User manual

Advantech SOM-Express Design Guide

44 Chapter 5 Carrier Board Design Guidelines

5.1.2.5 SOM-Express PCI interface supply voltage

The SOM-Express PCI interface is a 3.3 V signaling environment but has 5 V

tolerance for I/O signals. If a universal PCI connector is used at the carrier board, a

jumper design to select Vio for 5 V and 3.3 V is necessary. Otherwise, the suitable

Vio voltage should be designed for a 5 V or 3.3 V connector.

Table 5.6 Add-in Card Supplied Power Selection

Symbol 3.3 V Connector 5 V Connector Universal Connector

Vio 3.3 V 5 V Jumper select

Note:

1. Note the riser card supply voltage and do not plug in the wrong supply

voltage. If a universal connector is used, make sure the Vio jumper setting

is correct when plugged into the riser card.

2. Advantech’s demo carrier board provides a 5 V connector and 5 V Vio for

PCI slots. Plugging a 3.3 V riser card in the wrong direction will cause

carrier board or riser card damage.

5.1.3 Layout Guidelines

The following represents a summary of the routing guidelines for PCI devices.

Simulations assume that PCI cards follow the PCI Local Bus Specification, Revision

2.3, trace length guidelines.

5.1.3.1 PCI Bus Layout Example with IDSEL

The following guidelines apply to platforms with nominal impedances of 55 # ± 10%.

SOM-

Express

Connector

01 2

R_IDSEL

R_IDSEL R_IDSEL

W1 W2

AD BUS

L1 L2 L3 L4

W1 W2 W2 W2W1W1

AD BUS

AD BUS AD BUS

PCI AD Bus should be routed as daisy chain to PCI expansion

slots

Figure 5-3 PCI Bus Layout Example with IDSEL