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MC68302AG16C 产品描述及参数 - NXP（恩智浦）
制造商:
NXP（恩智浦）
分类:
微处理器
封装:
LQFP-144
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MC68302AG16C数据手册
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page 4
MC68302 AutoBaud Specificati
o
n - Rev.2.0
20 MHz MC68302 can support 115.2K AutoBaudrate with 2 low-speed channels. The
performance can vary depending on system loading, configuration, and echoing mode.
It is important that the highest priority SCC be used for the AutoBaud function, since it is
running at a very high rate. Any SCC that is guaranteed to be idle during the search operation of
the AutoBaud process will not impact the performance of Autobaud in an application. Idle is
defined as not having any transmit or receive requests to/from the SCC FIFOs.
1.2 AutoBaud Channel Reception Process
The interface between the AutoBaud controller and the host processor is implemented with
shared data structures in the SCC Parameter RAM and in external memory and through the use of
a special command to the SCC.
The AutoBaud Controller uses Receive Buffer Descriptor number 7 (Rx BD7) for the
AutoBaud Command Descriptor. This Rx BD is initialized by the Host to contain a pointer to a
lookup table residing in the external RAM (contains the maximum and nominal START bit length
for each Baudrate). The Host also prepares two characters against which the AutoBaud Controller
will compare the received character (usually these characters are ‘a’ and ‘A’ ), and the host
initializes a pointer to a buffer in external memory where the assembled characters will be stored
until the host stops the AutoBaud process. Finally, the host initializes the SCC Data
Synchronization Register (DSR) to $7FFF in order to synchronize on the falling edge of the start
bit.
Once the data structures are initialized, the host programs the SCON register to provide a
sampling clock that is 16X the maximum supported baudrate. The Host then issues the
Enter_Baud_Hunt command and enables the SCC in the BISYNC mode.
The AutoBaud Controller reception process starts when the START bit arrives. The
AutoBaud Controller then starts to measure the START bit length. With each byte received from
the SCC that “belongs” to the START bit, the AutoBaud Controller increments the Start Length
Counter and compares it to the current lookup table entry. If the Start Length Counter passed the
maximum bit length defined by the current table entry, the AutoBaud Controller switches to the
next lookup table entry (the next slower baudrate). This process goes on until the AutoBaud
Controller recognizes the end of the Start bit. Then, the AutoBaud controller starts the character
assembly process.
The character assembly process uses the nominal bit length, taken from the current lookup
table entry, to sample each incoming bit in it’s center. Each bit received is stored to form an 8 bit
character. When the assembly process is completed (a Stop bit is received), the character is
compared against two user-defined characters.
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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