7480 Group and 7481 Group User's Manual
1-132
HARDWARE
(3)
Operations of UART Reception
Receive Operation
When a falling edge of the RxD pin input is detected, this input level to the RxD pin is identified
according to the subsequent rising edge of the synchronous clock as follows:
As the start bit when the level is LOW.
As noise when the level is HIGH. In this case, the CPU suspends the receive operation and
enters the waiting state for the next start bit.
Synchronized with the rising edge of the synchronous clock, transmitted data is received on the
RxD pin by the bit and stored in the most significant bit (MSB) of the receive shift register. Every
time a data bit is received, the contents of the receive shift register are shifted by one bit to the
low-order direction.
The receive shift operation of ‘Receive Operation ’ is performed ‘n’ times (‘n’: the number of bits
set by the character length selection bit of the UART control register), and the received data is
stored completely in the receive shift register (Note 1).
The received data stored completely in the receive shift register is transferred to the receive buffer
register.
The parity bit and the stop bit are input to the RxD pin synchronized with rising edges of the
synchronous clock. When the last stop bit (the HIGH level) is input to the RxD pin, the receive
buffer full flag of the serial I/O status register is set to ‘1’ at the subsequent falling edge of the
synchronous clock (Note 2).
At this time, error flags are checked.
Notes 1: When the character length selection bit is ‘1’ (7 bits wide), the MSB of the receive buffer
register becomes ‘0’.
2: If the next data is stored completely in the receive shift register before the data transferred
from the receive shift register to the receive buffer register is read through (the receive
buffer full flag is ‘1’), the overrun error is generated. At this time, the overrun error flag and
the summing error flag of the serial I/O status register is set to ‘1’. Refer to (5) Notes on
Usage of UART.
When the receive buffer register is read, the receive buffer full flag is cleared to ‘0’.
Serial I/O Receive Interrupt
When the receive buffer full flag goes to ‘1’ (‘Receive Operation ’), the serial I/O receive interrupt
request bit of interrupt request register 1 is set to ‘1’; then the interrupt request is generated.
Figure 1.14.19 shows the receive operation of UART and Figure 1.14.20 shows a receive timing
example in UART.
1.14 Serial I/O