Kontron COMe Starterkit Eval T2 - Manual (Page 60)

COM Express Interfaces
A Dual-mode source Module requires level shifters on the Carrier to convert the low-swing AC
coupled differential pairs from the video source to DVI compliant current mode differential
outputs. The example schematics use a Chrontel CH7318C translator which supports data rates
up to 1.65GB/s per lane. FET based passive level translators can be used for lower data rates.
The DisplayPort AUX channel is configured as a DDC interface for HDMI. Further information on
pre-emphasis as well as output current trim capabilities of the CH7318C can be found in the
Chrontel datasheet.
See http://www.chrontel.com/index.php/ch7318c-hdmi-hdcp-dvi-transmitters for more
information.
ESD clamping diodes D125, D126 and D204 protect the Module from external ESD events and
should be placed near the DVI connector. The pin-out of the ESD clamp diodes allows for a
trace to run under the chip connector to two pins.
DVI uses I2C signaling for the DDC. Resistors 1031 and 1032 provide the necessary pull-up.
The FETs Q226 and Q227 provide the Hot Plug Detect signal
The Carrier provides 5V power to the DVI connector.
Series diodes (D207, D208, D209) should be used to prevent back feeding of power in the event
that the monitor is powered up when the Carrier is powered down.
The DDI1 Hot Plug Detect signal is buffered by two FETs Q226 and Q227 which prevent back
feeding of power from the display to the Module as well as level translation to 3.3V levels.
Note: The reference schematics assume that the Module’s DDI ports are dual-source
capable – dual source indicates that the Module can output DisplayPort or
HDMI/DVI based on the DDC_AUX_SEL signal.
Other DisplayPort Output Options: LVDS, VGA, etc.
Other display interfaces can be created from DisplayPort, but this needs an active interface
change. DisplayPort to LVDS can be created with interface chips from multiple vendors. One
example is the Chrontel CH7511.
DisplayPort to VGA interface chips are available from multiple vendors including Chrontel or
NXP.
Note: Please also follow the design guidelines from the chip vendor
2.5.1.3. Routing Considerations
For the DisplayPort interconnection between the COM Express Module and the DisplayPort
connector or the level shifter, refer to Section 6.5.6 'DisplayPort Trace Routing Guidelines' on
page 186 for details.
The Digital Video Interface (DVI) and the High Definition Multimedia Interface (HDMI) are based
on the differential signaling method TMDS. To achieve the full performance and reliability of
HDMI and DVI, the TDMS differential signals between the level shifter and the DVI connector
have to be routed in pairs with a differential impedance of 100Ω. The length of the differential
signals must be kept as close to the same as possible. The maximum length difference must not
exceed 100mils for any of the pairs relative to each other. Pair to pair spacing should be more
than 2x the trace width to reduce trace-to-trace couplings. For example, having wider gaps
between differential pair DVI traces will minimize noise coupling. It is also strongly advised that
ground not be placed adjacent to the DVI traces on the same layer. There should be a minimum
distance of 30mils between the DVI trace and any ground on the same layer.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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