Kontron COMe Starterkit Eval T2 - Manual (Page 103)

COM Express Interfaces
2.13.4. Routing Considerations
2.13.4.1. RGB Analog Signals
The RGB signal interface of the COM Express Module consists of three identical 8-bit digital-to-
analog converter (DAC) channels. One each for the red, green, and blue components of the
monitor signal. Each of these channels should have a 150Ω ±1% pull-down resistor connected
from the DAC output to the Carrier Board ground. A second 150Ω ±1% termination resistor
exists on the COM Express Module itself. An additional 75Ω termination resistor exists within the
monitor for each analog DAC output signal.
Since the DAC runs at speeds up to 350MHz, special attention should be paid to signal integrity
and EMI. There should be a PI-filter placed on each RGB signal that is used to reduce high-
frequency noise and EMI. The PI-filter consists of two 10pF capacitors with a 120Ω @ 100MHz
ferrite bead between them. It is recommended to place the PI-filters and the terminating resistors
as close as possible to the standard VGA connector.
2.13.4.2. HSYNC and VSYNC Signals
The horizontal and vertical sync signals 'VGA_HSYNC' and 'VGA_VSYNC' provided by the COM
Express Module are 3.3V tolerant outputs. Since VGA monitors may drive the monitor sync
signals with 5V tolerance, it is necessary to implement high impedance unidirectional buffers.
These buffers prevent potential electrical over-stress of the Module and avoid that VGA monitors
may attempt to drive the monitor sync signals back to the Module.
For optimal ESD protection, additional low capacitance clamp diodes should be implemented on
the monitor sync signals. They should be placed between the 5V power plane and ground and
as close as possible to the VGA connector.
2.13.4.3. DDC Interface
COM Express provides a dedicated I2C bus for the VGA interface. It corresponds to the VESA™
defined DDC interface that is used to read out the CRT monitor specific Extended Display
Identification Data (EDID™). The appropriate signals 'VGA_I2C_DAT' and 'VGA_I2C_CK' of the
COM Express Module are supposed to be 3.3V tolerant. Since most VGA monitors drive the
internal EDID™ EEPROM with a supply voltage of 5V, the DDC interface on the VGA connector
must also be sourced with 5V. This can be accomplished by placing a 100kΩ pull-up resistors
between the 5V power plane and each DDC interface line. Level shifters for the DDC interface
signals are required between the COM Express Module signal side and the signals on the
standard VGA connector on the Carrier Board.
Additional Schottky diodes must be placed between 5V and the pull-up resistors of the DDC
signals to avoid backward current leakage during Suspend operation of the Module.
2.13.4.4. ESD Protection/EMI
All VGA signals need ESD protection and EMI filters. This can be provided by using a VGA port
companion circuit or similar protective components. The Carrier Board sample VGA schematic
shown above uses a “VGA companion” protection circuit, the CM2009 from California Micro
Devices. The companion circuit implements ESD protection for the analog DAC output, DDC
and SYNC signals through the use of low-capacitance current steering diodes. Additionally, it
incorporates level shifting for the DDC signals and buffering for the SYNC signals. For more
details, refer to the 'CM2009' data sheet.
Many other protection and level shifting solutions are possible. Semtech offers a wide variety of
low capacitance ESD suppression parts suitable for high speed signals. One such Semtech part
is the RCLAMP502B.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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