XC886/888CLM
Serial Interfaces
User’s Manual
12-10
V1.3, 2010-02
Serial Interfaces, V 1.0
12.1.4
Baud Rate Generation
There are several ways to generate the baud rate clock for the serial ports, depending
on the mode in which they are operating.
The baud rates in modes 0 and 2 are fixed, so they use the
•
Fixed clock, (see
In modes 1 and 3, the variable baud rate is generated using the
•
Dedicated baud-rate generator (see
Additionally for UART module, the variable baud can also be generated using
•
Timer 1 (see
This selection between the different variable baud rate sources is performed by bit BGS
in UART module’s FDCON register.
12.1.4.1
Fixed Clock
The baud rates in modes 0 and 2 are fixed. However, for the case of UART module, while
the baud rate in mode 0 can only be
f
PCLK
/2, the baud rate in mode 2 can be selected as
either
f
PCLK
/64 or
f
PCLK
/32 depending on bit SMOD. Bit SMOD in the PCON register acts
as a double baud rate selector in modes 1, 2 and 3. In modes 1 and 3, only the variable
baud rate supplied by Timer 1 is dependent on SMOD. The baud rate supplied by the
dedicated baud-rate generator is independent of SMOD.
“Baud rate clock” and “baud rate” must be distinguished from each other. The serial
interface requires a clock rate that is 16 times the baud rate for internal synchronization.
Therefore, the dedicated baud-rate generator and Timer 1 must provide a “baud rate
clock” to the serial interface where it is divided by 16 to obtain the actual “baud rate”. The
abbreviation f
PCLK
refers to the input clock frequency.
PCON
Power Control Register
Reset Value: 00
H
7
6
5
4
3
2
1
0
SMOD
0
GF1
GF0
0
IDLE
rw
r
rw
rw
r
rw
*
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