C8051F38C-GQR View Datasheet(PDF) - Silicon Laboratories

Part Name

Description

MFG CO.

C8051F38C-GQR

Full Speed USB Flash MCU Family

Silicon Laboratories 

C8051F380/1/2/3/4/5/6/7/C

23.1. 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 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.

Timer 1

UART

TL1

Overflow

2

TX Clock

Start

Detected

TH1

RX Timer

Overflow

2

RX Clock

Figure 23.2. UART0 Baud Rate Logic

Timer 1 should be configured for Mode 2, 8-bit auto-reload (see Section “26.1.3. Mode 2: 8-bit

Counter/Timer with Auto-Reload” on page 267). 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. For any given Timer 1 clock source, the UART0 baud rate is determined by Equation 23.1-A and

Equation 23.1-B.

A)

UARTBaudRate = 12--  T1_Overflow_Rate

B)

T1_Overflow_Rate = 2----5-T--6-1----C–----L-T---KH-----1-

Equation 23.1. 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 “26. Timers” on page 263. A quick ref-

erence for typical baud rates and system clock frequencies is given in Table 23.1. The internal oscillator

may still generate the system clock when the external oscillator is driving Timer 1.

Rev. 1.4

233

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