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
D7
D6
D5
D4
D3
D2
D1
D0
D7
D6
D5
D4
D3
D2
D1
D7
D6
D5
D4
D3
D2
D7
D6
D5
D4
D3
Transmit data
MSB
b7
b0
D0
D1
D2
D7
LSB
UARTi transmit register