MC68HC05B6_13 Datasheet, PDF (91/302 Pages) Freescale Semiconductor, Inc – Microcontrollers

SCR2, SCR1, SCR0 — SCI rate select bits (receiver)
These three read/write bits select the baud rates for the receiver. The prescaler output described
above is divided by the factors shown in Table 6-5.
Table 6-5 Second prescaler stage (receiver)
SCR2
SCR1
SCR0
Receiver
division ratio (NR)
0
0
0
1
0
0
1
2
0
1
0
4
0
1
1
8
1
0
0
16
1
0
1
32
1
1
0
64
1
1
1
128
6
The following equations are used to calculate the receiver and transmitter baud rates:
baudTx
=
--------------f--^o---^p---------------
16 • NP • NT
where:
baudRx
=
---------------f--^o---^p---------------
16 • NP • NR
NP = prescaler divide ratio
NT = transmitter baud rate divide ratio
NR = receiver baud rate divide ratio
baudTx = transmitter baud rate
baudRx = receiver baud rate
f_OSC = oscillator frequency
6.12
Baud rate selection
The flexibility of the baud rate generator allows many different baud rates to be selected. A
particular baud rate may be generated in several ways by manipulating the various prescaler and
division ratio bits. Table 6-6 shows the baud rates that can be achieved, for five typical crystal
frequencies. These are effectively the highest baud rates which can be achieved using a given
crystal.
MC68HC05B6
Rev. 4.1
SERIAL COMMUNICATIONS INTERFACE
Freescale
6-19