Seven-segment display

Each of the four digits of the seven- segment LED display is composed of seven LED segments arranged in a “figure8” pattern. Segment LEDs can be individually illuminated, so any one of 128 patterns can be displayed on a digit by illuminating certain LED segments and leaving the others dark. Of these 128 possible patterns, the ten corresponding to the decimal digits are the most useful.

The anodes of the seven LEDs forming each digit are tied together into one common anode circuit node, but the LED cathodes remain separate. The common anode signals are available as four “digit enable” input signals to the 4-digit display. The cathodes of similar segments on all four displays are connected into seven circuit nodes labeled CA through CG (so, for example, the four “D” cathodes from the four digits are grouped together into a single circuit node called “CD”). These seven cathode signals are available as inputs to the 4-digit display. This signal connection scheme creates a multiplexed display, where the cathode signals are common to all digits but they can only illuminate the segments of the digit whose corresponding anode signal is asserted.

A scanning display controller circuit can be used to show a four-digit number on this display. This circuit drives the anode signals and corresponding cathode patterns of each digit in a repeating, continuous succession, at an update rate that is faster than the human eye response. Each digit is illuminated just one-quarter of the time, but because the eye cannot perceive the darkening of a digit before it is illuminated again, the digit appears continuously illuminated. If the update or “refresh” rate is slowed to a given point (around 45 hertz), then most people will begin to see the display flicker.

For each of the four digits to appear bright and continuously illuminated, all four digits should be driven once every 1 to 16ms (for a refresh frequency of

1KHz to 60Hz). For example, in a 60Hz refresh scheme, the entire display would be refreshed once every 16ms, and

each digit would be illuminated for ¼ of the refresh cycle, or 4ms. The controller must assure that the correct cathode pattern is present when the corresponding anode signal is driven. To illustrate the process, if AN1 is asserted while CB and CC are asserted, then a “1” will be displayed in digit position 1. Then, if AN2 is asserted while CA, CB and CC are asserted, then a “7” will be displayed in digit position 2. If A1 and CB, CC are driven for 4ms, and then A2 and CA, CB, CC are driven for 4ms in an endless succession, the display will show “17” in the first two digits. Figure 8 shows an example timing diagram for a four-digit seven-segment controller.

User I/O

inputs are normally low and driven high only when the pushbutton is pressed. Slide switches generate constant high or low inputs depending on position. Pushbuttons and slide switches all have series resistors 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).

Eight LEDs and a four-digit seven- segment LED display are provided for circuit outputs. LED anodes are driven from the FPGA via current-limiting resistors, so they will illuminate when a logic ‘1’ is written to the corresponding FPGA pin. A ninth LED is provided as a power-indicator LED, and a tenth LED

(LD-D) illuminates any time the FPGA has been successfully programmed