Toradex Colibri - Analog VGA; VGA Signals; Reference Schematics

Colibri Carrier Board Design Guide
Toradex AG l Altsagenstrasse 5 l 6048 Horw l Switzerland l +41 41 500 48 00 l www.toradex.com l info@toradex.com
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2.6 Analog VGA
Some Colibri modules feature a dedicated VGA interface on the HDMI FFC connector. For systems
that need to be compatible with a wide range of Colibri modules, it is recommended to use a
parallel RGB to VGA DAC instead of the dedicated VGA interface.
2.6.1 VGA Signals
Colibri
FFC Pin
Colibri
Signal Name
I/O
Type
Power
Rail
Description
18
VGA_1_R
O
Analog
Analog red video (0 to 0.7V)
20
VGA_1_G
O
Analog
Analog green video (0 to 0.7V)
22
VGA_1_B
O
Analog
Analog blue video (0 to 0.7V)
24
VGA_1_HSYNC
O
CMOS
3.3V
Horizontal sync
23
VGA_1_VSYNC
O
CMOS
3.3V
Vertical sync
16
HDMI_DDC_SDA
I/O
OD
3.3V/
5V tolerant
I
2
C interface for reading the extended display identification data
(EDID) over DDC. Signal shared with the HDMI interface
15
HDMI_DDC_SCL
O
OD
3.3V/
5V tolerant
Table 10: VGA Signals
2.6.2 Reference Schematics
The horizontal and vertical sync signals need to be level-shifted on the baseboard. The DDC
signals on the FFC connector do not require a level shifter since these signals are 5V tolerant. If a
different I
2
C interface is used as DDC, the shifters are needed. In the VGA connector standard, the
carrier board needs to provide a 5V power supply for the EDID memory on the DDC. This allows
the system to read out the EDID information of an attached display even if it is not powered.
Unfortunately, some displays source the 5V internally and also provide internal pull-up resistors to
the I
2
C lines. This can cause back-feeding problems. Therefore we recommend connecting the
display and pull-up resistor 5V supply over a diode to the module supply.
It is mandatory to place on every analog RGB signal a 150Ω resistor to the ground. Place this
resistor as close to the VGA connector as possible. Before this resistor, the signal trace can be
routed with 50Ω impedance. After the resistor, the signal should be routed with a 75Ω impedance.
Depending on the layer stack-up, 75Ω traces cannot be reached since the trace width is getting too
small. In this case, a lower trace impedance (e.g., 50Ω) can be used, but the trace length should
be kept short.
All signals on the VGA D-SUB connector need to be ESD protected. TSR diodes can be used. It is
recommended to add a PI-filter to the analog RGB signals. The values for the capacitors and
inductors depend on the maximum required display resolution. The PI-filter reduces EMI problems
but also limits the maximum bandwidth of the VGA signal.