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==Introduction==
This page provides useful information and resources to system designers in order to design carrier boards hosting '''DAVE Embedded Systems''' system-on-modules (SOM).
These guidelines are provided with the goal to help designers to design compliant systems with '''DAVE Embedded Systems''' modules and they cover schematics and PCB aspects. They apply to several products that are listed on the top right corner of this page (see "Applies to" boxes).
== Basic guidelines ==
In this section basic hardware guidelines valid for all '''DAVE Embedded Systems''' SOMs are detailed.
=== Schematics ===
* Check mirroring and pinout of '''DAVE Embedded Systems''' system-on-modules (SOM) connector* Properly decouple '''DAVE Embedded Systems''' system-on-modules (SOM) power supply with large bulk capacitor and small bypass capacitor
* Use low-ESR X7R capacitor if possible
* Check for correct connection of TX and RX lines
==== PCB Tecnology ====
Use a PCB technology as advised in the following table
{| {{table class="wikitable" border="1}} "
| align="center" style="background:#f0f0f0;"|'''Parameter'''
| align="center" style="background:#f0f0f0;"|'''Min'''
* Avoid voids on planes
* Use Solid Connection for on plane vias
* Place bulk and ByPass capacitor near '''DAVE Embedded Systems''' system-on-modules (SOM) power supply pins
* Place series terminator resistor near the related transmitter
=== SOM Connectors ===
This section provides information and suggestionsregarding the SOM mating connectors.
==== SO-DIMM ====
SO-DIMM mating connectors from different vendors may have slight differences in mechanical characteristics. For example, One critical point is the position of the end of the mating area (please see the picture below) , that can be slightly shifted inwards or outwards in respect to the retention holes on the carrier board. This can lead to a misalignment with the holes on the SO-DIMM modules, making difficult or impossible to insert the retentions screws or locking supports.
[[File:So-dimm-mating.png|250px]]
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 ==
This section provides guidelines for the most used interfaces on '''DAVEEmbedded Systems'''s SOMs module. <br/>
Please refer to SOM's detailed pages for specific additional information.
=== 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:
** Connect connector shield to an adeguate GND or shield Plane
===== PCB =====
'''Refer to this table for 10/100 differential pairs routing'''<br/>{| {{table class="wikitable" border="1}}"| align="center" style="background:#f0f0f0;"|'''Parameterfor 10/100 differential pair'''
| align="center" style="background:#f0f0f0;"|'''Min'''
| align="center" style="background:#f0f0f0;"|'''Typ'''
| Gap than other signals ||2xgap||4xgap||-
|-
| Intra pair matching(mils)*||0||5025||240150
|-
| TX and RX via # mismatch*||0||0||1
|-
|}
'''Refer to this table for Gigabit differential pairs routing''' <br/>
{| class="wikitable" border="1"
| align="center" style="background:#f0f0f0;"|'''Parameter for Gigabit Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
| align="center" style="background:#f0f0f0;"|'''Typ'''
| align="center" style="background:#f0f0f0;"|'''Max'''
|-
| Differential Impedance(ohm)||-||100||-
|-
| Common Mode Impedance||-||50||-
|-
| Gap than TX and RX signals ||2xgap||2xgap||-
|-
| Gap than other signals ||2xgap||4xgap||-
|-
| Intra pair matching(mils)*||0||10||10
|-
| Max PCB trace length ||3"||5"||-
|-
| TX and RX via # mismatch*||0||0||1
|-
|}
<br/>
<nowiki>*</nowiki> Not mandatory but recommended.
* If LAN connector with integrated magnetic is used:
** do not route traces under the connettor, neither on opposite side
* Properly separate analog Supply Rails
===== PCB =====
{| {{table class="wikitable" border="1}}"
| align="center" style="background:#f0f0f0;"|'''Parameter for RGMII interface '''
| align="center" style="background:#f0f0f0;"|'''Min'''
|}
<br/>
{| {{table class="wikitable" border="1}}"
| align="center" style="background:#f0f0f0;"|'''Parameter for Gigabit Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
|-
| Max PCB trace length ||3"||5"||-
|-
| TX and RX via # mismatch*||0||0||1
|-
|}
<nowiki>*</nowiki> Not mandatory but recommended.
<br/>
* Ground and VCC planes must be as large as possible
* Use a standard RMII PHY that supports correct clock mode (see SOM specification for further details)
===== PCB =====
{| {{table class="wikitable" border="1}}"
| align="center" style="background:#f0f0f0;"|'''Parameter for RMII interface '''
| align="center" style="background:#f0f0f0;"|'''Min'''
* Keep as best as possibile the same routing for all RMII traces
<br/>
{| {{table class="wikitable" border="1}}"
| align="center" style="background:#f0f0f0;"|'''Parameter for Ethernet Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
| Gap than other signals ||2xgap||4xgap||-
|-
| Intra pair matching(mils)*||0||25||150|- | TX and RX via # mismatch*||0||0||1|-
|}
<nowiki>*</nowiki> Not mandatory but recommended.
