C8051F060/1/2/3/4/5/6/7

C8051F060/1/2/3/4/5/6/7

23.1. 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 23.2), 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.

Figure 23.2. UART1 Baud Rate Logic

Timer 1 UART1

Overflow

TL1

2

TX Clock

TH1

Start

Detected

RX Timer

Overflow

2

RX Clock

Timer 1 should be configured for Mode 2, 8-bit auto-reload (see Section “24.1.3. Mode 2: 8-bit Counter/

Timer with Auto-Reload” on page 289 ). 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 23.1.

Equation 23.1. UART1 Baud Rate

UartBaudRate

=

(

T1

256 –

T1H

)

×

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 “24.1. Timer 0 and Timer 1” on page 287

. A quick reference for typical baud rates and system clock frequencies is given in Table 23.1

through Table 23.6. Note that the internal oscillator may still generate the system clock when the external

oscillator is driving Timer 1 (see Section “24.1. Timer 0 and Timer 1” on page 287

for more details).

278 Rev. 1.2