PC87591L datasheet(153/437 Pages) NSC

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Embedded Controller Modules (Continued)

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4.9.3

Operation

The USART has two basic modes of operation; Synchronous and Asynchronous. In addition, two special Synchronous and

Asynchronous modes, attention and diagnostic, are available. This section describes the operating modes of the USART.

Asynchronous Mode

USART Asynchronous mode enables the device to communicate with other devices using two communication signals: trans-

mit (UTXD) and receive (URXD).

In Asynchronous mode, Transmit Shift register (TSFT) and Transmit Buffer register (UTBUF) double buffer data for trans-

mission. To transmit a character, a data byte is loaded into UTBUF register. The data is then transferred to TSFT register.

While TSFT register is shifting out the current character (LSB first) on the UTXD pin, UTBUF register is loaded by software

with the next byte to be transmitted. When TSFT completes transmitting the last stop bit of the current frame, the contents

of UTBUF are transferred to TSFT register and the Transmit Buffer Empty flag (TBE) is set. The TBE flag is automatically

reset by the USART when the software loads a new character into UTBUF register. During transmission, the XMIP bit is set

high by the USART. This bit is reset only after the USART has sent the last stop bit of the current character and UTBUF

In Asynchronous mode, the input frequency to the USART is 16 times the baud rate, i.e., there are 16 clock cycles per bit

time. In Asynchronous mode, the baud rate generator is always used as the UART clock.

Receive Shift register (RSFT) and Receive Buffer register (URBUF) double buffer the data being received. The USART re-

ceiver continually monitors the signal on the URXD pin for a low level to detect the beginning of a start bit. On sensing this

low level, the USART waits for seven input clock cycles and samples again three times. If all three samples still indicate a

valid low, the receiver considers this to be a valid start bit, and the remaining bits in the character frame are each sampled

three times around the mid-bit position. For any bit following the start bit, the logic value is found by majority voting, i.e., the

two samples with the same value define the value of the data bit. Figure 53 illustrates the process of start-bit detection and

bit sampling. Serial data input on the URXD pin is shifted into RSFT register. On receiving the complete character, the con-

tents of RSFT register are copied into URBUF register and Receive Buffer Full flag (RBF) is set. The RBF flag is automati-

cally reset when software reads the character from URBUF register. The RSFT register is not user accessible.

Figure 53. USART Asynchronous Communication

Synchronous Mode

The USART Synchronous mode enables the device to communicate with other devices using three communication signals:

transmit (UTXD), receive (URXD) and clock (USCLK). In this mode, data bits are transferred synchronously using the

USART clock signal. As shown in Figure 54, the data is transmitted on the rising edge and received on the falling edge of

the clock. Data is transmitted and received with the LSB first.

Sample

STARTBIT

DATA (LSB)

Sample

DATABIT

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