Freescale Semiconductor, Inc.
Nominal START length = (Sampling Rate/Recognized baudrate) / 2 (EQ 4)
8.
For the 115.2K example, in the first entry of the table, this would be 1.8432 MHz/115.2K/2 =
The structure of the LookUp table is shown in Table 3 on page 12. The table starts with the maximum UART baudrate supported and ends with the minimum UART baudrate supported.
Table 3. AutoBaud LookUp Table Format
OFFSET from LookUp Table
Pointer
0
2
4
6
•
•
(LookUp Table Size - 1) * 4
[(LookUp Table Size - 1) * 4] + 2
DESCRIPTION
Maximum START Length
Nominal START Length
Maximum START Length
Nominal START Length
Maximum START Length
Nominal START Length
Maximum START Length
Nominal START Length
NOTE
If less margin is used in the calculation of the Maximum START length above, it is possible to distinguish between close UART rates such as 64K and 57.6K. However variations in RS232 drivers of up to 4%, plus nominal clocking rate variations of 3%, plus the fact that the sampling rate may not perfectly divide into the desired UART rate, can make this distinction difficult to achieve in some scenarios.
1.5.5.1
LookUp Table Example
Table 4 is an example AutoBaud Lookup Table. The Maximum Start and Nominal Start values are derived assuming a 1.8432 MHz sampling clock on TIN1 or RCLK and a shift factor of 5%.
Table 4. LookUp Table Example
Desired
Baudrate
115200
57600
38400
28800
19200
14400
12000
9600
Maximum
Start
17
34
50
67
101
134
161
202
Nominal
Start
8
16
24
32
48
64
77
96
MC68302 AutoBaud Specificati o n - Rev.2.0
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page 12
