User’s Manual
6-33
05.99
On-Chip Peripheral Components
C513AO
6.3.3 Baudrates
There are several possibilities for generating the baudrate clock for the serial interface, depending
on the mode in which it is operated.
To clarify the terminology, something should be said about the differences between “baudrate
clock” and “baudrate”.
The serial interface requires a clock rate which is 16 times the baudrate for internal synchronization.
Therefore, the baudrate generators must provide a “baudrate clock” to the serial interface results in
the actual “baudrate” after being divided by 16 by the serial interface.
Note: All formulae given below already include the factor and calculate the final baudrate.
The baudrate generation is further controlled by bit SMOD which is located in SFR PCON.
Special Function Register PCON (Address 87H) Reset Value: 0XX00000B
Mode 0
The baudrate in Mode 0 is fixed:
Mode 0 baudrate = oscillator frequency/12 =
f
OSC
/12
Mode 2
The baudrate in Mode 2 depends on the value of bit SMOD in special function register PCON (87H).
If SMOD = 0 (the value on reset), the baudrate is
f
OSC
/64. If SMOD = 1, the baudrate is
f
OSC
/32.
Mode 2 baudrate = 2
SMOD
/64
×
(
f
OSC
)
Modes 1 and 3
The baudrates in Mode 1 and 3 are determined by the timer overflow rate. These baudrates can be
determined by Timer 1 or by Timer 2 or both (one for transmit, the other for receive).
Symbol
Function
SMOD
Baudrate double bit
When set, the baudrate of the serial channel in Mode 1,2,or 3 is doubled.
MSB
LSB
87H
PCON
SMOD
–
–
SD
GF1
GF0
PDE
IDLE
7
6
5
4
3
2
1
0
Bit No.
The functions of the shaded bits are not described in this section.