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CHAPTER 13
SERIAL INTERFACE SIO0
User’s Manual U12790EJ2V0UD
13.4.3 SBI mode operation
SBI (Serial Bus Interface) is a high-speed serial interface that complies with the NEC Electronics serial bus format.
SBI uses a single master device and employs the clocked serial I/O format with the addition of a bus configuration
function. This function enables devices to communicate using only two lines. Thus, when configuring a serial bus
with two or more microcontrollers and peripheral ICs, the number of ports to be used and the number of wires on
the board can be decreased.
The master device outputs three kinds of data to slave devices on the serial data bus: “addresses” to select the
device to communicate with, “commands” to instruct the selected device, and “data”, which is the data actually sent.
The slave device can identify the received data as “address”, “command”, or “data” by hardware. This function
enables the application program that controls the serial interface SIO0 to be simplified.
The SBI function is incorporated into various devices including 75XL Series and 78K Series devices.
Figure 13-11 shows a serial bus configuration example when a CPU having a serial interface compliant with SBI
and peripheral ICs are used.
In SBI, the SB0 (SB1) serial data bus pin is an open-drain output pin and therefore the serial data bus line behaves
in the same way as a wired-AND configuration. In addition, a pull-up resistor must be connected to the serial data
bus line.
When using the SBI mode, refer to (d) in (11) SBI mode cautions described later.
Figure 13-11. Example of Serial Bus Configuration with SBI
Caution
When exchanging the master CPU/slave CPU, a pull-up resistor is necessary for the serial clock
line (SCK0) as well because serial clock line (SCK0) I/O switching is carried out asynchronously
between the master and slave CPUs.
Master CPU
SCK0
SB0 (SB1)
SCK0
SB0 (SB1)
SCK0
SB0 (SB1)
SCK0
SB0 (SB1)
Slave CPU
Address 1
Slave CPU
Address 2
Slave IC
Address N
Serial clock
Serial data bus
VDD