IEI Integration ICE-DB-9S - Manual (Page 49)

ICE Module
Page 36
high-speed periodic signals
Spacing between differential pairs and
low-speed non periodic signals
Min. 20mils
Length matching between differential pairs
(intra-pair)
Max. 5mils
Length matching between RX and TX pairs
(inter-pair)
No strict electrical requirements. Keep
difference within a 3.0 inch delta to minimize
latency.
Length matching between reference clock
differential pairs REFCLK+ and REFCLK-
(intra-pair)
Max. 5mils
Length matching between reference clock
pairs (inter-pair)
No electrical requirements.
Reference plain GND referenced preferred
Spacing from edge of plane Min. 40mils
Via Usage
Max. 2 vias per TX trace Max. 4 vias per RX
trace
AC coupling capacitors
The AC coupling capacitors for the TX lines
are incorporated on the COM Express
module. The AC coupling capacitors for RX
signal lines have to be implemented on the
customer COM Express" carrier board.
Capacitor type: X7R
4.1.6.2 AC Coupling Capacitors
TX AC coupling capacitor is already embedded in the ICE modules. Users only need
to add the RX AC coupling capacitor on the baseboard. The PCI Express specification
requires that each lane of a PCI Express link be AC coupled between the driver and
receiver. The specification allows for the AC coupling capacitors to be located either
on or off the die. However, it is anticipated that in most cases the AC coupling will be
separated from the die and in the form of discrete capacitors on the motherboard itself.
While the 0603 size capacitors are acceptable, size 0402 capacitors are strongly
encouraged. — The smaller package size reduces the series inductance. — The
smaller package size reduces the overall board area needed to place the capacitors.