9202 datasheet(96/175 Pages) STMICROELECTRONICS

96 / 175 page

UPSD3254A, UPSD3254BV, UPSD3253B, UPSD3253BV

96/175

Transceiver

USB Physical Layer Characteristics. The

fol-

lowing section describes the uPSD325X devices

compliance to the Chapter 7 Electrical section of

the USB Specification, Revision 1.1. The section

contains all signaling, and physical layer specifica-

tions necessary to describe a low speed USB

function.

Low Speed Driver Characteristics. The

uPSD325X devices use a differential output driver

to drive the Low Speed USB data signal onto the

USB cable. The output swings between the differ-

ential high and low state are well balanced to min-

imize signal skew. The slew rate control on the

driver minimizes the radiated noise and cross talk

on the USB cable. The driver’s outputs support

three-state operation to achieve bi-directional half

duplex operation. The uPSD325X devices driver

tolerates a voltage on the signal pins of -0.5V to

3.6V with respect to local ground reference without

damage. The driver tolerates this voltage for

10.0µs while the driver is active and driving, and

tolerates this condition indefinitely when the driver

is in its high impedance state.

A low speed USB connection is made through an

unshielded, untwisted wire cable a maximum of 3

meters in length. The rise and fall time of the sig-

nals on this cable are well controlled to reduce RFI

emissions while limiting delays, signaling skews

and distortions. The uPSD325X devices driver

reaches the specified static signal levels with

smooth rise and fall times, resulting in segments

between low speed devices and the ports to which

they are connected.

Figure 44. Low Speed Driver Signal Waveforms

AI06629

Signal pins

pass output

spec levels

with minimal

reflections and

ringing

One Bit

Time

1.5 Mb/s

VSS

Driver

Signal Pins

VSE(min)

VSE(max)

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