mpc823rg Freescale Semiconductor, Inc, mpc823rg Datasheet - Page 608

• Manufacturer Part Number: mpc823rg
• Description: Mpc823 Powerquicc Integrated Communications Processor For Portable Systems
• Manufacturer: Freescale Semiconductor, Inc

Communication Processor Module
16.8.1 Autobaud Operation
Using the autobaud process, a UART determines the baud rate of its received character
stream by examining the pattern received and the timing information of that pattern. The
MPC823 baud rate generators have a built-in autobaud control function that automatically
measures the length of a start bit and modifies the baud rate accordingly. If the ATB bit in
the baud rate generator configuration registers is set, the autobaud control block starts
searching for a low level on the RXDx signal, which it assumes is the beginning of a start bit
and begins counting the start bit length. During this time, the BRG output clock toggles for
16 BRG clock cycles at the BRG input clock rate and then stops with the BRGOx output
clock in the low state.
After the RXDx signal changes back to the high level, the autobaud control block rewrites
the CD and DIV16 bits in the baud rate generator configuration registers to the divide ratio
it found. Due to a measurement error that occurs at high baud rates, this divide ratio written
by the autobaud controller may not be the precise baud rate you prefer (56,600 could be the
resulting baud rate, rather than 57,600). Thus, an interrupt is provided in the SCCE–UART
register (described in Section 16.9.15.18 SCCx UART Event Register) to signify that the
baud rate generator configuration registers were rewritten by the autobaud controller. When
this interrupt is recognized, you must rewrite the baud rate generator configuration registers
with the value you prefer. It is recommended that this be done as quickly as possible (even
prior to the first character being fully received) to ensure that all characters are recognized
correctly by the UART.
Once a full character is received, you can check the software to verify that the received
character matches a predefined value (such as “a” or “A”). The software must then check
for other characters (such as “t” or “T”) and program a serial communication controller to the
preferred parity mode. You can change the parity mode in the PSMR–SCC UART register,
which is described in Section 16.9.3 Protocol-Specific Mode Register.
Note: The serial communication controller associated with the baud rate generator
must be programmed to UART mode and have the TDCR and RDCR fields in
the GSMR_L set to the 16 option for the autobaud function to operate correctly.
Input frequencies, such as 1.32MHz, 3.68MHz, 7.36 MHz, and 14.72MHz, must
be used. For autobaud to operate successfully, the serial communication
controller must be connected to a baud rate generator. For instance, SCC2 must
be clocked by BRG2 to successfully perform the autobaud function. Also, for a
serial communication controller to correctly detect an autobaud lock and
generate an interrupt, it must receive three full receive clocks from the baud rate
generator before the autobaud process begins. To do this, set the ATB bit to 0
in the baud rate generator configuration register and enable the BRGx receive
clock to the highest frequency. Immediately prior to the start of the autobaud
process (after device initialization), set the ATB bit to 1.
Freescale Semiconductor, Inc.
For More Information On This Product,
MPC823 REFERENCE MANUAL
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