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C8051F340/1/2/3/4/5/6/7
220 Rev. 0.5
19.1. Baud Rate Generator
The UART1 baud rate is generated by a dedicated 16-bit timer which runs from the controller’s core clock
(SYSCLK), and has prescaler options of 1, 4, 12, or 48. The timer and prescaler options combined allow
for a wide selection of baud rates over many SYSCLK frequencies.
The baud rate generator is configured using three registers: SBCON1, SBRLH1, and SBRLL1. The
UART1 Baud Rate Generator Control Register (SBCON1,
SFR Definition 19.4) enables or disables the
baud rate generator, and selects the prescaler value for the timer. The baud rate generator must be
enabled for UART1 to function. Registers SBRLH1 and SBRLL1 contain a 16-bit reload value for the dedi
-
cated 16-bit timer. The internal timer counts up from the reload value on every clock tick. On timer over-
flows (0xFFFF to 0x0000), the timer is reloaded. For reliable UART operation, it is recommended that the
UART baud rate is not configured for baud rates faster than SYSCLK/16. The baud rate for UART1 is
defined in
Equation 19.1.
Equation 19.1. UART1 Baud Rate
A quick reference for typical baud rates and system clock frequencies is given in Table 19.1.
Table 19.1. Baud Rate Generator Settings for Standard Baud Rates
Target Baud
Rate (bps)
Actual Baud
Rate (bps)
Baud Rate
Error
Oscillator
Divide
Factor
SB1PS[1:0]
(Prescaler Bits)
Reload Value in
SBRLH1:SBRLL1
SYSCLK = 12 MHz
230400 230769 0.16% 52 11 0xFFE6
115200 115385 0.16% 104 11 0xFFCC
57600 57692 0.16% 208 11 0xFF98
28800 28846 0.16% 416 11 0xFF30
14400 14388 0.08% 834 11 0xFE5F
9600 9600 0.0% 1250 11 0xFD8F
2400 2400 0.0% 5000 11 0xF63C
1200 1200 0.0% 10000 11 0xEC78
SYSCLK = 24 MHz
230400 230769 0.16% 104 11 0xFFCC
115200 115385 0.16% 208 11 0xFF98
57600 57692 0.16% 416 11 0xFF30
28800 28777 0.08% 834 11 0xFE5F
14400 14406 0.04% 1666 11 0xFCBF
9600 9600 0.0% 2500 11 0xFB1E
2400 2400 0.0% 10000 11 0xEC78
1200 1200 0.0% 20000 11 0xD8F0
SYSCLK = 48 MHz
230400 230769 0.16% 208 11 0xFF98
115200 115385 0.16% 416 11 0xFF30
57600 57554 0.08% 834 11 0xFE5F
28800 28812 0.04% 1666 11 0xFCBF
14400 14397 0.02% 3334 11 0xF97D
9600 9600 0.0% 5000 11 0xF63C
2400 2400 0.0% 20000 11 0xD8F0
1200 1200 0.0% 40000 11 0xB1E0
Baud Rate
SYSCLK
65536 (SBRLH1:SBRLL1)–()
---------------------------------------------------------------------------
1
2
---
×
1
Prescaler
----------------------
×=