Microchip USB2230 Manual

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SMSC AN 14.18 APPLICATION NOTE Revision 0.96 (06-01-06)
AN 14.18
Design Guidelines for
USB22XX/USB260X
High-Speed SD
1 Introduction
This application note provides information on Printed Circuit Board layout for High-Speed Secure-Digital
(SD) media sockets with USB22XX and USB260X.
2 Overview
Successful High-Speed operation of Secure-Digital media with USB22XX and USB260X requires
special consideration for Printed Circuit Board (PCB) layout. This application note describes the
important items to consider for layout of PCB.
3 PCB Layout Guidelines
The guidelines presented are applicable to all SMSC card-reader products that support High-Speed
Secure-Digital operation. Guidelines for both two-layer and four-layer PCBs are presented here.
3.1 Power and Ground Distribution to SD Socket
Ground connection between the card-reader and the media socket is important, both for supply return
current and signal return currents. The ground should be solid, have low impedance and few
constrictions between the card-reader and the socket. The card-reader supplies the power to the socket
in most applications since the power-FET is built-in to the SMSC card-reader.
SD socket VDD supply trace from card-reader should be 20 mils wide, 0.5 oz or thicker for a length
up to 3000 mils.
SD socket ground connection: plane, 0.5 oz or thicker, between socket and card-reader.
Note: No constrictions or cuts allowed in the ground between card-reader and socket.
3.2 Signal Traces to SD Socket
The Secure-Digital interface has a total of seven signals to the card-reader. Six of these signals are
critical for high-speed operation: SD_DAT[0:3], SD_CMD and SD_CLK and require special
considerations.
Signal traces must be above a solid and continuous ground plane along the path from card-reader
to socket.
SD_CLK trace requires two grounded guard-traces, one on each side spaced, at 3 times minimum
spacing.
SD_CLK guard traces must have a via connecting to the ground plane at both ends and every 500
mils along the path.
Design Guidelines for USB22XX/USB260X High-Speed SD
Revision 0.96 (06-01-06) 2 SMSC AN 14.18
APPLICATION NOTE
SD_CLK termination resistor must be placed close, within 400 mils to the SD_CLK pin on card-
reader for two layers PCB.
SD_CLK termination resistor must be placed close, within 400 mils to socket for four layers PCB.
SD_CLK must be buffered when trace length exceeds 1000 mils. A 74AHC1G125 or equivalent
buffer can be used that has less than 2.5 ns propagation delay.
3.3 Signal Trace Length
Trace length for SD signals must be less than the maximum length specified in Ta ble 3.1 and Table 3 .2.
An external buffer is required for SD_CLK when trace lengths exceed 1000 mils (800 mils excluding
the socket itself). Table 3 .1 provides the trace length when the unbuffered SD_CLK is driven directly
by the SMSC card-reader. Ta b le 3 .2 provides the trace length when an external clock buffer is used.
the buffer must be connected with a trace shorter than 50 mils to the card-reader SD_CLK pin. Skew
control between data lines is not critical within the limits given in the tables.
Figure 4.1 and Figure 4.2 show the suggested placement of a series termination resistor. Figure 4.3
shows the series termination resistor when an external buffer is used with a long SD_CLK trace. The
details for guarding of the SD_CLK are shown in Figure 5.1.
Note: Trace length includes 200 mils for trace inside the socket.
Table 3.1 Trace length and tolerance for unbuffered SD_CLK
Signal
Maximum
Trace Length
(including
socket) [mils]
PCB Trace
Impedance 2-
layer PCB
[Ohm]
PCB Trace
Impedance 4-
layer PCB
[Ohm]
Trace Length
Tolerance
[mils]
Maximum
Trace Length
difference to
SD_CLK [mils]
SD_CLK
1000 100 - 130 > 55 N/A N/A
SD_DAT[0:3]
2500 80 - 150 > 50 +/- 750 -250 to +1500
SD_CMD
2500 80 - 150 > 50 N/A -250 to + 1500
Table 3.2 Trace length and tolerance for buffered SD_CLK
Signal
Maximum
Trace Length
(including
socket) [mils]
PCB Trace
Impedance 2-
layer PCB
[Ohm]
PCB Trace
Impedance 4-
layer PCB
[Ohm]
Trace Length
Tolerance
[mils]
Maximum
Trace Length
difference to
SD_CLK [mils]
SD_CLK
3000 80 - 150 > 55 N/A N/A
SD_DAT[0:3]
3500 80 - 150 > 50 +/- 750 -500 to +500
SD_CMD
3500 80 - 150 > 50 N/A -500 to +500

Produkt Specifikationer

Mærke: Microchip
Kategori: Ikke kategoriseret
Model: USB2230

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