C8051F336_08 Datasheet, PDF (160/227 Pages) Silicon Laboratories – Mixed-Signal Byte-Programmable EPROM MCU

C8051F336/7/8/9
22.1. 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 22.2), 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.
Timer 1
TL1
Overflow
UART
2
TX Clock
Start
Detected
TH1
RX Timer
Overflow
2
RX Clock
Figure 22.2. UART0 Baud Rate Logic
Timer 1 should be configured for Mode 2, 8-bit auto-reload (see Section “24.1.3. Mode 2: 8-bit Coun-
ter/Timer with Auto-Reload” on page 184). The Timer 1 reload value should be set so that overflows will
occur at two times the desired UART baud rate frequency. Note that Timer 1 may be clocked by one of six
sources: SYSCLK, SYSCLK/4, SYSCLK/12, SYSCLK/48, the external oscillator clock/8, or an external
input T1. For any given Timer 1 clock source, the UART0 baud rate is determined by Equation 22.1-A and
Equation 22.1-B.
A) _{UartBaudRate =} ^1-- _{× T1_Overflow_Rate}
2
B) _{T1_Overflow_Rate = ------T---1---}^C_---^L_---^K_-------
256 – TH1
Equation 22.1. 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 “24. Timers” on page 180. A quick ref-
erence for typical baud rates and system clock frequencies is given in Table 22.1 through Table 22.2. The
internal oscillator may still generate the system clock when the external oscillator is driving Timer 1.
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