IEI Integration ICE-DB-9S - Manual (Page 103)

ICE Module
Page 90
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.
5.4 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. The +5V standby rail may be left unconnected on the Carrier Board if the
standby functions are not required by the application. Likewise, the +3V battery input
may be left open if the application does not require the RTC to keep time in the
absence of the main and standby sources. There may be module specific concerns
regarding storage of system setup parameters that may be affected by the absence of
the +5V standby and / or the +3V battery.
The rationale for this power-delivery scheme is:
Module pins are scarce. It is more pin-efficient to bring power in on a
higher voltage rail.
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.
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.