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Advantech SOM-Express Design Guide
Chapter 5 Carrier Board Design Guidelines
85
5.11.2.2 Indicated
LED
Implementation
SOM-Express provides a signal (ATA_ACT#) to indicate SATA activity. In order for
this signal to work in conjunction with Parallel ATA hard drives, it is recommended
that designers implement glue logic. An example is shown in the Figure 5-52.
When low, ATA_ACT# indicates SATA device activity and should activate the Hard
Drive LED. When tri-stated, the signal will not activate the LED. The Hard Drive LED
is active low. An external pull-up to Vcc3_3 on ATA_ACT# is required if implemented.
Vcc 3_3
Vcc 3_3
Hard Drive LED
From IDE
Connector
ATA_ACT #
Figure 5-52 ATA_ACT# Circuit Example
5.11.2.3 Terminating
Unused SATA interface Ports
If one of the SATA interface is not implemented, the unused port
’
s TX and RX signals
may be left unconnected on the carrier board.
5.11.3 Layout
Guidelines
5.11.3.1
General routing and placement
!
SATA signals must be ground referenced.
!
Route all traces over continuous GND planes, with no interruptions. Avoid crossing
over anti-etched areas if at all possible. Any discontinuity or split in the ground
plane can cause signal reflections and should be avoid.
!
Minimize layer changes. Use as few vias per SATA trace as possible (via count
should include through hole connectors as an effective via). If a layer change is
necessary, ensure that trace matching for either the TX or RX pair occurs within the
same layer.
!
Do not route SATA traces under crystals, oscillators, clock synthesizers, magnetic
devices or ICs that use and/or duplicate clocks.
!
Avoid stubs whenever possible. Utilize vias and connector pads as test points
instead.
!
In SOM-Express platforms, the SATA differential trace impendence target is 100
Ω
±
20%. Use an impedance calculator to determine the trace width and spacing
required for the specific board stack-up being used, keeping in mind that the target
is a 100
Ω
±
20%. Please refer to chapter 4 to get more information.
5.11.3.2
Serial ATA Trace length
!
The length of the SATA differential pairs should be designed as short as possible.
For direct-connected topology where the SATA differential signal pair is routed
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