Silicon Laboratories C8051F340 Enhanced Baud Rate Generation, Equation 18.1. UART0 Baud Rate

C8051F340/1/2/3/4/5/6/7

18.1. 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 18.2), 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

TH1

Start

Detected

RX Timer

Overflow

2 RX Clock

Figure 18.2. UART0 Baud Rate Logic

Timer 1 should be configured for Mode 2, 8-bit auto-reload (see Section “21.1.3. Mode 2: 8-bit Counter/ Timer with Auto-Reload” on page 245). 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. For any given Timer 1 clock source, the UART0 baud rate is determined by Equation 18.1.

	T1CLK		1
UartBaudRate =	------------------------------	⋅	--
	(256 – T1H)		2

Equation 18.1. UART0 Baud Rate

Where T1CLK is the frequency of the clock supplied to Timer 1, and T1H is the high byte of Timer 1 (reload value). Timer 1 clock frequency is selected as described in Section “21. Timers” on page 243. A quick reference for typical baud rates using the internal oscillator is given in Table 18.1. Note that the internal oscillator may still generate the system clock if an external oscillator is driving Timer 1.

18.2. Operational Modes

UART0 provides standard asynchronous, full duplex communication. The UART mode (8-bit or 9-bit) is selected by the S0MODE bit (SCON0.7). Typical UART connection options are shown below.

212 Rev. 0.5