7470/7471/7477/7478 GROUP USER’S MANUAL
1-128
HARDWARE
1.13 Serial I/O
1.13B.3 Notes on use
(1) Notes on external clock selection
In the 7477/7478 group, either the internal clock or external clock can be selected as the synchronous
clock. When the external clock is selected as the synchronous clock, take the following points into
consideration.
2 Clock synchronous serial I/O
1 During data transmission, when setting the transmit enable bit to “1” or writing data into the
Transmit buffer register, perform a write operation while the synchronous clock is at “H.”
2 The transmission or the shift operation of the Receive shift register is continued while the
synchronous clock is input to the serial I/O circuit. When the external clock is selected, stop the
synchronous clock at the end of 8 cycles. When the internal clock is selected, the synchronous
clock stops automatically at the end of 8 cycles.
3 When the external clock is selected, set the “H” and “L” widths (TWH, TWL) of the pulse used
as the external clock source to TWH, TWL [s]
Q 8/(f(XIN) [Hz]). For example, when f(XIN) is 8
MHz, use a clock of 500 kHz or less (duty ratio 50 %).
2 UART
Set the “H” and “L” widths (T WH, TWL) of the pulse used as the external clock source to TWH,
TWL [s]
Q 2/(f(XIN) [Hz]). For example, when f(XIN) is 8 MHz, use a clock of 2 MHz or
less (duty ratio 50 %).
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(2) When the SRDY output is performed in the clock synchronous serial I/O
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When the receive side using the external clock performs an SRDY output, set the receive enable bit,
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the SRDY output enable and the transmit enable bit to “1” (transmit enable).
(3) When a serial I/O transmit interrupt or a serial I/O receive interrupt is caused
2 When using a serial I/O transmit interrupt
1 Clear the serial I/O transmit interrupt request bit (bit 6 of IR1) to “0” after one instruction or
more after setting a value in the Serial I/O control register.
2 After setting in 1, set the serial I/O transmit interrupt enable bit (bit 6 of IE1) to “1.”
2 When using a serial I/O receive interrupt
1 Clear the serial I/O receive interrupt request bit (bit 5 of IR1) to “0” after one instruction or more
after setting a value in the Serial I/O control register.
2 After setting in 1, set the serial I/O receive interrupt enable bit (bit 5 of IE1) to “1.”
(4) Transmit interrupt request in the transmit enable state
After the transmit enable bit is set to “1,” the transmit buffer empty flag and the transmit shift
completion flag are set to “1.” Accordingly, even if a transmit buffer empty state is selected or a
termination of shift operation of the Transmit shift register is selected as a transmit interrupt source,
an interrupt request is generated and the transmit interrupt request bit is set to “1.”
For this reason, when using a transmit interrupt, set the transmit enable bit to “1,” clear the transmit
interrupt request bit to “0,” and then set the transmit interrupt enable bit to “1” (enable state).