9 MSI Logic Circuits

Some of digital system operations: Decoding and encoding; multiplexing; demultiplexing; comparison; code converting; data busing.

We will study some ICs in MSI(medium-scale-integration) category which can provide such operations.

9-1 Decoders

A decoder accepts a set of inputs that represents a binary number and activates only the output that corresponds to that input number.

FIGURE 9-2 3-line-to-8-line (or 1-of-8) decoder.

FIGURE 9-1 General decoder diagram.

9-1 Decoders cont.

Some decoders have one or more ENABLE inputs used to control the operation of the decoder.

9-1 Decoders cont.

9-1 Decoders cont.FIGURE 9-6 Example 9-3: counter/decoder combination used to provide timing and sequencing operations.

9-2 BCD-TO-7-Segment Decoder/Drivers

FIGURE 9-7 (a) 7-segment arrangement; (b) active segments for each digit.

FIGURE 9-8 (a) BCD-to-7-segment decoder/driver driving a common-anode 7-segment LED display; (b) segment patterns for all possible input codes.

9-3 Liquid-Crystal Displays FIGURE 9-9 Liquid-crystal display: (a) basic arrangement; (b) applying a voltage between the segment and the backplane turns ON the segment. Zero voltage turns the segment OFF.

FIGURE 9-10 (a) Method for driving an LCD segment; (b) driving a 7-segment display.

9-4 EncoderFIGURE 9-12 General encoder diagram.

FIGURE 9-13 Logic circuit for an octal-to-binary (8-line-to-3-line) encoder. For proper operation, only one input should be active at one time.

Question: Determine the outputs of the encoder when A3 and A5 are simultaneously high.

9-4 Encoder cont.A priority encoder includes the necessary logic to ensure that when two or more inputs are activated, the output code will correspond to the highest-numbered input.

FIGURE 9-14 74147 decimal-to-BCD priority encoder.

9-4 Encoder cont.—Switch EncoderFIGURE 9-15 Decimal-to-BCD switch encoder.

9-5 TroubleshootingObservation/analysis is used to narrow the location of the fault to a small area of the circuits.

Divide-and-conquer is used to identify the location of the problem after observation/analysis has generated a number of possibilities.

Example 9-7

A technician tests the circuit of Figure 9-4 by using a set of switches to apply the input code at A4 through A0. She observes that all of the odd-numbered outputs respond correctly, but all of the even-numbered outputs fail to respond when their code is applied.

FIGURE 9-4 Four 74AS138s forming a 1-of-32 decoder.

9-5 Troubleshooting cont.

FIGURE 9-17 Troubleshooting circuitry in Example 9-7.

Solution: The most probable faults would be those that prevent A0 from going LOW. These include:

1. A fault switch connected to A0

2. A break in the path between the switch and the A0 line

3. An external short from the A0 line to VCC

4. An internal short to VCC at the A0 inputs of any one of the decoder chips

The left hand circuit is used to identify the cause.

9-6 Multiplexers (Data Selector)FIGURE 9-18 Functional diagram of a digital multiplexer (MUX).

9-6 Multiplexers cont.

FIGURE 9-20 Four-input multiplexer FIGURE 9-21 (a) Logic diagram for the 74ALS151 multiplexer; (b) truth table; (c) logic symbol. (Courtesy of Fairchild, a Schlumberger company)

9-6 Multiplexers cont.

FIGURE 9-22 Example 9-9; two 74HC151s combined to form a 16-input multiplexer.

9-7 Multiplexer Applications

FIGURE 9-24 System for displaying two multidigit BCD counters one at a time.

Data Routing:

9-7 Multiplexer Applications cont.

Parallel-to-Serial Conversion:

FIGURE 9-25 (a) Parallel-to-serial converter; (b) waveforms for X7X6X5X4X3X2X1X0 = 10110101.

9-7 Multiplexer Applications cont.Operation Sequencing:

FIGURE 9-26 Seven-step control sequencer.

9-7 Multiplexer Applications cont.

FIGURE 9-27 Multiplexer used to implement a logic function described by the truth table.

Logical Function Generation:

9-8 Demultiplexers (Data Distributors)

A DEMUX takes a single input and distributes it over several outputs.

9-8 Demultiplexers (Data Distributors) cont.

FIGURE 9-29 1-line-to-8-line demultiplexer.

9-8 Demultiplexers (Data Distributors) cont.

FIGURE 9-30 (a) The 74ALS138 decoder can function as a demultiplexer with E1 used as the data input. (b) Typical waveforms for a select code of A2 A 1 A 0 = 000 show that O0 is identical to the data input I on E1.

