C8051F52X Datasheet, PDF (144/220 Pages) Silicon Laboratories – 8/4/2 kB ISP Flash MCU Family

C8051F52x-53x
16.1. 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 16.2), 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 16.2. UART0 Baud Rate Logic
Timer 1 should be configured for Mode 2, 8-bit auto-reload (see Section “19.1.3. Mode 2: 8-bit
Counter/Timer with Auto-Reload” on page 187). 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. The UART0 baud rate is determined by Equation 16.1-A and Equation 16.1-B.
A)
UartBaudRate
=
1--
2
x
T1_Overflow_Rate
B) T1_Overflow_Rate = ------T----1---^C----^L--^K-------
256 – TH1
Equation 16.1. 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 “19. Timers”
on page 185. A quick reference for typical baud rates and system clock frequencies is given in Table 16.1.
Note that the internal oscillator may still generate the system clock when the external oscillator is driving
Timer 1.
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Rev. 0.3