NEC PD78F9488 - Manual (Page 186)

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User’s Manual U15331EJ4V1UD
CHAPTER 11 SERIAL INTERFACE 20
11.1 Serial Interface 20 Functions
Serial interface 20 has the following three modes.
• Operation stop mode
• Asynchronous serial interface (UART) mode
• 3-wire serial I/O mode
(1) Operation stop mode
This mode is used when serial transfer is not performed. Power consumption is minimized in this mode.
(2) Asynchronous serial interface (UART) mode
This mode is used to send and receive the one byte of data that follows a start bit. It supports full-duplex
communication.
Serial interface 20 contains a UART-dedicated baud rate generator, enabling communication over a wide
range of baud rates. It is also possible to define baud rates by dividing the frequency of the clock input to the
ASCK20 pin.
(3) 3-wire serial I/O mode (switchable between MSB-first and LSB-first transmission)
This mode is used to transmit 8-bit data, using three lines: a serial clock line (SCK20) and two serial data
lines (SI20 and SO20).
As it supports simultaneous transmission and reception, 3-wire serial I/O mode requires less processing time
for data transmission than asynchronous serial interface mode.
Because, in 3-wire serial I/O mode, it is possible to select whether 8-bit data transmission begins with the
MSB or LSB, serial interface 20 can be connected to any device regardless of whether that device is
designed for MSB-first or LSB-first transmission.
3-wire serial I/O mode is useful for connecting peripheral I/O circuits and display controllers having
conventional synchronous serial interfaces, such as those of the 75XL, 78K, and 17K Series devices.
11.2 Serial Interface 20 Configuration
Serial interface 20 includes the following hardware.
Table 11-1. Configuration of Serial Interface 20
Item Configuration
Registers Transmit shift register 20 (TXS20)
Receive shift register 20 (RXS20)
Receive buffer register 20 (RXB20)
Control registers Serial operation mode register 20 (CSIM20)
Asynchronous serial interface mode register 20 (ASIM20)
Asynchronous serial interface status register 20 (ASIS20)
Baud rate generator control register 20 (BRGC20)
Port mode register 2 (PM2)
Port 2 (P2)