5.1 Explain the term common cathode with reference to the seven-segment LED display - NSC Electrical Technology Digital - Question 5 - 2021 - Paper 1

A sinking digital output circuit involves a transistor (typically an NPN) that connects the output load to ground. When the output is activated (logic high), the transistor conducts, allowing current to flow from the load to ground, while preventing the flow of current when the output is inactive (logic low).

Here's a basic representation of the circuit:

   +V
    |\n    R (load)
    |
    |-----> To Output
    |
    Q1 (NPN transistor)
    |   |
   GND---

Current flows from the positive voltage through the load to the transistor and then to ground when the output activates.

The encoder in FIGURE 5.3 converts the decimal input data received from the switches (S1, S2, S3) into corresponding binary output signals at A0 and A1. The encoder recognizes which switch is pressed and outputs the appropriate binary value. For example, if S1 is pressed, the output will indicate the binary equivalent of '1'. This is useful for reducing multiple input signals into a compact digital representation.

When S1 is pressed, the encoder responds by outputting:

• A0 = 1
• A1 = 0
• A = 1
• B = 0
• C = 0
• D = 0
• E = 0

These outputs represent the binary equivalent of the active high input from S1.

The logic circuit can be constructed with the following components:

• Two AND gates to implement the 'SET' and 'CLK' functions.
• A NOR gate to implement the 'K' and the 'J' input logic.

Here is a basic textual representation:

        +----[AND]----> Q
 SET---|              |
        |            [NOR]---> Q'
        +----[AND]----> CLK
 K -------------------
 J -------------------

The half-adder can be constructed using an AND gate for the carry output and an Exclusive-OR gate for the sum output. The circuit can be illustrated as follows:

          A -----------
                      ┌───────[XOR]─────── Σ (Sum)
          B -----------|
                      ┌───────[AND]─────── Cₒ (Carry)

The truth table for a half-adder can be summarized as follows:

1. Frequency dividers, which are used in frequency synthesis and signal processing.
2. Binary Coded Decimal (BCD) counters, commonly used in digital clocks and calculators.

A full-sequence counter counts from 0 to its maximum value, often defined by its bit width, before returning to 0. In contrast, a truncated counter stops counting at a predetermined value that is less than its maximum. This allows the truncated counter to reset or recycle sooner than a full-sequence counter.

Positive edge triggering refers to the behavior of a circuit that reacts to the leading edge of a clock pulse. Specifically, it means that the circuit will respond when the clock signal transitions from low (0) to high (1). This is essential in synchronous systems where changes in states are synchronized with the clock signal.

The counter works by utilizing flip-flops FF0, FF1, and FF2 to count the clock pulses from LSB to MSB. Initially, FF0 increments on each clock pulse, while the output of FF0 subsequently feeds the next flip-flop, FF1. As the count progresses, the AND gate governs whether FF1 triggers FF2.

When the count reaches 510 (decimal), FF2 will disable the triggering of the flip-flops, effectively halting further counting and keeping the output stable until reset.

The four-bit shift register consists of four D flip-flops connected serially. Each flip-flop captures the input data on the rising edge of the clock. The diagram can be represented as follows:

      Serial Data In --->   D1 -->   D2 -->   D3 -->   D4 -->   Parallel Out
         |                  |           |           |           |
        CLK                CLK         CLK         CLK         CLK