Freescale Semiconductor, Inc.
1.0 AutoBaud controller
This document describes the operation, registers, and programming of the Autobaud
Microcode Package Revision 2.0. Below are the major features of Revision 2.0:
• Downloads into the MC68302 dual-port RAM.
• Requires 16 x the maximum baudrate for the sampling clock (can be internally or externally generated).
• Supports parity determination and character length detection on reception for 7-bit characters with parity, 8-bit characters with and without parity.
• Supports parity generation on transmission
• Supports 2 echo modes (automatic and smart echoing).
• Supports determining baudrates up to 115.2 kbaud.
1.1 Overview
The support for the AutoBaud function is contained in a RAM microcode that is downloaded into the dual-port RAM of the MC68302. The AutoBaud function determines the baudrate and format of an asynchronous data stream starting with a known character. This package may be used to implement the standard AT command set or other characters.
In order to use the AutoBaud mode, the Serial Communication Controller (SCC) is initially programmed to BISYNC mode. The SCC receiver then synchronizes on the falling edge of the
START bit. Once a start bit is detected, each bit received is processed by the AutoBaud controller.
The AutoBaud controller measures the length of the START bit to determine the receive baudrate and compares the length to values in a user supplied lookup table. After the baudrate is determined, the AutoBaud controller assembles the character and compares it against two userdefined characters. If a match is detected, the AutoBaud controller interrupts the host and returns the determined nominal start value from the lookup table. The AutoBaud controller continues to assemble the characters and interrupt the host until the host stops the reception process. The incoming message should contain a mixture of even and odd characters so that the user has enough information to decide on the proper character format ( length and parity). The host then uses the returned nominal start value from the lookup table, modifies the SCC Configuration
Register (SCON) to generate the correct baudrate, and reprograms the SCC to UART mode.
Many rates are supported including: 150, 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K,
38.4K, 57.6K, 64K, 96K, and 115.2K. To estimate the performance of the AutoBaud microcode package, the performance table in Appendix A of the MC68302 user’s manual can be used. The maximum full-duplex rate for a BISYNC channel is one-tenth of the system clock rate. So a 16.67
MHz 68302 can support 115.2K AutoBaudrate with another low-speed channel (<50 kbps) and a
MC68302 AutoBaud Specificati o n - Rev.2.0
For More Information On This Product,
Go to: www.freescale.com
page 3
