Changing baud rate calculation to always use double speed mode except for 57600 baud at 16 MHz.

http://code.google.com/p/arduino/issues/detail?id=394

1 files changed, 9 insertions, 15 deletions

diff --git a/cores/arduino/HardwareSerial.cpp b/cores/arduino/HardwareSerial.cpp
index 9257302..b25b4f1 100644
--- a/cores/arduino/HardwareSerial.cpp
+++ b/cores/arduino/HardwareSerial.cpp
@@ -194,22 +194,16 @@ HardwareSerial::HardwareSerial(ring_buffer *rx_buffer,
 void HardwareSerial::begin(long baud)
 {
   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);
+  bool use_u2x = true;
+
+#if F_CPU == 16000000UL
+  // hardcoded exception for compatibility with the bootloader shipped
+  // with the Duemilanove and previous boards and the firmware on the 8U2
+  // on the Uno and Mega 2560.
+  if (baud == 57600) {
+    use_u2x = false;
   }
+#endif
   
   if (use_u2x) {
     *_ucsra = 1 << _u2x;