How to Analyze USB PCB Layout with PollEx SI

Jaehoon

1. Introduction

This document shows how to analyze USB (Universal Serial Bus) PCB layout design with PollEx SI (Signal Integrity) tool.

2. Background: USB Specification Revision 2.0

• USB Physical Interface
  • The USB transfer signal (D+/D-) and power (VBus/GND) over a four-wire cable are shown in Figure 1. The signaling occurs over two wires on each point-to-point segment.

Figure 1. USB Cable

• • USB data rates
    • High-speed signaling bit rate: 480 Mbit/S <- applied for this SI analysis
    • Full-speed signaling bit rate:  12 Mbit/S
    • Low-speed signaling bit rate: 1.5 Mbit/S

 

• High-speed (480 Mb/s) Driver Characteristics
  • Figure 2 defines four test planes; TP1 and TP4 are the test points for the transceiver, and TP2 and TP3 are the test points at the connectors.

Figure 2. Measurement Planes.

• • When testing the hub circuit board at TP2 in Figure 2, measurements are made with the Transmitter/Receiver Test Fixture shown in Figure 3. The total terminating impedance of D+ is 45 ohm and that of D- is 45 ohm, too. (45 Ohm = 143//(15.8+50)).

Figure 3. Transmitter/Receive Test Fixture for TP2.

 

• • Figure 4 and Table 1 show the transmit waveform requirement for a hub measured at TP2.

Figure 4. Transmit waveform requirements (Eye Mask) for a hub measured at TP2.

 

Table 1. Eye Mask Values for Figure 4. 

3. SI Analysis: USB PCB Layout Design with PollEx

• Open USB PCB design file

• 1. PollEx PCB > File > Altair_PCB_USB_sample.pdbb (attached in the Project file)
  2. Save AS Project

• Check USB Layout (Pattern)
  1. Tool > PCB Explorer > Net > MCU_HOST_USB+/MCU_HOST_USB > Excl

Figure 5. USB D+/D- Layout Design (Exclusive View).

• 2. Option > Net 2D/3D Viewer : Tx= U1(Part: NXP-4330) D24 & D25 Pins, Rx= CN2(Part: 675031020) 2 & 3 Pins

Figure 6. USB Layout Design (Net 2D View).

• Include “IBIS model” for IC-NXP4330 and “Linear model” for 675031020

1. Properties > Parts > Double-click “IC-NXP4330” > Device Model Files > Add “NXP3330_USB.ibs” > Click “Display” to open IBIS Manager > USB20_HS

Figure 7. USB IBIS model for USB transmitter (NXP4330). 

             2. Properties > Parts > Double-click “675031020” > Device Model Files > Add “USB_Connector_linear.dmf” > Click                         “Display” to open Linear Device Modeler > USB_PD45

• 1. • Rin_g= 45 Ohm from  Figure 3

(a) Model Spec

(b) Model Data

Figure 8. Linear model for USB receiver (connector).

• Execute “SI analysis: Net Topology Analyzer” for one USB differential pair design

1. Analysis > Net Topology Analysis > Select Net for Analysis > Select “MCU_HOST_USB+” and “MCU_HOST_USB-” > Click “Analyze”

Figure 9. Net topology for USB differential pair layout

2. Analysis > Net Analysis in Net Topology Analyzer

• Active Driver Pin = U1_D25
• Pulse Period= 4.16 (nS) from the HS data rate = 480 Mbit/S (240 MHz= Pulse period 4.16 nS)”
• Device Models:  Input = USB_PD45 & Output = USB20_HS

(a) Overall setting

(b) Device Model setting

Figure 10. Setting for Topology Network Analysis.

 

                  3. Check the final simulation results: Eye diagram

• Waveform: CN2_3 & CN2_2 > Vdiff
• Eye Mask setting with reference to Figure 4 and Table 1: Height (V) = 0.6 (=0.3+0.3), Top/Bottom= 25 (=(0.625-0.375)*100), Middle width(%)= 85 (=(0.925-0.075)*100), Reference voltage = Ground

 

Figure 11. Eye diagram (green) at USB connector with Eye mask (red).

Reference

• Universal Serial Bus - Specification Revision 2.0, April 27, 2000.