MC68HC912BL16 Datasheet, PDF (89/128 Pages) Freescale Semiconductor, Inc – 16-Bit Microcontroller

Freescale Semiconductor, Inc.
13.2.2 SCI Baud Rate Generation
The basis of the SCI baud rate generator is a 13-bit modulus counter. This counter gives the generator
the flexibility necessary to achieve a reasonable level of independence from the CPU operating frequen-
cy and still be able to produce standard baud rates with a minimal amount of error. The clock source for
the generator comes from the P Clock.
Table 29 Baud Rate Generation
Desired BR Divisor for BR Divisor for
SCI Baud Rate P = 4.0 MHz P = 8.0 MHz
110
2273
4545
300
833
1667
600
417
833
1200
208
417
2400
104
208
4800
52
104
9600
26
52
14400
17
35
19200
13
26
38400
—
13
13.2.3 Register Descriptions
Control and data registers for the SCI subsystem are described below. The memory address indicated
for each register is the default address that is in use after reset. The entire 512-byte register block can
be mapped to any 2-Kbyte boundary within the standard 64-Kbyte address space.
SC0BDH — SCI Baud Rate Control Register
$00C0
Bit 7
6
5
4
3
2
1
Bit 0
BTST
BSPL
BRLD SBR12 SBR11 SBR10 SBR9
SBR8
High
RESET:
0
0
0
0
0
0
0
0
SC0BDL — SCI Baud Rate Control Register
$00C1
Bit 7
6
5
4
3
2
1
Bit 0
SBR7
SBR6
SBR5
SBR4
SBR3
SBR2
SBR1
SBR0
Low
RESET:
0
0
0
0
0
1
0
0
SC0BDH and SC0BDL are considered together as a 16-bit baud rate control register.
Read any time. Write SBR[12:0] anytime. Low order byte must be written for change to take effect. Write
SBR[15:13] only in special modes. The value in SBR[12:0] determines the baud rate of the SCI. The
desired baud rate is determined by the following formula:
SCI Baud Rate = ₁--^M--₆---^C-_×----^L-_B--^K--_R---
which is equivalent to:
BR = ₁----₆-----_×----_S----_C-^M---_I--^C--_B---^L-_a--^K-_u---_d-----_R----_a----_t-_e---
BR is the value written to bits SBR[12:0] to establish baud rate.
NOTE
The baud rate generator is disabled until the TE or RE bit in SC0CR2 register is set
for the first time after reset, and/or the baud rate generator is disabled when
SBR[12:0] = 0.
MC68HC912BL16TS/D
For More Information On This Product,
89
Go to: www.freescale.com