在USB設(shè)計(jì)中，時(shí)鐘頻率是產(chǎn)生能量的主要來(lái)源。USB差分DP/DM對(duì)以480Mbps數(shù)據(jù)率工作于高速模式，系統(tǒng)時(shí)鐘可工作于12MHz、48MHz和60MHz。由于USB電纜擔(dān)當(dāng)了單極天線的角色，必須防止RF電流耦合到電纜上。
設(shè)計(jì)USB PCB板時(shí)，最關(guān)注的信號(hào)有：
·器件接口信號(hào)：在PCB上的器件之間運(yùn)行的時(shí)鐘和其他信號(hào)/數(shù)據(jù)線。
·進(jìn)出電纜的電力：USB連接器插座的管腳1(VBUS)可能被嚴(yán)重濾波，需要僅僅負(fù)載低于100KHz的低頻信號(hào)。USB插座的管腳4（analog ground）在數(shù)據(jù)傳輸期間必須能夠負(fù)荷返回電流，且必須保證基本濾波。
·經(jīng)電纜、DP和DM流出的差雙鉸對(duì)信號(hào)：依據(jù)數(shù)據(jù)傳輸率的不同，這些器件端點(diǎn)上的信號(hào)帶有240MHz (高速)、6MHz (全速)、750kHz (低速)的基頻成份。
·外部晶體電路（器件端點(diǎn)XI and X0）：12MHz、19.2MHz、24MHz、48MHz基頻。如果使用外接晶體作為參考時(shí)鐘，強(qiáng)烈推薦采用24MHz和更高基頻的晶體。
進(jìn)一步信息，請(qǐng)閱讀德州儀器（TI）公司用戶指南：USB 2.0 Board Design and Layout Guidelines
1 Background
Contents
1 Background.......................................................................................... 1
2 USB PHY Layout Guide ........................................................................... 2
3 Electrostatic Discharge (ESD) .................................................................... 8
4 References......................................................................................... 10
List of Figures
1 Suggested Array Capacitors and a Ferrite Bead to Minimize EMI........................... 2
2 Four-Layer Board Stack-Up ....................................................................... 3
3 USB Connector ..................................................................................... 4
4 3W Spacing Rule ................................................................................... 4
5 Power Supply and Clock Connection to the USB PHY........................................ 5
6 USB PHY Connector and Cable Connector..................................................... 6
7 Do Not Cross Plane Boundaries.................................................................. 7
8 Do Not Overlap Planes ............................................................................ 7
9 Do Not Violate Image Planes ..................................................................... 8
下載英文文件，請(qǐng)?jiān)L問(wèn)http://focus.ti.com.cn/cn/lit/an/spraar7/spraar7.pdf。
