18

MITSUBISHI MICROCOMPUTERS

7477/ 7478 GROUP

SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

SERIAL I/O

Serial I/O can be used as either clock synchronous or asynchro-

nous (UART) serial I/O. A dedicated timer (baud rate generator) is

also provided for baud rate generation.

Clock Synchronous Serial I/O Mode

Clock synchronous serial I/O mode can be selected by setting the

mode selection bit of the serial I/O control register 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 transmit or receive buffer.

Fig. 8 Clock synchronous serial I/O block diagram

Fig. 9 Operation of clock synchronous serial I/O function

Transfer shift clock

(1/8 to 1/8192 of the internal

clock, or an external clock)

Serial output TxD

Serial input RxD

Write signal to

receive/transmit buffer

TBE = 0

TBE = 1

TSC = 0

RBF = 1

TSC = 1

Overrun error (OE) detection

Notes 1 : The transmit interrupt request (TI) can be selected to occur 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/O control

register.

2 : If data is written to the transmit buffer when TSC = 0, the transmit clock is generated continuously and serial data is output

continuously from the TxD pin.

3 : The receive interrupt request (RI) is set when the receive buffer full flag (RBF) becomes “1”.

Data bus

Data bus

Receive buffer register

Clock control circuit

Serial I/O control register

Fall detect

Serial I/O status register

F/F

1/4

1/4

1/4

Transmit shift completion flag (TSC)

Transmit interrupt request (TI)

Transmit buffer empty flag (TBE)

Receive buffer full flag (RBF)

Receive interrupt request (RI)

Serial I/O synchronous

clock selection

bit (SCS)

Frequency dividing

ratio 1/(n+1)

Shift clock

Clock control circuit

Receive shift register

Transmit shift register

Transmit buffer register

SRDY

CSS

TE

RE

SIOE

Baud rate generator

TIC

Shift clock