APPLICATION
2.5 Serial I/O1
3820 GROUP USER’S MANUAL
2–106
I
Transmit operation in the clock synchronous mode
Transmit operation in the clock synchronous mode is described below.
G
Start of transmit operation
A transmit operation is started by writing transmit data into the transmit buffer register (address
0018
16
) in the transmit enable state.
8
1
G
Transmit operation
By writing transmit data into the transmit buffer
register, the transmit buffer empty flag (bit 0)
of the serial I/O1 status register (address
0019
16
) is cleared to “0.”
The transmit data written in the transmit buffer
register is transferred to the transmit shift
register.
8
2
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.”
8
3
The transmit data transferred to the transmit
shift register is output from the P4
5
/TxD pin
in synchronization with the falling of the shift
clocks.
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.
8
1: Initialization of register or others for a trans-
mit operation. Refer to
“2.5.4 Register set-
ting example.”
8
2: When the transmit interrupt source selec-
tion bit (bit 3) of the serial I/O1 control
register (address 001A
16
) is set to “0,” a
serial I/O1 transmit interrupt request occurs
immediately after transfer in
. When this
bit is set to “1,” a transmit interrupt re-
quest occurs at the time of
.
8
3: While the transmit buffer empty flag is “1,” it is possible to write the next transmit data into the
transmit/receive buffer register.
Serial I/O1 status
register
[Address 19
16
]
1
b0
0
Transmit buffer register
Serial I/O1 status
register
[Address 19
16
]
Write transmit data
Data bus
0
b0
1
[Address 18
16
]
Transmit shift register
Transfer transmit data
Transmit buffer register
Transmit shift register
P4
5
/TxD
D
0
D
1
D
2
D
3
D
4
D
5
D
6
D
7
b0
Transmit shift register
P4
5
/TxD
D
1
b0
D
2
D
3
D
4
D
5
D
6
D
7