Automatic baud rate detection (ABR, autobaud) refers to the process by which a receiving device (such as a
modem
The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...
) determines the speed, code level, start bit, and
stop bit
Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Synchronization ( clock recovery) is done by data-embedded signal ...
s of incoming data by examining the first
character, usually a preselected sign-on character (
syncword
In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identif ...
) on a
UART
A universal asynchronous receiver-transmitter (UART ) is a peripheral device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significant to ...
connection. ABR allows the receiving device to accept data from a variety of transmitting devices operating at different speeds without needing to establish data rates in advance.
Process
During the autobaud process, the
baud rate of received character stream is determined by examining the received pattern and its timing, and the length of a start bit. These types of baud rate detection mechanisms are supported by many hardware chips including processors such as STM32 MPC8280, MPC8360, and so on.
When start bit length is used to determine the baud rate, it requires the character to be odd since UART sends
LSB bit firstthis particular bit order scheme is referred to as
little-endian
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In computing, endianness is the order in which bytes within a word (data type), word of d ...
. Often
ASCII
ASCII ( ), an acronym for American Standard Code for Information Interchange, is a character encoding standard for representing a particular set of 95 (English language focused) printable character, printable and 33 control character, control c ...
symbols 'a' or 'A' (0x61 or 0x41) are used. For example, the MPC8270 SCC tries to detect the length of the UART start bit for autobaud.
Many protocols begin each frame with a preamble of alternating 1 and 0 bits that can be used for automatic baud rate detection. For example, the TI PGA460 uses a ASCII ''U'' ( or ) sync byte for automatic baud rate detection as well as
frame synchronization
In telecommunications, frame synchronization or framing is the process by which, while receiving a stream of fixed-length frames, the receiver identifies the frame boundaries, permitting the data bits within the frame to be extracted for decodin ...
, and so does the LIN header (
Local Interconnect Network#Header).
For example, the UART-based FlexWire protocol begins each frame with a ''U'' ()
sync byte
In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identif ...
. FlexWire receivers use the sync byte to precisely set their UART bit-clock frequency without a high-precision oscillator.
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Texas Instruments]
"TPS929121-Q1 12-Channel Automotive 40-V High-Side LED Driver With FlexWire"
2021.
p. 37 and p. p. 39.
For example, the
Ethernet preamble contains 56 bits of alternating 1 and 0 bits for synchronizing bit clocks.
Support
Most
modem
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s currently on the market support autobaud. Before receiving any input data, most modems use a default baud rate of 9600 for output. For example, the following modems have been verified for autobaud and default output baud rate 9600:
* USRobotics USR5686G 56K Serial Controller Fax modem
* Hayes V92 External modem
* Microcom DeskPorte 28.8P
The baud rate of modems are adjusted automatically after receiving input data by the autobaud process.
See also
*
Autonegotiation
Autonegotiation is a signaling mechanism and procedure used by Ethernet over twisted pair by which two connected devices choose common transmission parameters, such as speed, Duplex_(telecommunications), duplex mode, and Flow_control_(data), flow ...
*
Telecommunications
Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...
References
* "17.2 Autobaud Operation on a UART in MPC8280 PowerQUICC™ II Family Reference Manual" http://www.nxp.com/files/netcomm/doc/ref_manual/MPC8280RM.pdf
* "Automatic Baud Rate Detection on the MSP430" https://web.archive.org/web/20161026080239/http://www.ti.com/lit/an/slaa215/slaa215.pdf
* "How to implement “auto baud rate detection” feature on Cortex-M3" https://stackoverflow.com/q/38979647
* "mpc8270 SCC2 UART issue" https://community.nxp.com/message/906833
{{refend
Data transmission
Units of measurement
Telecommunications techniques