From 12ea12d14cfe1c7ae208651210a0cb1f73c5977f Mon Sep 17 00:00:00 2001
From: "David A. Mellis" <d.mellis@arduino.cc>
Date: Sun, 12 Jul 2009 02:58:59 +0000
Subject: Adding improved baud rate calculation, including use (or not) of the
U2X bit. (Code from gabebear).
---
cores/arduino/HardwareSerial.cpp | 35 +++++++++++++++++++++++++++++++----
1 file changed, 31 insertions(+), 4 deletions(-)
(limited to 'cores/arduino')
diff --git a/cores/arduino/HardwareSerial.cpp b/cores/arduino/HardwareSerial.cpp
index d7a2d6e..1af6a66 100755
--- a/cores/arduino/HardwareSerial.cpp
+++ b/cores/arduino/HardwareSerial.cpp
@@ -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);
--
cgit v1.2.3-18-g5258