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SERIAL I/O

7721 Group User’s Manual

11–21

11.3 Clock synchronous serial I/O mode

11.3.3 Transmit operation

When the transmit conditions described in section “11.3.2 Method of transmission” are satisfied in the

case of selecting an internal clock, a transfer clock is generated and the following operations are automatically

performed after 1 cycle of the transfer clock has passed. When the transmit conditions are satisfied and

the external clock is input to the CLKi pin in the case of selecting an external clock, the following operations

are automatically performed.

•The UARTi transmit buffer register’s contents are transferred to the UARTi transmit register.

•The transmit buffer empty flag is set to “1.”

•The transmit register empty flag is cleared to “0.”

•8 transfer clocks are generated (when an internal clock is selected).

•A UARTi transmit interrupt request occurs, and the interrupt request bit is set to “1.”

The transmit operations are described below:

Data in the UARTi transmit register is transmitted from the TxDi pin synchronously with the falling edge

of the transfer clock.

This data is transmitted bit by bit sequentially beginning with the least significant bit.

When 1-byte data has been transmitted, the transmit register empty flag is set to “1.” This indicates the

completion of transmission.

Figure 11.3.4 shows the transmit operation.

When an internal clock is selected, when the transmit conditions for the next data are satisfied at completion

of the transmission, the transfer clock is generated continuously. Accordingly, when performing transmission

continuously, set the next transmit data to the UARTi transmit buffer register during transmission (when the

transmit register empty flag = “0”). When the transmit conditions for the next data are not satisfied, the

transfer clock stops at “H” level.

Figures 11.3.5 and 11.3.6 show examples of transmit timing.

Fig. 11.3.4 Transmit operation

Transfer clock

UARTi transmit buffer register

Transmit data

MSB

LSB

UARTi transmit register

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