NEC PD750008 - Manual (Page 171)

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CHAPTER 5 PERIPHERAL HARDWARE FUNCTIONS
Cautions 1. In the SBI mode, the serial data bus pin SB0 (or SB1) is an open-drain output. So
the serial data bus line is placed in the wired OR state. A pull-up resistor is
required for the serial data bus line.
2. To switch between the master and slave, a pull-up resistor is required also for the
serial clock line (SCK) because SCK input/output switching is performed between
the master and slave asynchronously.
(1) SBI functions
Conventional serial I/O methods provide only data transfer functions. Therefore, many ports and wires
are required to identify chip select signals, commands, and data, and to detect busy states, when the serial
bus is configured with multiple devices. Also, these processes are too burdensome to be controlled by
software.
The SBI method can configure a serial bus with two signal lines: Serial clock SCK and serial data bus
(SB0 or SB1). For this reason, the number of ports on a microcomputer can be reduced and the wiring
on a circuit board can be simplified.
SBI functions are described below.
(a) Address/command/data identification function
Serial data is classified into three types: Address, command, and data.
(b) Address-based chip select function
The master selects a chip for a slave by address transfer.
(c) Wake-up function
A slave can easily check address reception (for chip select identification) with the wake-up function.
This function can be set or released by software.
When the wake-up function is set, an interrupt (IRQCSI) is generated when a match address is
received. For this reason, in communication with multiple devices, a CPU other than a selected slave
can operate independently of serial communication.
(d) Acknowledge signal (ACK) control function
The acknowledge signal, which is used to confirm the reception of serial data, can be controlled.
(e) Busy signal (BUSY) control function
The busy signal, which is used to post the busy state of a slave, can be controlled.