📑 Table of Contents▼

Automatic baud rate detection (ABR, autobaud) refers to the process by which a receiving device (such as a modem) determines the speed, code level, start bit, and stop bits of incoming data by examining the first character, usually a preselected sign-on character (syncword) on a UART 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.[1]

Process

edit

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,[2] 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 first – this particular bit order scheme is referred to as little-endian.[3] Often ASCII 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 an ASCII U (5516 or 10101012) sync byte for automatic baud rate detection as well as frame synchronization,[4] and so does the LIN header (Local Interconnect Network#Header).

For example, the UART-based FlexWire protocol begins each frame with a U (5516) sync byte. FlexWire receivers use the sync byte to precisely set their UART bit-clock frequency without a high-precision oscillator.[5] For example, the Ethernet preamble contains 56 bits of alternating 1 and 0 bits for synchronizing bit clocks.

Support

edit

Most modems 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

edit

References

edit
  1. ^ This article is based on material taken from Automatic+baud+rate+detection at the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.
  2. ^ STM32 USART automatic baud rate detection
  3. ^ "MIT — 6.111 Introductory Digital Systems Laboratory" (PDF). MIT 6.111 Fall 2019. Retrieved July 15, 2023.
  4. ^ "Sync Field" section of "PGA460 Ultrasonic Signal Processor and Transducer Driver". 2017. p. 35.
  5. ^ Texas Instruments. "TPS929121-Q1 12-Channel Automotive 40-V High-Side LED Driver With FlexWire". 2021. p. 37 and p. p. 39.

Further reading

edit

📚 Artikel Terkait di Wikipedia

Symbol rate

In a digitally modulated signal or a line code, symbol rate, modulation rate or baud is the number of symbol changes, waveform changes, or signaling events

Universal asynchronous receiver-transmitter

machines (e.g. Raspberry Pi Pico's PIO or NXP's FlexIO). Automatic baud rate detection Baud Bit rate Comparison of synchronous and asynchronous signalling

Clock synchronization

synchronization is used in synchronization in telecommunications and automatic baud rate detection. Plesiochronous or isochronous operation refers to a system

Hayes AT command set

modem into acting properly. The Hayes command set facilitated automatic baud rate detection as "A" and "T" happen to have bit patterns that are very regular;

Asynchronous serial communication

to the Baud rate when each symbol represents one bit). Some systems use automatic speed detection, also called automatic baud rate detection. Whether

ABR

Adaptive bit rate, a method of video transmission Available bit rate, a service used in ATM networks Average bitrate Automatic baud rate detection Anaerobic

Syncword

Ethernet and other protocols, the preamble can also be used for automatic baud rate detection. Copy protection § Early ages Magic number (programming) Out-of-band

TI MSP430

(USCI_A) supports UART mode; SPI mode; pulse shaping for IrDA; and automatic baud-rate detection for LIN communications. The synchronous channel (USCI_B) supports