NEC mPD789101, User Manual

Page: 163 / 265

User’s Manual U13045EJ2V0UM00
163
CHAPTER 13 SERIAL INTERFACE 20
13.1 Functions of Serial Interface 20
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 channel 0 contains a dedicated UART 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 input clock
pulse at the ASCK20 pin.
It is recommended that ceramic/crystal oscillation be used for the system clock in the UART mode. Because
the frequency deviation is large in RC oscillation, if an internal clock is selected as the source clock for the
baud rate generator, there may be problems in send/receive operations.
(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 (SCK20) line 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, channel 0 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 clock synchronous serial interfaces, such as those of the 75XL, 78K, and 17K Series devices.
13.2 Serial Interface 20 Configuration
Serial interface 20 consists of the following hardware.
Table 13-1. Serial Interface 20 Configuration
Item Configuration
Register Transmit shift register 20 (TXS20)
Receive shift register 20 (RXS20)
Receive buffer register 20 (RXB20)
Control register Serial operating 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)