Board Power

The Basys2 board is a circuit design and implementation platform that anyone can use to gain experience building real digital circuits. Built around a Xilinx Spartan-3E Field Programmable Gate Array and a Atmel AT90USB2 USB controller, the Basys2 board provides complete, ready-to-use hardware suitable for hosting circuits ranging from basic logic devices to complex controllers. A large collection of on-board I/O devices and all required FPGA support circuits are included, so countless designs can be created without the need for any other components.

Four standard expansion connectors allow designs to grow beyond the Basys2 board using breadboards, user-designed circuit boards, or Pmods (Pmods are inexpensive analog and digital I/O modules that offer A/D & D/A conversion, motor drivers, sensor inputs, and many other features). Signals on the 6-pin connectors are protected against ESD damage and short-circuits, ensuring a long operating life in any environment. The Basys2 board works seamlessly with all versions of the Xilinx ISE tools, including the free WebPack. It ships with a USB cable that provides power and a programming interface, so no other power supplies or programming cables are required.

The Basys2 board can draw power and be programmed via its on-board USB2 port. Digilent’s freely available PC-based Adept software automatically detects the Basys2 board, provides a programming interface for the FPGA and Platform Flash ROM, and allows user data transfers (see www.digilentinc.com for more information).

Board Power

The Basys2 board is typically powered from a USB cable, but a battery connector is also provided so that external supplies can be used. To use USB power, simply attach the USB cable. To power the Basys2 using a battery or other external source, attach a 3.5V-5.5V battery pack (or other power source) to the 2-pin, 100-mil spaced battery connector (three AA cells in series make a good 4.5+/- volt supply). Voltages higher than 5.5V on either power connector may cause permanent damage.

Input power is routed through the power switch (SW8) to the four 6-pin expansion connectors and to a Linear Technology LTC3545 voltage regulator. The LTC3545 produces the main 3.3V supply for the board, and it also produces 2.5V and 1.2V supply voltages required by the FPGA. Total board current is dependent on FPGA configuration, clock frequency, and external connections. In test circuits with roughly 20K gates routed, a 50MHz clock source, and all LEDs illuminated, about 100mA of current is drawn from the 1.2V supply, 50mA from the 2.5V supply, and 50mA from the 3.3V supply. Required current will increase if larger circuits are configured in the FPGA, or if peripheral boards are attached.

The Basys2 board uses a four layer PCB, with the inner layers dedicated to VCC and GND planes. The FPGA and the other ICs on the board have large complements of ceramic bypass capacitors placed as close as possible to each VCC pin, resulting in a very clean, low-noise power supply.

Configuration

After power-on, the FPGA on the Basys2 board must be configured before it can perform any useful functions. During configuration, a “bit” file is transferred into memory cells within the FPGA to define the logical functions and circuit interconnects. The free ISE/WebPack CAD software from Xilinx can be used to create bit files from VHDL, Verilog, or schematic-based source files.

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