
Advantech SOM-Express Design Guide
42 Chapter 5 Carrier Board Design Guidelines
SOM-Express
Module
Pin C43
Pin D40
Pin C42
Pin D39
Pin C49
Pin C50
Pin D46
Pin D47
PCI Slot / Device 0 PCI Slot / Device 1 PCI Slot / Device 2 PCI Slot / Device 3
Pin A26
IDSEL
Pin A26
IDSEL
Pin A26
IDSEL
Pin A26
IDSEL
Pin A6
Pin B7
Pin A7
Pin B8
Pin A6 Pin A6Pin A6
Pin B7
Pin B7
Pin B7
Pin A7
Pin A7 Pin A7
Pin B8 Pin B8
Pin B8
AD23
AD22
AD21
AD20
INTA#
INTB#
INTC#
INTD#
Figure 5-1 Routing PCI Slot/Device CSB Interrupt
Due to different system configurations, IRQ line routing to the PCI slots should be
made to minimize the sharing of interrupts between both internal chipset functions
and PCI functions. However, the INTA# pin of the device should not necessarily be
connected to the SOM-Express INTA# signal. Please refer to 2.3.1 System Interrupt
for more details.
5.1.2.2 PCI Clock and Clock Skew
The trace length for all PCI clocks should be matched and controlled. PCI clock
routes should be separated as far from other signal traces as possible. PCI clock
signals should be routed as controlled-impedance traces, with trace impedance of 55
#. Only one PCI device or slot should be driven from the SOM-Express PCI clock
output.
The maximum allowable clock skew is 2 ns. This specification applies not only at a
single threshold point, but at all points on the clock edge that fall in the switching
range. The maximum skew is measured between any two components rather than
between connectors. To correctly evaluate clock skew, the system designer must
take into account clock distribution on the add-in card.
Table 5.4 Clock Skew Parameters
Symbol 3.3 V Signaling 5 V Signaling Units
Vtest 0.4 Vcc 1.5 V
Tskew 2 (max) 2 (max) ns
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