32
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
MITSUBISHI MICROCOMPUTERS
3850 Group (Spec. H/A)
SERIAL I/O
G
SERIAL I/O1
Serial I/O1 can be used as either clock synchronous or asynchro-
nous (UART) serial I/O. A dedicated timer is also provided for
baud rate generation.
(1) Clock Synchronous Serial I/O Mode
Clock synchronous serial I/O mode can be selected by setting the
serial I/O1 mode selection bit of the serial I/O1 control register (bit
6 of address 001A
16
) to
“
1
”
.
For clock synchronous serial I/O, the transmitter and the receiver
must use the same clock. If an internal clock is used, transfer is
started by a write signal to the TB/RB.
Fig. 26 Block diagram of clock synchronous serial I/O1
Fig. 27 Operation of clock synchronous serial I/O1 function
1/4
1/4
F/F
P2
6
/S
CLK
Serial I/O1 status register
Serial I/O1 control register
P2
7
/S
RDY1
P2
4
/R
X
D
P2
5
/T
X
D
X
IN
Receive buffer register
Address 0018
16
Receive shift register
Receive buffer full flag (RBF)
Receive interrupt request (RI)
Clock control circuit
Shift clock
Serial I/O1 synchronous
clock selection bit
Frequency division ratio 1/(n+1)
Baud rate generator
Address 001C
16
BRG count source selection bit
Clock control circuit
Falling-edge detector
Transmit buffer register
Data bus
Address 0018
16
Shift clock
Transmit shift completion flag (TSC)
Transmit buffer empty flag (TBE)
Address 0019
16
Transmit interrupt request (TI)
Transmit interrupt source selection bit
Data bus
Address 001A
16
Transmit shift register
D
7
D
7
D
0
D
1
D
2
D
3
D
4
D
5
D
6
D
0
D
1
D
2
D
3
D
4
D
5
D
6
RBF = 1
TSC = 1
Overrun error (OE)
detection
TBE = 0
TBE = 1
TSC = 0
Transfer shift clock
(1/2 to 1/2048 of the internal
clock, or an external clock)
Serial output TxD
Serial input RxD
Write pulse to receive/transmit
buffer register (address 0018
16
)
Notes 1:
As the transmit interrupt (TI), either when the transmit buffer has emptied (TBE=1) or after the transmit shift operation has
ended (TSC=1), by setting the transmit interrupt source selection bit (TIC) of the serial I/O1 control register.
2:
If data is written to the transmit buffer register when TSC=0, the transmit clock is generated continuously and serial data
is output continuously from the TxD pin.
3:
The receive interrupt (RI) is set when the receive buffer full flag (RBF) becomes
“
1
”
.
Receive enable signal
S
RDY1