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5-24 Lucent Technologies Inc.
USS-720
Instant USB
Preliminary Data Sheet, Rev. 5
USB-to-
IEEE
1284 Bridge September 1999
13
External Circuitry Requirements
The USS-720 is intended to be a single-chip solution.
As such, the USB transceiver and the
IEEE
1284 buff-
ers have been integrated on-chip. External require-
ments include a 3.3 V supply and a 1.5 kΩ±5% pull-up
resistor for the DPLS pin. If the internal oscillator is
used, a 12 MHz crystal along with bias capacitors are
needed (see Figure 3). If the internal oscillator is not
used, a 12 MHz clock signal should be supplied to
CLK_LO, and CLK_HI should be left unconnected.
1.2 kΩ ± 5% pull-up resistors are needed on all
IEEE
1284 signals (except PLH), unless used in the High
Drive Mode (see the following page). A 5 V supply and
USB and/or
IEEE
1284 connectors might also be
needed, depending on the application.
Figure 3. External Crystal Connection
Figure 4 shows the normal USS-720 connection to the
USB. Both DPLS and DMNS require 24 Ω ± 1% series
resistors for USB impedance matching. Additionally, a
1.5 kΩ pull-up resistor is required on DPLS for full-
speed/low-speed differentiation.
Figure 4. USB Transceiver Connection
(Normal Mode)
For using the USS-720 in a self-powered device, there
are some additional considerations. The device must
refrain from supplying power through the pull-up
resistor when the device is plugged into an unpowered
bus. The USS-720 device circuit must also ensure that
the DPLS and DMNS lines are in an appropriate state
when the device is powered but not plugged in.
Figure 5 shows an example connection to meet these
requirements.
Figure 5. Self-Powered Device Example Connection
Figure 6 shows a USS-720-to-
IEEE
1284 parallel port
connection that complies with the
IEEE
1284 specifica-
tion. Other connections are also possible. While the
IEEE
specification requires these resistors, developers
must make their own design decision against the
500 µA suspend mode current requirements required
by the USB specification.
Figure 6. USS-720 Connection to
IEEE
1284
5-5259.r2
CRYSTAL
12 MHz
CLK_LO
15 pF
CLK_HI
15 pF
TO INTERNAL
USS-720
TO INTERNAL
USS-720
5-5404.r1
+3.3 V VDD
DPLS
DMNS
1.5 kΩ ± 5%
24 Ω± 1%
24 Ω± 1%
5-5506a.r4
DPLS
DMNS
24 Ω± 1%
24 Ω± 1%
+3.3 V
1.5 MΩ
SWITCH ON WHEN VBUS
IS POWERED
1.5 kΩ± 5%
1.5 MΩ
+5 V
PLH
7.5 kΩ± 5%
1.2 kΩ± 5%
ALL PARALLEL PORT
INPUT SIGNALS
EXCEPT PLH
+5 V
1.2 kΩ± 5%
ALL PARALLEL PORT
OUTPUT SIGNALS
24 Ω± 5%
5-5505a.r1