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 36 of 64
NXP Semiconductors
UM10204
I
2
C-bus specification and user manual
5.1 Fast-mode
Fast-mode devices can receive and transmit at up to 400 kbit/s. The minimum
requirement is that they can synchronize with a 400 kbit/s transfer; they can then prolong
the LOW period of the SCL signal to slow down the transfer. The protocol, format, logic
levels and maximum capacitive load for the SDA and SCL lines are the same as the
Standard-mode I
2
C-bus specification. Fast-mode devices are downward-compatible and
can communicate with Standard-mode devices in a 0 to 100 kbit/s I
2
C-bus system. As
Standard-mode devices, however, are not upward compatible; they should not be
incorporated in a Fast-mode I
2
C-bus system as they cannot follow the higher transfer rate
and unpredictable states would occur.
The Fast-mode I
2
C-bus specification has the following additional features compared with
the Standard-mode:
• The maximum bit rate is increased to 400 kbit/s.
• Timing of the serial data (SDA) and serial clock (SCL) signals has been adapted.
There is no need for compatibility with other bus systems such as CBUS because
they cannot operate at the increased bit rate.
• The inputs of Fast-mode devices incorporate spike suppression and a Schmitt trigger
at the SDA and SCL inputs.
• The output buffers of Fast-mode devices incorporate slope control of the falling edges
of the SDA and SCL signals.
• If the power supply to a Fast-mode device is switched off, the SDA and SCL I/O pins
must be floating so that they do not obstruct the bus lines.
The external pull-up devices connected to the bus lines must be adapted to accommodate
the shorter maximum permissible rise time for the Fast-mode I
2
C-bus. For bus loads up to
200 pF, the pull-up device for each bus line can be a resistor; for bus loads between
200 pF and 400 pF, the pull-up device can be a current source (3 mA max.) or a switched
resistor circuit (see Section 7.2.4
).
5.2 Fast-mode Plus
Fast-mode Plus (Fm+) devices offer an increase in I
2
C-bus transfer speeds and total bus
capacitance. Fm+ devices can transfer information at bit rates of up to 1 Mbit/s, yet they
remain fully downward compatible with Fast- or Standard-mode devices for bidirectional
communication in a mixed-speed bus system. The same serial bus protocol and data
format is maintained as with the Fast- or Standard-mode system. Fm+ devices also offer
increased drive current over Fast- or Standard-mode devices allowing them to drive
longer and/or more heavily loaded buses so that bus buffers do not need to be used.
The drivers in Fast-mode Plus parts are strong enough to satisfy the Fast-mode Plus
timing specification with the same 400 pF load as Standard-mode parts. To be backward
compatible with Standard-mode, they are also tolerant of the 1 μs rise time of
Standard-mode parts. In applications where only Fast-mode Plus parts are present, the
high drive strength and tolerance for slow rise and fall times allow the use of larger bus
capacitance as long as set-up, minimum LOW time and minimum HIGH time for
Fast-mode Plus are all satisfied and the fall time and rise time do not exceed the 300 ns t
f
and 1 μs t
r
specifications of Standard-mode. Bus speed can be traded against load
capacitance to increase the maximum capacitance by about a factor of ten.