPU / 16 + speed / 2) / speed - 1);
cbi(*_ucsra, _u2x);
uint16_t baud_setting;
bool use_u2x;
// U2X mode is needed for baud rates higher than (CPU Hz / 16)
if (baud > F_CPU / 16) {
use_u2x = true;
} else {
// figure out if U2X mode would allow for a better connection
// calculate the percent difference between the baud-rate specified and
// the real baud rate for both U2X and non-U2X mode (0-255 error percent)
uint8_t nonu2x_baud_error = abs((int)(255-((F_CPU/(16*(((F_CPU/8/baud-1)/2)+1))*255)/baud)));
uint8_t u2x_baud_error = abs((int)(255-((F_CPU/(8*(((F_CPU/4/baud-1)/2)+1))*255)/baud)));
// prefer non-U2X mode because it handles clock skew better
use_u2x = (nonu2x_baud_error > u2x_baud_error);
}
if (use_u2x) {
*_ucsra = 1 << _u2x;
baud_setting = (F_CPU / 4 / baud - 1) / 2;
} else {
*_ucsra = 0;
baud_setting = (F_CPU / 8 / baud - 1) / 2;
}
// assign the baud_setting, a.k.a. ubbr (USART Baud Rate Register)
*_ubrrh = baud_setting >> 8;
*_ubrrl = baud_setting;
sbi(*_ucsrb, _rxen);
sbi(*_ucsrb, _txen);
sbi(*_ucsrb, _rxcie);