Lucent Technologies Exploring and Their Drivers for Enhanced Device Functionality

Application Note

February 1999

Typical Circuit Showing the USS-720

Bridging USB to Parallel Port

Description

The USS-720 can be used in a variety of applica- tions, such as bus-powered devices, self-powered devices, hubs, and embedded printer controllers. The following describes using the device in a bus- powered application (see attached schematic).

Note: The attached schematic depicts a typical func- tional circuit using the USS-720 as a bridge between the USB and a parallel port. Actual applications may require additional protection circuitry. The schematic and circuit description provided in this application note are for refer- ence purposes only. Neither Lucent nor In- System Design warrants their suitability for any particular purpose.

The USS-720 is a dual-powered chip requiring both 5 V and 3.3 V supplies. The 3.3 V is generated using a low dropout regulator. The USS-720 must operate with a USB supply (VBUS) of 4.4 V to 5.25 V. Using a low dropout regulator ensures a solid 3.3 V supply even at the lowest limits of the 5 V VBUS. The 5 V supply is used by the 1284 printer port drivers.

The USS-720 also requires a 12 MHz ± 0.25% crystal. In embedded applications, an oscillator output should be connected to clk_lo (pin 21), and pin 22 should be left unconnected.

The USS-720 requires a 1.5 kΩ pull-up resistor attached to the DPLS signal to indicate that it is a high-speed 12 Mbits/s device as per the USB specifi- cation. There is also a USB differential driver imped- ance specification of 30 Ω to 42 Ω. A 24 Ω series resistor, when added to the output impedance of the USS-720 USB drivers, puts the total output imped- ance in the middle of that range.

A similar driver output impedance requirement is true for the IEEE* 1284 printer signals. A 24 Ω series resistor is used on all the 1284 signals that are driven by the USS-720 to give each signal a total output impedance of 45 Ω to 55 Ω . The IEEE 1284 specifi- cation also requires 1.2 kΩ pull-up resistors on all the printer signals, except PLH. PLH only requires a 7.5 kΩ pull-down resistor.

While the IEEE 1284 specification requires these resistors, developers must make their own design decisions against the 500 μA suspend mode current requirements of the USB specification. The following schematic does not show the pull-down resistor on the PLH signal. The internal pull-down on the USS- 720 can be used for this purpose. Similarly, the pull- up resistor values may have to be modified to higher values than the IEEE 1284 specification allows in order to meet the USB requirements.

Standard decoupling should be used on the board. It is recommended that 0.1 μF capacitors are placed between VCC_5V/VCC and GND and that they are located as close as possible to the power pins on the USS-720. Sufficient grounding must be implemented on the board to ensure proper functionality. A four- layer board design is recommended with two of the layers used for power and ground planes.

The test and scan signals (pins 23, 24, 31, 32, and

33)are used for functional testing after fabrication and are tied low during normal operation. It is recom- mended that typical bypass techniques be used on the voltage supply pins (see ASIC BYPASS CAPS on attached schematic).

The USS-720 contains a small amount of ROM space that is used to store descriptor data. This data is used during the Plug and Play mode in a Microsoft Windows† operating system to identify the product. This onboard ROM data can be used during the ini- tial development phase, but unique descriptor data must be provided by the USB peripheral developer for each design.

*IEEE is a registered trademark of The Institute of Electrical and Electronics Engineers, Inc.

†Microsoft and Windows are registered trademarks of Microsoft Corporation.

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