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SERIAL I/O

7721 Group User’s Manual

11–27

11.3 Clock synchronous serial I/O mode

11.3.5 Receive operation

In the case of selecting an internal clock, when the receive conditions described in section “11.3.4 Method

of reception” are satisfied, a transfer clock is generated and the reception is started after 1 cycle of the

transfer clock has passed.

In the case of selecting an external clock, when the receive conditions are satisfied, the UARTi enters the

receive enable state and reception is started by input of an external clock to the CLKi pin.

____

In the case of selecting an external clock and the RTS function, when the UARTi enters the receive enable

____

state, the RTSi pin’s output level becomes “L” to inform the transmitter side that reception is enabled. When

____

reception is started, the RTSi pin’s output level becomes “H.” Accordingly, by connecting the RTSi pin to

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the CTSi pin of the transmitter side, the timing of transmission and that of reception can be matched. When

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an internal clock is selected, do not use the RTS function. It is because the RTS output becomes undefined.

Figure 11.3.10 shows a connection example.

The receive operations are described below:

The input signal of the RxDi pin is taken into the most significant bit of the UARTi receive register

synchronously with the rising edge of the transfer clock.

The contents of the UARTi receive register are shifted by 1 bit to the right.

Steps and are repeated at each rising edge of the transfer clock.

When 1-byte data is prepared in the UARTi receive register, the contents of this register are transferred

to the UARTi receive buffer register.

Simultaneously with step , the receive complete flag is set to “1,” and a UARTi receive interrupt request

occurs and its interrupt request bit is set to “1.”

The receive complete flag is cleared to “0” when the low-order byte of the UARTi receive buffer register

____

is read out. The RTSi pin outputs “H” level until the receive conditions are next satisfied (when selecting

____

the RTS function). Figure 11.3.11 shows the receive operation, and Figure 11.3.12 shows an example of

receive timing (when selecting an external clock).

Fig. 11.3.10 Connection example

TxDi

RxDi

CLKi

TxDi

RxDi

CLKi

Transmitter side

Receiver side

CTSi

RTSi

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