Seven-Segment Displays
Introduction
The 7-segment display will be used throughout the class for general output of numbers. This tutorial covers how the common anode displays in the lab work, how to identify your part, and how to wire one to the FPGA.
Common Anode Displays
On a common anode display, the anode (positive terminal) of all of the LEDs is tied to 3.3 V through a single (“common”) pin. Each segment’s cathode (negative terminal) is connected to a pin. Therefore, you will need 7 separate control signals.
All seven segments of a digit share the same anode, which should be connected to 3.3 V. Each segment has its own cathode, which can be pulled to 0 to turn the segment on.
Because the segments are common anode, you drive each segment by pulling its cathode low. A logic 0 turns a segment on and a logic 1 turns it off — the opposite of what you might assume.
Identifying Your Part
The MAN6410 is a two-digit, common anode display from the Fairchild MAN6400 series (datasheet). If you have a single-digit display instead, the common anode member of the same series is the MAN6460, and the pinout section of the datasheet covers both. The HDSP series of 7-segment displays from Broadcom are also common in the lab (datasheet). Check your part for its number, but note that the designs are mostly pin and current level compatible.
Whatever part you have, confirm it is common anode before wiring anything. Both series also include common cathode models (MAN6440, MAN6980) that are wired the opposite way and will not work with the circuit described here.
If you have a two-digit part, each digit has its own common anode pin, so a digit you don’t use simply stays unpowered.
Limiting Current
Each segment of the display works as an independent LED. Therefore, the same current-limiting concern with the LEDs applies to the display as to the on-board bank of LEDs. You can limit the current into each segment of the display the same way you did for the LEDs on board, adding a suitable resistor in series to provide roughly 5-20 mA of current. (Beware: A LED connected in parallel with your pin (from pin to ground) will not limit current!) You can find resistors and other such components in the supply cabinet or in the stockroom.
Wiring the Display
Be sure to turn power off before wiring circuits on your board.
Connect the common anode pin for your chosen digit to 3.3 V. Then connect that digit’s seven segment pins to the FPGA header pins you selected during pin assignment, with a suitable current-limiting resistor in series with each one. Consult the datasheet pinout to identify which physical pin is which segment, since the numbering is not in alphabetical order.
You have finished the last tutorial. Go back to Lab 1 and continue with Part 2: Design.