C8051F120 Datasheet, PDF (304/356 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F120/1/2/3/4/5/6/7
C8051F130/1/2/3
22.1. Enhanced Baud Rate Generation
The UART1 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
UART1
2
TX Clock
Start
Detected
TH1
RX Timer
Overflow
2
RX Clock
Figure 22.2. UART1 Baud Rate Logic
Timer 1 should be configured for Mode 2, 8-bit auto-reload (see Section “23.1.3. Mode 2: 8-bit Counter/
Timer with Auto-Reload” on page 315). The Timer 1 reload value should be set so that overflows will occur
at two times the desired baud rate. 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 UART1 baud rate is determined by Equation 22.1.
Equation 22.1. UART1 Baud Rate
UARTBaudRate
=
--------^T----1---^C---^L---^K---------
(256 – T1H)
×
1--
2
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 “23.1. Timer 0 and Timer 1” on
page 313. A quick reference for typical baud rates and system clock frequencies is given in Table 22.1
through Table 22.5. Note that the internal oscillator or PLL may still generate the system clock when the
external oscillator is driving Timer 1 (see Section “23.1. Timer 0 and Timer 1” on page 313 for more
details).
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Rev. 1.3