Digilent Ceres - Manual (Page 2)

Ceres Reference Manual Digilent, Inc.
www.digilentinc.com ® Digilent, Inc. Page 2
Once the C-Mod board is connected to the PC
via the JTAG cable, the configuration software
will automatically detect the CPLD. For more
information, see the “Configuration” section of
the C-Mod reference manual.
Oscillator
The Ceres board provides an user-adjustable
oscillator that can produce a clock signal in the
0.5 to 4KHz range. The oscillator circuit uses
an auto-feedback Schmidt-trigger inverter, with
a variable resistor and fixed 4.7uF capacitor in
the feedback path. The variable resistor is a
15-turn precision potentiometer that can be
adjusted from 0 to 500K ohms, providing an
RC time constant that varies by several orders
of magnitude. This clock source provides an
adequate frequency range for experiments that
run from “human time” (i.e., less than 1 Hz) to
the audio range. The oscillator output drives
the CLK0 input of the CPLD via a second
Schmidt trigger.
Power Supplies
The CERES board can be powered from any
wall-plug transformer that uses a 2.1mm
center-positive jack, and that produces at least
100mA in the 5VDC to 9VDC range. The board
can also be powered from a secondary power
connector that bypasses the onboard
regulator. This 2-pin header can accommodate
a 2AA battery pack or any other power source
that outputs at least 100mA at 2.5VDC to
4.0VDC (if a supply of more than 4VDC is
applied to the secondary connector, the C-Mod
will be permanently damaged). During
operation, the board consumes less than
80mA with all LEDs and LED segments
illuminated.
Discrete LEDs
Eight individual LEDs are provided for circuit
outputs. The LED cathodes are tied to Vdd via
240-ohm resistors, and the LED cathodes are
driven directly from the C-Mod board.
Button Inputs
The Ceres contains four normally open
pushbuttons. Button outputs are pulled to Vdd
by a 47K resistor. When the button is pressed,
the output is connected to GND via a 4.7K
resistor. This results in a logic signal that is low
(1/10
th of Vdd) only while the button is actively
pressed, and high at all other times. The
buttons are debounced with an RC filter and
Schmidt-trigger inverter as shown in the figure
below. This circuit creates a logic high signal
when the button is pressed. The debounce
circuit provides ESD protection and creates a
signal with clean edges, so the BTN# signals
can be used as clock signals if desired.
4.7K
Vdd
47K
.1uF
Figure 4. Pushbutton circuit
Slide Switches
Eight slide switches are provided for circuit
inputs. The slide switches use a 4.7Kohm
series resistor for nominal input protection.
MCLK
User
Figure 2. Ceres Oscillator
240 ohms
Figure 3. LED circuit