APPLICATION
2.5 Serial I/O
3825 GROUP USER’S MANUAL
2–115
sTransmit operation in the UART mode
Transmit operation in the UART mode is described below.
qStart of transmit operation
A transmit operation is started by writing transmit data into the transmit buffer register (address
001816) in the transmit enable state.V1
qTransmit operation
By writing transmit data into the transmit buffer
register, the transmit buffer empty flag (bit 0)
of the serial I/O status register (address
001916) is cleared to “0.”
The transmit data written in the transmit buffer
register is transferred to the transmit shift
register.V2
When a data transfer from the transmit buffer
register to the transmit shift register is com-
pleted, the transmit buffer empty flag is
set to “1.”V3
The transmit data transferred to the transmit
shift register is output from the P45/TxD pin
in synchronization with the falling of the shift
clock, beginning with the start bit. A start bit,
a parity bit and a stop bit are automatically
generated and output in accordance with the
contents set in the UART control register.
The data is output from the least significant
bit of the transmit shift register. Each time 1-
bit data is output, the data of the transmit
shift register is shifted by 1 bit toward the
least significant bit.
V1: Initialization of register or others for a trans-
mit operation. Refer to “2.5.4 Register set-
ting example.”
V2: When the transmit interrupt source selection bit (bit 3) of the serial I/O control register (address
001A16) is set to “0,” a serial I/O transmit interrupt request occurs immediately after transfer in .
When this bit is set to “1,” a transmit interrupt request occurs at the time of .
V3: While the transmit buffer empty flag is “1,” it is possible to write the next transmit data into the
transmit/receive buffer register.
Transmit shift register
P45/TxD
ST
D1
D2
D3
D4
D5
D6
D7
b0
D0
Serial I/O status
register
[Address 1916]
b0
0
1
Transmit buffer register
Write transmit data
Data bus
Serial I/O status
register
[Address 1916]
b0
[Address 1816]
1
0
Transmit shift register
Transfer transmit data
Transmit buffer register
Transmit shift register
P45/TxD
D0
b0
D2
D3
D4
D5
D6
D7
D1