<br/>
* Ground and VCC planes must be as large as possible
=== USB ===
==== Schematics ====
* Create schematic in accordance with '''DAVE Embedded Systems''' system-on-modules (SOM) USB specification ( see SOM detailed pages )
==== PCB ====
{| {{table class="wikitable" border="1}}"
| align="center" style="background:#f0f0f0;"|'''Parameter for USB Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
* Connector shield must be properly connected
==== PCB ====
{| {{table class="wikitable" border="1}}"
| align="center" style="background:#f0f0f0;"|'''Parameter for HDMI Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
* Use certified SATA connector
==== PCB ====
{| {{table class="wikitable" border="1}}"
| align="center" style="background:#f0f0f0;"|'''Parameter for SATA Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
* Minimized vias use
* No strong matching required between TX and RX, but keep same route for every differential pair
=== PCI Express ===
==== PCB ====
{| class="wikitable" border="1"
| align="center" style="background:#f0f0f0;"|'''Parameter for PCI Express Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
| align="center" style="background:#f0f0f0;"|'''Typ'''
| align="center" style="background:#f0f0f0;"|'''Max'''
|-
| Differential Impedance [Ohm]||-||100||-
|-
| Common Mode Impedance [Ohm]||-||60||-
|-
| Gap than other signals (reccomended)||-||2xgap||-
|-
| Intra pair matching [mils]*||-||-||10
|-
| Max Total Length [in]*||-||-||12
|-
| Maximum allowed stub||-||-||0
|-
| Max allowed vias||-||-||6
|}
* * 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 ===
==== Schematics ====
* Please refer to '''DAVE Embedded Systems''' system-on-modules (SOM) carrier board documentationfor further information
* Predispose series resistor terminator (RPACK for LCD data and single resistor for Clock and H-SYNC and V-SYNC)
* Series resistor value may vary depending by PCB and schematic
=== VIN Interface ===
==== Schematics ====
* Please refer to '''DAVE Embedded Systems''' system-on-modules (SOM) carrier board documentationfor further information
* Predispose series resistor terminator (RPACK for LCD data and single resistor for Clock and H-SYNC and V-SYNC)
* Series resistor value may vary depending PCB and schematic
=== TVOUT ===
==== Schematics ====
* Please refer to '''DAVE Embedded Systems''' system-on-modules (SOM) carrier board documentationfor further information
==== PCB ====
{| {{table class="wikitable" border="1}}"
| align="center" style="background:#f0f0f0;"|'''Parameter for SATA Differential Pairs'''
| align="center" style="background:#f0f0f0;"|'''Min'''
=== I2C Interface ===
==== Schematics ====
* Predispose properly pullup resistors on line in accordance with '''DAVE Embedded Systems''' system-on-modules (SOM)
* Do not overload I2C lines with too much devices
* Ensure that I2C devices are being properly initialized during power up
* Isolate I2C clock from noise sensitive signals
* Avoid stub
=== SD/MMC Interface ===
{| class="wikitable" border="1"
| align="center" style="background:#f0f0f0;"|''' '''
| align="center" style="background:#f0f0f0;"|''' Min '''
| align="center" style="background:#f0f0f0;"|''' Typ '''
| align="center" style="background:#f0f0f0;"|''' Max '''
|-
| Common Mode impedance SOM(ohm)||align="center"|-||align="center"|50||align="center"|60
|-
| Matching required*||align="center"|-||align="center"|-||align="center"|-
|-
| Max allowed parallel routing(mils)||align="center"|-||align="center"|-||align="center"|1000
|-
| Max trace Length**||align="center"|-||align="center"|-||align="center"|-
|-
| Max # of vias allowed||align="center"|-||align="center"|-||align="center"|-
|-
|}
<nowiki>*</nowiki>This is not mandatory, however it is suggested in case trace length exceeds 10cm
<nowiki>**</nowiki>Overall trace length - i.e. Bora + carrier board - should not exceed 10cm. If this is not possible, try to avoid parallel routing in order to reduce crosstalk, and refer them to a ground plane.
=== CAN Interface ===
{| class="wikitable" border="1"
| align="center" style="background:#f0f0f0;"|''' '''
| align="center" style="background:#f0f0f0;"|''' Min '''
| align="center" style="background:#f0f0f0;"|''' Typ '''
| align="center" style="background:#f0f0f0;"|''' Max '''
|-
| Differential Mode impedance(ohm)||align="center"|108||align="center"|120||align="center"|132
|-
| Matching required||align="center"|-||align="center"|-||align="center"|-
|-
| Min Interpair spacing||align="center"|-||align="center"|4xgap||align="center"|-
|-
| Max allowed parallel routing(mils)||align="center"|-||align="center"|-||align="center"|-
|-
| Max trace Length||align="center"|-||align="center"|-||align="center"|-
|-
| Max via allowed||align="center"|-||align="center"|-||align="center"|-
|-
|}
==Functional guidelines==
===Sudden power off management===
* 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.
