IEI Integration ICE-DB-T6 - Manual (Page 80)

Type 6 Carrier Board Design Guide
Page 70
strip-line layers.
ƒ For high-speed signals transitioning between layers next to the
component, the signal pins should be accounted for by the GND
stitching vias that would stitch all the GND plane layers in that area of
the board.
ƒ High-speed routing on external layers should be minimized in order to
avoid EMI. Routing on external layers also introduces different delays
compared to internal layers. This makes it extremely difficult to do
length matching if routing is done on both internal and external layers.
4.4 ATX Power Delivery Guidelines
The COM Express module uses a single main power rail with a nominal value of +12V.
Two additional rails are specified: a +5V standby power rail and a +3V battery input to
power the module Real-time Clock (RTC) circuit in the absence of other power sources. If
the standby functions are not required by the application, the +5V standby rail may be left
unconnected on the carrier board. Likewise, if the application does not require the RTC to
keep time in the absence of the main and standby sources, the +3V battery input may be
left open. There may be module-specific concerns regarding storage of system setup
parameters which may be affected by the absence of the +5V standby and/or the +3V
battery.
The rationale for this power-delivery scheme is:
ƒ Contemporary chipsets have no power requirements for +5V other than to
provide a reference voltage for +5V tolerant inputs. No COM Express module
pins are allocated to accept +5V except for the +5V standby pins. In the case
of an ATX supply, the switched (non standby) +5V line would not be used for
the COM Express module, but it might be used elsewhere on the carrier
board.
ƒ Module pins are scarce. It is more pin-efficient to bring power in on a higher
voltage rail.
ƒ Lithium ion battery packs for mobile systems are most prevalent with a
+14.4V output. This is well suited for the +12V main power rail.