Data Sheet

UM10204 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
User manual Rev. 6 — 4 April 2014 32 of 64
NXP Semiconductors
UM10204
I
2
C-bus specification and user manual
4. Other uses of the I
2
C-bus communications protocol
The I
2
C-bus is used as the communications protocol for several system architectures.
These architectures have added command sets and application-specific extensions in
addition to the base I
2
C specification. In general, simple I
2
C-bus devices such as I/O
extenders could be used in any one of these architectures since the protocol and physical
interfaces are the same.
4.1 CBUS compatibility
CBUS receivers can be connected to the Standard-mode I
2
C-bus. However, a third bus
line called DLEN must then be connected and the acknowledge bit omitted. Normally, I
2
C
transmissions are sequences of 8-bit bytes; CBUS compatible devices have different
formats.
In a mixed bus structure, I
2
C-bus devices must not respond to the CBUS message. For
this reason, a special CBUS address (0000 001X) to which no I
2
C-bus compatible device
responds has been reserved. After transmission of the CBUS address, the DLEN line can
be made active and a CBUS-format transmission sent. After the STOP condition, all
devices are again ready to accept data.
Master-transmitters can send CBUS formats after sending the CBUS address. The
transmission is ended by a STOP condition, recognized by all devices.
Remark: If the CBUS configuration is known, and expansion with CBUS compatible
devices is not foreseen, the designer is allowed to adapt the hold time to the specific
requirements of the device(s) used.
4.2 SMBus - System Management Bus
The SMBus uses I
2
C hardware and I
2
C hardware addressing, but adds second-level
software for building special systems. In particular, its specifications include an Address
Resolution Protocol that can make dynamic address allocations.
Dynamic reconfiguration of the hardware and software allow bus devices to be
‘hot-plugged’ and used immediately, without restarting the system. The devices are
recognized automatically and assigned unique addresses. This advantage results in a
plug-and-play user interface. In both those protocols, there is a very useful distinction
made between a System Host and all the other devices in the system that can have the
names and functions of masters or slaves.
SMBus is used today as a system management bus in most PCs. Developed by Intel and
others in 1995, it modified some I
2
C electrical and software characteristics for better
compatibility with the quickly decreasing power supply budget of portable equipment.
SMBus also has a ‘High Power’ version 2.0 that includes a 4 mA sink current that cannot
be driven by I
2
C chips unless the pull-up resistor is sized to I
2
C-bus levels.
4.2.1 I
2
C/SMBus compliancy
SMBus and I
2
C protocols are basically the same: A SMBus master is able to control I
2
C
devices and vice versa at the protocol level. The SMBus clock is defined from 10 kHz to
100 kHz while I
2
C can be 0Hz to 100kHz, 0Hz to 400kHz, 0Hz to 1MHz and
0 Hz to 3.4 MHz, depending on the mode. This means that an I
2
C-bus running at less
than 10 kHz is not SMBus compliant since the SMBus devices may time-out.