URL:
<http://savannah.nongnu.org/bugs/?51255>
Summary: Calculations in util/setbaud.h are incorrect
Project: AVR C Runtime Library
Submitted by: dense505
Submitted on: Sat 17 Jun 2017 05:56:37 AM UTC
Category: Header
Severity: 3 - Normal
Priority: 5 - Normal
Item Group: Header files
Status: None
Percent Complete: 0%
Assigned to: None
Originator Email:
Open/Closed: Open
Discussion Lock: Any
Release: 2.0.0
Fixed Release: None
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Details:
Calculations are wrong, the error becomes severe at high baud rates. Correct
formula in attached file includes correct calculation and finesse of (ratio *
100 + 50)/100 to achieve correct rounding to nearest integer value, not just
truncation.
This should also address bug #37758 as the issue is more general than just a
problem for Xmega devices. I personally find the updated formula version
easier to follow than the suggestion in bug #37758.
table of results from attached spreadsheet, looks weird with the limited width
to display here, hope it comes out at your end. The corrected formula agrees
exactly with the corresponding table in an Atmel datasheet.
Osc 16000000 error % after rounding
Baud rate UBRR bad UBRR error % Rounded UBRR Rounded bad
UBRR Correct UBRR bad
UBRR
1200 832.3 833.9 0.19 832 834 0.04 -0.20
2400 415.7 417.2 0.36 416 417 -0.08 -0.32
4800 207.3 208.8 0.72 207 209 0.16 -0.80
9600 103.2 104.7 1.45 103 105 0.16 -1.78
14400 68.4 69.9 2.19 68 70 0.65 -2.27
19200 51.1 52.6 2.94 51 53 0.16 -3.75
28800 33.7 35.2 4.45 34 35 -0.82 -3.79
38400 25.0 26.5 5.99 25 27 0.17 -7.82
57600 16.4 17.9 9.17 16 18 2.21 -10.02
76800 12.0 13.5 12.48 12 14 0.17 -16.46
115200 7.7 9.2 19.53 8 9 -4.16 -17.18
230400 3.3 4.8 44.91 3 5 10.19 -49.69
250000 3.0 4.5 50.00 3 5 0.00 -66.67
500000 1.0 2.5 150.00 1 3 0.00 -200.00
_______________________________________________________
File Attachments:
-------------------------------------------------------
Date: Sat 17 Jun 2017 05:56:37 AM UTC Name: setbaud corrected.h Size: 7kB
By: dense505
<http://savannah.nongnu.org/bugs/download.php?file_id=40929>
-------------------------------------------------------
Date: Sat 17 Jun 2017 05:56:37 AM UTC Name: setbuad errors.xlsx Size: 14kB
By: dense505
<http://savannah.nongnu.org/bugs/download.php?file_id=40930>
_______________________________________________________
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