Display Driver Circuit with GAL 22V10
In the realm of display drivers, various standard implementations exist. This particular circuit employs a GAL 22V10 to efficiently drive two 7-segment displays without resorting to multiplexing techniques. At its core, a 4-bit binary code fed into inputs A/B/C/D is skillfully converted into a corresponding decimal number. One practical application of this setup is the ‘Audio Limiter (for DVD)’; however, careful attention must be paid to the voltage levels. It’s worth noting that in that specific application, multiplexers are propelled by a 4-bit binary counter, adding to the circuit’s complexity and functionality.
This circuit will give a better an indication of the behavior and settings of the limiter. The segments that have to light up with the various input bit combinations are shown on the accompanying table.
Equation Formulation and Output Configuration
This table serves as the foundational framework for crafting the equations that eventually translate into the program for the GAL. Depending on which approach results in the smallest number, either the max- or min-terms can be utilized. The outputs in this setup operate in an active low configuration, meaning they can effectively sink more current than they can source.
Segment Current Control
To ensure optimal functioning, resistors R1 through R9 are meticulously chosen to regulate the current flowing through each of the segments, stabilizing it at approximately 3mA. Notably, the displays in this context must adhere to a common-anode configuration, where the anodes of the LEDs are shared while individual cathodes control the segments. These displays are notably diminutive, boasting characters a mere 7 mm tall, with the entire display towering to 10 mm in height.
Printed Circuit Board Considerations
While the provided printed circuit board serves as a practical illustration, it’s crucial to acknowledge its exemplar status. In the ultimate application, a more favorable approach might involve housing LD1 and LD2 on a distinct PCB. This flexibility enables customization based on specific application requirements and spatial constraints.
Special Input Feature and Dash Display
A distinctive feature can be discerned from the table when all the inputs are configured as 0000. In this particular scenario, only a solitary dash is displayed, exclusively illuminating segment g of LD1. If the corresponding input is left unconnected, this dash becomes visibly apparent, signifying a low level or ‘0’. In the context of the limiter discussed earlier, this signifies that no attenuation is currently in effect.
Integrated Voltage Regulation
This circuit incorporates a dedicated 5-V regulator, the 78L05 (it’s essential to take note of the heat dissipation). Consequently, securing an appropriate power supply is not expected to pose any challenges. The power consumption ranges from a minimum of roughly 60 mA (corresponding to the ‘–’ indication) to a maximum of around 85 mA (corresponding to the ’10’ indication).
Program File and PCB Availability
The requisite jedec file for programming the GAL can be conveniently obtained as a Free Download from the Elektor Electronics website. Regrettably, the PCB showcased here is not readily available as a pre-made component.