MITSUBISHI MICROCOMPUTERS
7477/7478 GROUP
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
Asynchronous Serial I/O (UART) Mode
Clock asynchronous serial I/O mode (UART) can be selected by
clearing the serial I/O mode selection bit of the serial I/O control
register to “0”.
Eight serial data transfer formats can be selected, and the transfer
formats used by a transmitter and receiver must be identical. The
transmit and receive shift registers each have a buffer, but the two
buffers have the same address in memory. Since the shift register
cannot be written to or read from directly, transmit data is written
to the transmit buffer, and receive data is read from the receive
buffer. The transmit buffer can also hold the next data to be trans-
mitted, and the receive buffer can hold a character while the next
character is being received.
Data bus
Serial I/O control register
P1
4
Address 00E216
RXD
RE
Address 00E016
Receive buffer register
Receive buffer full flag (RBF)
Receive interrupt request (RI)
OE
ST detection
7-bit
8-bit
Receive shift register
UART control register
SP detection
1/16
PE FE
Address 00E316
Clock control circuit
Serial I/O synchronous clock
selection bit
S
CLK
Frequency dividing ratio 1/(n+1)
f(XIN
)
1/4
P1
Baud rate generator
1/4
1/16
ST/SP/PA generation
Transmit shift completion flag (TSC)
Transmit interrupt request (TI)
TE
TIC
Transmit shift register
TXD
Character
length
selection
Transmit buffer register
Transmit buffer empty flag (TBE)
Serial I/O status register
P1
5
6
Address 00E116
bit
Address 00E016
Data bus
Fig. 10 UART serial I/O block diagram
Transmit or receive clock
Transmit buffer write signal
TBE=0
TBE=0
TSC=0
TBE=1
✽
TBE=1
TSC=1
SP
Serial output TxD
D
0
D
1
D0
D
1
ST
ST
ST
SP
1 start bit
✽
Generated at 2nd bit in 2-stop-bit mode
7 or 8 data bits
1 or 0 parity bit
1 or 2 stop bit(s)
Receive buffer read signal
RBF=0
RBF=1
SP
RBF=1
SP
Serial input RxD
ST
D
0
D
0
D1
D
1
Notes 1 : Error flag detection occurs at the same time that the RBF flag becomes “1” (at 1st stop bit during reception).
2 : The transmit interrupt (TI) can be selected to occur when either the TBE or TSC flag becomes “1,” depending on the
setting of the transmit interrupt source selection bit (TIC) of the serial I/O control register.
3 : The receive interrupt (RI) is set when the RBF flag becomes “1”.
Fig. 11 Operation of UART serial I/O function
18
MITSUBISHI [ Mitsubishi Group ]
