NEC PD75402A User Manual page 105

(1) SBI functions
Since conventional serial I/O methods have only data transfer functions, when a serial bus is configured with
multiple devices connected a large number of ports and wires are required for Chip Select signal and command/
data differentiation, busy status recognition, etc. If these controls are performed by software, the load incurred by
software is very large.
When the SBI, a serial bus can be configured using only two lines: The serial clock, SCK, and the serial data bus,
SB0. As a result, the number of microcomputer ports and the amount of substrate wiring can be significantly
reduced.
SBI functions are described below.
(a) Address/command/data differentiation function
Identifies serial data as an address, command or actual data.
(b) Chip selection by address
The master performs chip selection by address transmission.
(c) Wake-up function
A slave can identify address reception (chip selection) easily by means of the wake-up function (settable/
releasable by software).
When the wake-up function is set, an interrupt (IRQCSI) is generated when a matching address is received.
As a result, non-selected CPUs can operate without regard to serial communications even when commu-
nication with multiple devices is performed.
(d) Acknowledge signal (ACK) control function
Controls the acknowledge signal used to confirm serial data reception.
(e) Busy signal (BUSY) control function
Controls the busy signal used to give notification of a slave busy status.
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CHAPTER 5. PERIPHERAL HARDWARE FUNCTIONS