Digilent Ceres User Manual

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Revision: August 11, 2005 246 East Main | Pullman, WA 99163

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Overview

The Ceres board provides a ready -made
development platform for Digilent C-Mod
boards. It contains a 40-pin DIP socket to hold
C-Mods, and a collection of useful I/O devices
connected to the socket pins. The Ceres/C­Mod board combination makes an ideal
platform for students or engineers who want to
experiment with modern design technologies
and/or Xilinx CPLD devices. The Ceres board
can operate from a wall-plug power supply
(included), or up to 40 hours on two AA
batteries. Ceres features include:

• An user-settable oscillator (0.5Hz - 4KHz);

• 4-digit, high-bright seven-segment display;

• 4 debounced buttons and 8 slide switches;

• 8 LEDs in three different colors (red, green,

and yellow);

• 40-pin Dip format receptacle for use with
the C-Mod line of boards;

• 40-pin expansion connector capability.

• Attached C-Mod boards can be

programmed with JTAG3 cable.

The Ceres/C-Mod board makes an excellent
platform for introducing programmable
technologies in instructional lab settings. The
C-mod boards can host a wide range of
designs, from simple logic circuits to complex
state machines. The CPLDs use non-volatile
configuration memory, so designs can be
completed outside the lab and brought in for
evaluation. The Ceres/C-mod combination is
fully compatible with all versions of the Xilinx
CAD tools, including the free WebPack tools
available at the Xilinx website. When
purchased in combination, the Ceres/C-Mod
ships with a programming cable and a power
source, so designs can be implemented
immediately without the need for any additional
hardware.

Power/Battery

Jack

Four 7-seg.

displays

Figure 1. Ceres Block Diagram

Expansion

Connector

C-Mod Board Socket

40-pin 600-mil DIP

8 LEDs

8 switches

Functional description

The Ceres board provides a power supply and
collections of I/O devices that can be used by
C-Mod boards. Ceres boards can host different
C-Mod boards, so designs can easily be
retargeted to different CPLD devices for
experimentation. Current C-Mod boards
include the C-Mod 95 (XC9572XL), the C-Mod
C2 (XC2C64), and the C-Mod XCR
(XCR3064XL).

The Ceres board can be powered from a wall­plug power supply or from a 2AA battery pack.
A 40-pin expansion connector makes all DIP
connector signals available to external circuits.

Ceres/C-Mod JTAG Configuration

C-Mod boards contain a JTAG port for
programming, but the Ceres board does not.
The C-Mod JTAG port consists of a pattern of
six offset holes that are not normally loaded
with header pins. A row of 6 unattached
header pins can be inserted into the end of a
JTAG3 cable, and the other side of the header
can be inserted into the offset hole patterns.

Adjustable clock

(0.5Hz to 4KHz)

Debounce

4 buttons

© Digilent, Inc. 4 pages Doc: 502-048

Ceres Reference Manual Digilent, Inc.

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.

User

MCLK

Figure 2. Ceres Oscillator

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.

240 ohms

Figure 3. LED circuit

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
(1/10th 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.

Vdd

47K

4.7K

.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.

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