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==Introduction==
If you plan to use the holes as additional retention system, we recommend to pay attention to the mechanical characteristics when evaluating the SO-DIMM mating connectors to be mounted on the carrier board.
== Power-up sequence ==
In order to prevent back powering effects, DAVE Embedded Systems' SOMs provide the signals required to handle power-up sequence properly. For instance, see the recommended sequence for the Bora SOM [[Power_(Bora)|here]].
In case the power-up sequence is not managed properly, the circuitry populating the SOM '''may be damaged'''.
== Interfaces Guidelines ==
=== Ethernet 10/100/1000 ===
====Case #1: PHY is integrated on SOM====
This section refers to the case of PHY integrated on SOM such as [[:Category:Lizard SOM|Lizard]] and [[:Category:Maya SOM|MayaMAYA]].
===== Schematics =====
* If LAN connector with integrated magnetic is used:
* * Including SoM trace length
* Preferred underneath plane over entire trace length GND.
=== LVDS ===
==== PCB ====
{| class="wikitable" border="1"
| align="center" style="background:#f0f0f0;"|'''Parameter for LVDS Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
| align="center" style="background:#f0f0f0;"|'''Typ'''
| align="center" style="background:#f0f0f0;"|'''Max'''
|-
| Differential Impedance [Ohm]||85||100||115
|-
| Common Mode Impedance [Ohm]||46.75||55||63.25
|-
| Gap than other signals (reccomended)||-||2xgap||-
|-
| Intra pair skew [mils]*||-||-||5
|-
| Inter pair skew [mils]**||-||400||-
|-
| Maximum allowed stub||-||-||0
|-
|}
* Prefer to route traces on TOP layer, referring them to a continuos GND plane.
* * Not includes SOM's length.
* ** Typical value can be relaxed depending on LVDS clock frequency
=== LCD Interface ===
| Max via allowed||align="center"|-||align="center"|-||align="center"|-
|-
|}
==Functional guidelines==
* they likely run complex operating systems that derive from desktop world (linux, Android, Windows CE etc.)
* they implement complex storage schemes (raw NAND, SSD, eMMC etc.).
One of the main difference between such systems and PCs is that the formers are - if appropriately designed - inherently resilient to sudden power fails (see for example this presentation: [http://events.linuxfoundation.org/slides/2010/linuxcon_japan/linuxcon_jp2010_jung.pdf Application of UBIFS for Embedded Linux Products]).
In any case, system designer should take into account these events and decide if and how manage them explicitly. Here are some typical techniques used to deal with this situation:
* in case the system is used by human operators, the use of clean shutdown - triggered by the user himself - should be encouraged to prevent sudden power off. Technically speaking, this can be done via GUI (soft button) or mechanical device (push buttons and alike). In the latter case, puch push button controllers such as [http://www.linear.com/product/LTC2954 Linear LTC2954] can be very useful to implement this feature
* in case no human operators interact with the system, more complex solutions might be required. This strategy is strongly dependent on hardware characteristics of SOM and must be approached on a case-by-case basis.
