C8051F52X_12 Datasheet, PDF (145/221 Pages) Silicon Laboratories – 8/4/2 kB ISP Flash MCU Family

C8051F52x/F53x
15.1. 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 15.2), 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
UART
TL1
Overflow
2
TX Clock
Start
Detected
TH1
RX Timer
Overflow
2
RX Clock
Figure 15.2. UART0 Baud Rate Logic
Timer 1 should be configured for Mode 2, 8-bit auto-reload (see Section “18.1.3. 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 exter-
nal input T1. The UART0 baud rate is determined by Equation 15.1-A and Equation 15.1-B.
A) _{UartBaudRate =} ^1-- _{x T1_Overflow_Rate}
2
B) _{T1_Overflow_Rate = ------T----1---}^C_----^L_--^K_-------
256 – TH1
Equation 15.1. 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 (8-bit
auto-reload mode reload value). Timer 1 clock frequency is selected as described in Section “18. Timers”
on page 182. A quick reference for typical baud rates and system clock frequencies is given in Table 15.1.
Note that the internal oscillator may still generate the system clock when the external oscillator is driving
Timer 1.
Rev. 1.4
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