Advantech SOM-Express Design Guide
Chapter 5 Carrier Board Design Guidelines
89
Keyboard Connector
Mini-dim 6P
KBDAT
VCC
KBCLK
GND
Mouse Connector
Mini-dim 6P
KBDAT
VCC
KBCLK
GND
Digital Ground
Chassis Ground
Fuse 1A
Fuse 1A
Ferrite
Bead
Ferrite
Bead
1A Trace Wide
1A Trace Wide
SOM-
Express
LPC_AD[0]
LPC_AD[1]
LPC_AD[2]
LPC_AD[3]
LPC_FRAME#
LPC_DRQ[0]
LPC_DRQ[1]
LPC_SERIRQ
LPC_CLK
VCC
VCC
KB_DAT
KB_CLK
MS_DAT
MS_CLK
Super
I/O
Figure 5-56 Keyboard and Mouse connection
To avoid EMI and ESD, the ground plane of the keyboard/mouse connector and
other digital ground planes, should be separated with an isolation moat (which is
recommended to be at least 40 mils) to the power planes. Digital ground and chassis
ground should be connected via screw holes to assure the integrity of the ground
plane.
5.12.4.3 LPT/Floppy
5.12.4.4 EMI
Considerations
I/Os like LPT ports/Floppy and COM ports, should be physically isolated from digital
circuitry, analog circuitry, and power and ground planes. This isolation prevents noise
sources located elsewhere on the PCB from corrupting susceptible circuits. An
example is power plane noise from digital circuits entering the power pins of analog
devices, audio components, I/O filters and interconnects, and so on.
Each and every I/O port (or section) must have a partitioned ground/power plane.
Lower frequency I/O ports may be bypassed with high-frequency capacitors located
near the connectors.
Trace routing on the PCB must be controlled to avoid recouping RF currents into the
cable shield. A clean ground must be located at the point where cables leave the
system. Both power and ground planes must be treated equally, as these planes act
as a path for RF return currents.
To implement a clean ground, use of a partition or moat is required. The clean area
should be:
1. 100% isolated with I/O signals entering and exiting via an isolation
transformer or an optical device.
2. Data line filtered; or
3. Filtered through a high-impedance common-mode inductor (choke) or
protected by a ferrite bead-on-lead component.
