SERIAL I/O
7902 Group User’s Manual
12-50
12.4 Clock asynchronous serial I/O (UART) mode
12.4.6 Receive operation
When the receive enable bit is set to “1,” the UARTi enters the receive-enabled state. Then, reception will
start when ST (’s falling edge) is detected and a transfer clock is generated.
If the RTS function selected, when connecting the RTSi pin to the CTSi pin of the transmitter side, the
timing of transmission and that of reception can be matched. If the RTS function selected, the RTSi pin’s
output level becomes as described below.
When the receive enable bit = “0,” if one of the following is performed, the RTSi pin’s output level becomes
“L” and informs of the transmitter side that reception has become enabled:
The receive enable bit is set to “1.”
The low-order byte of the UARTi receive buffer register is read out.
When the receive enable bit = “1,” if the low-order byte of the UARTi receive buffer register is read out,
the RTSi pin’s output level becomes “L.”
Accordingly, when performing reception continuously, an overrun occurrence can be avoided because the
RTS output level does not become “L” until the receive data is read out.
When reception has started, the RTSi pin’s output level becomes “H.”
Figure 12.4.12 shows a connection example.
Fig. 12.4.12 Connection example
The receive operation is described below.
The signal input to the RxDi pin is taken into the most significant bit of the UARTi receive register,
synchronously with the transfer clock’s rising edge.
The contents of the UARTi receive register are shifted, bit by bit, to the right.
Steps and are repeated at each rising edge of the transfer clock.
When one set of data has been prepared, in other words, when the shift operation has been performed
several times according to the selected data format, the UARTi receive register’s contents are transferred
to the UARTi receive buffer register.
Simultaneously with step , the receive complete flag is set to “1.” Additionally, when the receive
interrupt is selected (UARTi receive interrupt mode select bit = “0”), 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
has been read out. Figure 12.4.13 shows an example of receive timing when the transfer data length = 8
bits.
TxDi
RxDi
TxDi
RxDi
Transmitter side
Receiver side
CTSi
RTSi