MC68LC302 Datasheet, PDF (83/169 Pages) Motorola, Inc – Low Power Integrated Multiprotocol Processor Reference Manual

Communications Processor (CP)
assuming that the DIV bit in SCON is set to 0, (otherwise an additional “divide-by-4” must
be included).
Sampling Rate = BRG Clk Rate, where BRG Clk rate >= (Max Desired UART Baud Rate) x 16
(EQ 2)
For instance, if a 115.2K baud rate is desired, with a 16.67 MHz system clock, the minimum
sampling rate possible is 1.843 MHz = 115.2K x 16. This exact frequency can be input to
RCLK1 or TIN1 as the sample clock. If the system clock is to be used, a 16.67 MHz system
clock cannot produce an exact baud rate clock of 1.843 MHz. The lowest one that can be
used is Baud Rate = 16.67 MHz / (7+1) = 2.083 MHz. Thus, 2.083 MHz is the sampling rate,
and the SCON should be set to $000E to produce this.
Once the sampling rate is known, the other two equations follow easily. The maximum
START bit length is calculated by the following equation:
Maximum start length = (Sampling Rate/Recognized baud rate) x 1.05
(EQ 3)
Thus, for the first entry in the table, the maximum start length is 1.8432 Mhz/115200 x 1.05
= 17 for an external sample clock. The value 1.05 is a suggested margin that allows char-
acters 5% larger than the nominal character rate to be accepted. In effect, the margin deter-
mines the “split point” between what is considered to be a 56.7K character rate and what is
a 38.4K character rate. The margin should not normally be less than 1.03 due to clocking
differences between UARTs.
The nominal START bit length is calculated by:
Nominal start length = (Sampling Rate/Recognized baud rate) / 2
(EQ 4)
For the 115.2K example in the first table entry, this would be 1.8432 MHz/115.2K/2 = 8.
The structure of the lookup table is shown in Table 4-5. The table starts with the maximum
UART baud rate supported and ends with the minimum UART baud rate supported.
Table 4-5. 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 driv-
ers of up to 4%, plus nominal clocking rate variations of 3%, plus
4-16
MC68LC302 REFERENCE MANUAL
MOTOROLA