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Clean up the bitrate calculations per the Atmel datasheet. 

Verified that when configured for 115200, the actual output as seen on a scope is around 117600Hz consistent with the +2.1% error in the datasheet.

git-svn-id: https://arducopter.googlecode.com/svn/trunk@542 f9c3cf11-9bcb-44bc-f272-b75c42450872
This commit is contained in:
DrZiplok@gmail.com 2010-09-23 05:41:30 +00:00
parent 15e6469ca9
commit f72129bdb3
1 changed files with 31 additions and 24 deletions
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55
libraries/FastSerial/FastSerial.cpp
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@ -121,9 +121,11 @@ void FastSerial::begin(long baud)
void FastSerial::begin(long baud, unsigned int rxSpace, unsigned int txSpace)
{
uint16_t baud_setting;
bool use_u2x;
uint16_t ubrr;
bool use_u2x = false;
int ureg, u2;
long breg, b2, dreg, d2;
// if we are currently open, close and restart
if (_open)
end();
@ -136,33 +138,38 @@ void FastSerial::begin(long baud, unsigned int rxSpace, unsigned int txSpace)
}
_open = true;
// U2X mode is needed for baud rates higher than (CPU Hz / 16)
// U2X mode is needed for bitrates higher than (F_CPU / 16)
if (baud > F_CPU / 16) {
use_u2x = true;
ubrr = F_CPU / (8 * baud) - 1;
} else {
// figure out if U2X mode would allow for a better connection
// Determine whether u2x mode would give a closer
// approximation of the desired speed.
// 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);
// ubrr for non-2x mode, corresponding baudrate and delta
ureg = F_CPU / 16 / baud - 1;
breg = F_CPU / 16 / (ureg + 1);
dreg = abs(baud - breg);
// ubrr for 2x mode, corresponding bitrate and delta
u2 = F_CPU / 8 / baud - 1;
b2 = F_CPU / 8 / (u2 + 1);
d2 = abs(baud - b2);
// Pick the setting that gives the smallest rate
// error, preferring non-u2x mode if the delta is
// identical.
if (dreg <= d2) {
ubrr = ureg;
} else {
ubrr = u2;
use_u2x = true;
}
}
if (use_u2x) {
*_ucsra = _BV(_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;
*_ucsra = use_u2x ? _BV(_u2x) : 0;
*_ubrrh = ubrr >> 8;
*_ubrrl = ubrr;
*_ucsrb |= _portEnableBits;
}