C8051F300/1/2/3/4/5
132
Rev. 2.9
14.1. Enhanced Baud Rate Generation
The UART0 baud rate is generated by Timer 1 in 8-bit auto-reload mode. The TX clock is generated by
TL1; the RX clock is generated by a copy of TL1 (shown as RX Timer in Figure 14.2), which is not user
accessible. Both TX and RX Timer overflows are divided by two to generate the TX and RX baud rates.
The RX Timer runs when Timer 1 is enabled, and uses the same reload value (TH1). However, an
RX Timer reload is forced when a START condition is detected on the RX pin. This allows a receive to
begin any time a START is detected, independent of the TX Timer state.
Figure 14.2. UART0 Baud Rate Logic
Timer 1 should be configured for Mode 2, 8-bit auto-reload (see
Section “15.1.3. Mode 2: 8-bit
Counter/Timer with Auto-Reload” on page 145
). The Timer 1 reload value should be set so that over-
flows will occur at two times the desired UART baud rate frequency. Note that Timer 1 may be clocked by
one of five sources: SYSCLK, SYSCLK / 4, SYSCLK / 12, SYSCLK / 48, or the external oscillator clock / 8.
For any given Timer 1 clock source, the UART0 baud rate is determined by Equation 14.1.
Equation 14.1. UART0 Baud Rate
Where
T1
CLK
is the frequency of the clock supplied to Timer 1, and
T1H
is the high byte of Timer 1 (reload
value). Timer 1 clock frequency is selected as described in
Section “15.2. Timer 2” on page 151
reference for typical baud rates and system clock frequencies is given in Tables 14.1 through 14.6. Note
that the internal oscillator may still generate the system clock when the external oscillator is driving Timer 1
(see
Section “15.1. Timer 0 and Timer 1” on page 143
for more details).
RX Timer
Start
Detected
Overflow
Overflow
TH1
TL1
TX Clock
2
RX Clock
2
Timer 1
UART0
UartBaudRate
T
1
CLK
256
T
1
H
–
(
)
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2
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