PIC microprocessor drives 20-LED dot- or bar-graph display

The circuit in Figure 1 uses only five I/O lines to drive a dot- or bar-graph display of 20 LEDs. Although this version of the design uses a small and inexpensive one-time-programmable microprocessor, such as a Microchip PIC12C508A, you can use other microprocessors with N I/O lines to drive as many as N×(N–1) LEDs. For software development or modification, you can use a PIC12C508A-JW reprogrammable-EPROM version of the PIC12C508A, or you can substitute a less expensive PIC16F84A with flash memory.

To avoid application of excessive reverse voltage to the LEDs, the circuit's power supply, V_DD, must not exceed 3V dc. You can drive other types of loads and provide electrically isolated interfaces by replacing the LEDs with appropriately rated optocouplers. For demonstration purposes, IC₁'s input line, GP3, connects to a pushbutton display-mode-selector switch and a pulldown resistor that simulates a digital-input-signal source with a voltage amplitude of 3V p-p.

Listing 1 performs a variety of functions. To conserve battery power, the basic software drives one LED at a time in dot or bar mode with a minimum amount of current. Approximately 2 mA flashes a high-brightness LED. The software includes a delay routine that solves the problem of contact bounce. Tests show that a miniature pushbutton switch requires a delay of at least 1 msec for successful debouncing.

Consuming fewer than 256 words, the software avoids a C12C508A programming restriction that requires placement of subroutines only in page 0. Other features of the software include a two-level stack, the use of files common to both the PIC12C508A and the PIC16F84A in the 0C_Hto 1F_H range, use of the RETLW 00_H instruction instead of Return, and avoidance of the ADDLW and SUBLW instructions. The software defaults to the dot-display mode. Pressing and holding S₁ before and during power application selects the bar-display mode. Note that the configuration settings for the MPASM assembler vary according to which microprocessor you select for programming.