C8051F524-C-IM 查看數據表（PDF） - Silicon Laboratories

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C8051F524-C-IM

8/4/2 kB ISP Flash MCU Family

Silicon Laboratories 

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 T1CLK 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

145

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