C8051F970-A-GM Datasheet, PDF(380/454 Page) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

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C8051F970-A-GM Datasheet, PDF (380/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

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C8051F97x

31. Universal Asynchronous Receiver/Transmitter (UART0)

UART0 is an asynchronous, full duplex serial port offering modes 1 and 3 of the standard 8051 UART. Enhanced

baud rate support allows a wide range of clock sources to generate standard baud rates (details in Section

â31.1. Enhanced Baud Rate Generationâ on page 381). Received data buffering allows UART0 to start reception of

a second incoming data byte before software has finished reading the previous data byte.

UART0 has two associated SFRs: Serial Control Register 0 (SCON0) and Serial Data Buffer 0 (SBUF0). The

single SBUF0 location provides access to both transmit and receive registers. Writes to SBUF0 always access

the transmit register. Reads of SBUF0 always access the buffered receive register; it is not possible to

read data from the transmit register.

With UART0 interrupts enabled, an interrupt is generated each time a transmit is completed (TI is set in SCON0),

or a data byte has been received (RI is set in SCON0). The UART0 interrupt flags are not cleared by hardware

when the CPU vectors to the interrupt service routine. They must be cleared manually by software, allowing soft-

ware to determine the cause of the UART0 interrupt (transmit complete or receive complete).

UART0

TB8

TI, RI

(9th bit)

Interrupts

Output Shift

R e g is te r

TX

Control /

C o n fig u ra tio n

SBUF (8 LSBs)

Baud Rate

G e n e ra to r

(Timer 1)

TX Clk

RX Clk

RB8

(9th bit)

Input Shift

R e g is te r

RX

START

D e te c tio n

Figure 31.1. UART0 Block Diagram

31.1. 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 31.2), 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.

Rev 1.0

381

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