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Power supply, User i/o – Digilent Basys Board Rev.C User Manual

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Digilent

Basys Reference Manual

www.digilentinc.com

Copyright Digilent, Inc.

Page 3/12

Doc: 502-107

Power Supply

The Basys board can be powered from any DC supply that produces a voltage in the 5VDC-12VDC
range. The power jack on the Basys board requires a center-positive, 2.1mm power supply connector
as is commonly found on wall-plug power supplies (Basys ships with a 5VDC wall-plug supply).
Voltages higher than 12V may permanently damage Basys.

The voltage input to the power jack is routed to the four 6-pin expansion connectors and to a LM1117
voltage regulator that produces the 3.3V supply for the board. The 2.5V and 1.2V supplies required by
the FPGA are produced by post-regulating the 3.3V supply. Total board current is dependant 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 200mA of current is drawn from the
1.2V supply, 50mA from the 2.5V supply, and 100mA from the 3.3V supply. Required current will
increase if larger circuits are configured in the FPGA, and if peripheral boards are attached.

The Basys 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 all have a large complement of bypass capacitors placed as
close as possible to each VCC pin. The power supply routing and bypass capacitors result in a very
clean, low-noise power supply.


Oscillators


The Basys board includes a primary, user-
settable silicon oscillator that produces 25MHz,
50MHz, or 100MHz based on the position of the
clock select jumper at JP3. A socket for a second
oscillator is also provided at IC7 (the IC7 socket
can accommodate any 3.3V CMOS oscillator in a
half-size DIP package).

The primary and

secondary oscillators are connected to global
clock input pins at pin 36 and pin 84 respectively.
Both clock inputs can drive the clock synthesizer
DLL on the Spartan 3E, allowing for a wide range
if internal frequencies, from 4 times the input
frequency to any integer divisor of the input
frequency.



User I/O

The Basys board includes several Input and Output devices, and several data ports so that many
designs can be implemented without the need for any other components.


Inputs: Slide Switches and Pushbuttons

Four pushbuttons and eight slide switches are provided for circuit inputs. Pushbutton inputs are
normally low, and they are driven high only when the pushbutton is pressed. Slide switches generate
constant high or low inputs depending on their position. Pushbutton and slide switch inputs use a
series resistor for protection against short circuits (a short circuit would occur if an FPGA pin assigned
to a pushbutton or slide switch was inadvertently defined as an output).