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2.12 General Purpose I2C Bus Interface
The I2C port
shall
be available in addition to the SMBus. The I2C clock
shall
support 100kHz and
should
support 400kHz operation. The maximum capacitance on the Carrier Board
shall
not exceed 100pF. The
I2C interface
should
support multi-master operation. This capability will allow a Carrier to read an
optional Module EEPROM before powering up the Module.
Revision 1.0 of the specification placed the I2C interface on the non-standby power domain. With this
connection, the I2C interface can only be used when the Module is powered on. Since the I2C interface
is used to connect to an optional Carrier EEPROM and since it is desirable to allow a Module based
board controller access to the optional Carrier EEPROM before the Module is powered on, revision 3.x
of this specification changes the power domain of the I2C interface to standby-power allowing access
during power down and suspend states. There is a possible leakage issue that can arise when using a
R3.x Module with a R2.1 Carrier that supports I2C devices. The R2.1 Carrier will power any I2C devices
from the non-standby power rail. A R3.x Module will pull-up the I2C clock and data lines to the
standby-rail through a 2.2K resistor. The difference in the power domains on the Module and Carrier can
provide a leakage path from the standby power rail to the non-standby power rail.
2.12.1 Signal Definitions
The general purpose I2C Interface is powered from 3.3V suspend rail. The I2C_DAT is an open collector
line with a pull-up resistor located on the Module. The I2C_CK has a pull-up resistor located on the
Module. The Carrier should not contain pull-up resistors on the I2C_DAT and I2C_CK signals. Carrier
based devices should be powered from 3.3V suspend voltage. The use of main power line for a Carrier
I2C device will require a bus isolator to prevent leakage to other I2C devices on 3.3V power.
At this time, there is no allocation of I2C addresses between the Module and Carrier. Carrier designers
will need to consult with Module providers for address ranges that can be used on the Carrier.
