Kontron COMe Starterkit Eval T2 - Manual (Page 121)

COM Express Interfaces
2.18. General Purpose I2C Bus Interface
The I2C (Inter-Integrated Circuit) bus is a two-wire serial bus originally defined by Philips. The
bus is used for low-speed (up to 400kbps) communication between system ICs. The bus is often
used to access small serial EEPROM memories and to set up IC registers. The COM Express
Specification defines several I2C interfaces that are brought to the Module connector for use on
the Carrier. Some of these interfaces are for very specific functions (VGA, LVDS, and DDIX),
one interface is the SMBus used primarily for management and one other interface is a general
purpose I2C interface. Since COM.0 Rev. 2.0 this 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 2.0 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 R2.0 Module with a R1.0 Carrier that
supports I2C devices. The R1.0 Carrier will power any I2C devices from the non-standby power
rail. A R2.0 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.
Vendor interoperability is given via EAPI – Embedded Application Programming Interface, which
allows and easier interoperability of COM Express Modules.
2.18.1. 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.
A reference to the I2C source specification can be found in Section 8 'Applicable Documents and
Standards' on page 199.
The COM Express general purpose I2C pins are on the B row of the COM Express A-B
connector as shown in Table 38 below.
Table 37: General Purpose I2C Interface Signal Descriptions
Signal Pin Description I/O Pwr Rail Comment
I2C_CK B33 General Purpose I2C Clock output I/O OD
CMOS 3.3V
Suspend
I2C_DAT B34 General Purpose I2C data I/O line. I/O OD
CMOS 3.3V
Suspend
2.18.2. Reference Schematics
The COM Express specification recommends implementing a serial I2C EEPROM of at least
2kbit on the Carrier Board where all the necessary system configuration can be saved. For more
information about the content of this system configuration EEPROM, refer to the COM Express
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
121/218