Lucent Technologies USS-720 manual External Circuitry Requirements, External Crystal Connection

USS-720Instant USB	Preliminary Data Sheet, Rev. 5
USB-to-IEEE1284 Bridge	September 1999

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.

1.2kΩ ± 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.

TO INTERNAL		TO INTERNAL
	USS-720		USS-720
	CLK_LO		CLK_HI

12MHz

CRYSTAL

15 pF 15 pF

5-5259.r2

Figure 3. External Crystal Connection

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.

+3.3 V

SWITCH ON WHEN VBUS IS POWERED

1.5 kΩ ± 5%

DPLS

24 Ω ± 1%

1.5 MΩ

DMNS

24 Ω ± 1%

1.5 MΩ

5-5506a.r4

Figure 5. Self-Powered Device Example 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

1.5kΩ pull-up resistor is required on DPLS for full- speed/low-speed differentiation.

+3.3 V VDD

1.5kΩ ± 5%

DPLS

24 Ω ± 1%

DMNS 24 Ω ± 1%

5-5404.r1

Figure 4. USB Transceiver Connection (Normal Mode)

5-24

Figure 6 shows a USS-720-to-IEEE1284 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.

	+5 V
ALL PARALLEL PORT	1.2 kΩ ± 5%
OUTPUT SIGNALS

24 Ω ± 5%

+5 V

ALL PARALLEL PORT

INPUT SIGNALS1.2 kΩ ± 5%

EXCEPT PLH

PLH

7.5 kΩ ± 5%

5-5505a.r1

Figure 6. USS-720 Connection to IEEE 1284

Lucent Technologies Inc.