7480 Group and 7481 Group User's Manual
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1.14 Serial I/O
HARDWARE
(5)
Notes on Usage of Clock Synchronous Serial I/O
Pay attention to the following notes when clock synchronous serial I/O is selected.
Selecting External Clock as Synchronous Clock
Perform the following operations while the SCLK pin input is HIGH during transmission:
Write ‘1’ to the transmit enable bit
Write transmit data to the transmit buffer register
The shift operations of the transmit shift register and the receive shift register are performed while
the synchronous clock is being input to the serial I/O circuit. Stop the synchronous clock with 8
cycles.
Keep the HIGH- and the LOW- width (TWH and TWL) of the pulses used as the external clock
source TWH, TWL [s]
≥ (8/f(XIN) [Hz]). For example, use a frequency of 500 kHz or less (50% duty
cycle) as the external clock source when f(XIN) = 8 MHz.
Set the transmit enable bit to ‘1’ as well as the receive enable bit and the SRDY output enable bit
of the serial I/O control register when the receiver outputs the SRDY signal.
Handling Recovering from Errors Generated
Handling when overrun error is generated
If the next data is stored completely word in the receive shift register before the data transferred
from the receive shift register to the receive buffer register is read through, the overrun error is
generated. At this time, the overrun error flag and the summing error flag of the serial I/O status
register are set to ‘1’. The contents of the receive shift register are not transferred to the receive
buffer register, so that the contents of the receive buffer register remain unaffected. As a result,
if the contents of the receive buffer register are read, the data of the receive shift register is not
transferred to the receive buffer register and becomes invalid.
When the overrun error occurs, clear the overrun error flag to ‘0’ by any of the following operations,
and perform receive preparation again.
Clear the serial I/O enable bit of the serial I/O control register to ‘0’. (In this case, only the
overrun error flag returns to ‘0’.)
Clear the receive enable bit of the serial I/O control register to ‘0’.
Write dummy data into the serial I/O status register.
Referring to Transmit Shift Completion Flag
The transmit shift completion flag changes from ‘1’ to ‘0’ with a delay of 0.5 to 1.5 clocks of the
synchronous clock. Therefore, pay attention to this delay when data transmission is controlled, by
referring to the transmit shift completion flag after the transmit data is written to the transmit buffer
register.