9-8 Demultiplexers (Data Distributors) cont.

FIGURE 9-32 Security monitoring system.

9-8 Demultiplexers (Data Distributors) cont.FIGURE 9-33 Synchronous data transmission system that is used to serially transmit four four-bit data words from a transmitter to a remote receiver.

9-8 Demultiplexers (Data Distributors) cont.FIGURE 9-34 Waveforms during one complete transmission cycle.

9-9 More Troubleshooting

Example 9-14 Suppose that the synchronous data transmission system of Figure 9-33 is malfunctioning as follows: the Z waveform is correct, but the O0 waveform is identical to the Z waveform at all times while the other outputs are constantly LOW. Assume that the receiver circuit is soldered on a PC board with no IC sockets.

Solution: Observation/analysis should be used to determine the possible causes. Divide-and-conquer should be used to isolate the problem. The most obvious cause appears to be that S and S of the DEMUX are always LOW as data are transmitted. Assuming that this is true, there are many possible causes for this symptom. Let’s list them:

1. S0 of the MUX or Q0 of the word counter could be shorted to ground, preventing the counter from incrementing.

2. The word counter could be defective(shorted or open CLK line, MR shorted to ground, or internal faults).

3. The bit counter could be defective(shorted or open CLK line or Q1, MR shorted to ground, or internal faults).

4. The INVERTER or the AND gate could be defective (shorted or open outputs or inputs, or internal faults).

5. FF Y could be defective (D, CLR, or Y shorted to ground, or internal faults).

6. The one-short could be defective .

7. The transmitted clock line may be open between the transmitter and the receiver.

9-9 More Troubleshooting cont.

FIGURE 9-35 Example 9-14: a troubleshooting tree diagram.

9-10 Magnitude ComparatorsFIGURE 9-36 Logic symbol and truth table for a 74HC85 (7485, 74LS85) four-bit magnitude comparator.

9-10 Magnitude Comparators cont.

FIGURE 9-37 (a) 74HC85 wired as a four-bit comparator; (b) two 74HC85s cascaded to perform an eight-bit comparison.

9-10 Magnitude Comparators cont.

FIGURE 9-38 Magnitude comparator used in a digital thermostat.

9-11 Code ConvertersA code converter is a logic circuit that changes data presented in one type of binary code to another type of binary code.

FIGURE 9-39 Basic idea of a two-digit BCD-to-binary converter.

9-11 Code ConvertersFIGURE 9-40 BCD-to-binary converter implemented with 74HC83 four-bit parallel adders.

9-12 Data Busing

FIGURE 9-41 Three different devices can transmit eight-bit data over an eight-line data bus to a microprocessor; only one device at a time is enabled so that bus contention is avoided.

9-13 The 74LS173/HC173 Tristate RegisterFIGURE 9-42 Truth table and logic diagram for the 74ALS173 tristate register. (Courtesy of Fairchild, a Schlumberger company)

9-13 The 74LS173/HC173 Tristate RegisterFIGURE 9-43 Logic symbol for the 74ALS173/HC173 IC.

9-14 Data Bus OperationFIGURE 9-44 Tristate registers connected to a data bus.

9-14 Data Bus Operation cont.

FIGURE 9-45 Signal activity during the transfer of the data 1011 from register A to register C.

9-14 Data Bus Operation cont.FIGURE 9-46 Simplified way to show signal activity on data bus lines.

9-14 Data Bus Operation cont.

FIGURE 9-47 A 74HC541 octal bus driver connects the outputs of an analog-to-digital converter (ADC) to an eight-line data bus. The D0 output connects directly to the bus showing the capacitive effects.

Simplified Bus Representation

9-14 Data Bus Operation cont.FIGURE 9-48 Simplified representation of bus arrangement.

9-14 Data Bus Operation cont.FIGURE 9-49 Bundle method for simplified representation of data bus connections. The “/8” denotes an eight-line data bus.

9-14 Data Bus Operation cont.FIGURE 9-50 Bidirectional register connected to data bus.

9-15 PLDs and Truth Table Entry

FIGURE 9-52 Three different ways to enter an XOR truth table in CUPL.

9-15 PLDs and Truth Table Entry cont.

FIGURE 9-53 An active-HIGH output 1-of-8 decoder using CUPL table entry format.

9-15 PLDs and Truth Table Entry cont.FIGURE 9-54 74LS138 decoder implemented on a GAL 16V8.

9-15 PLDs and Truth Table Entry cont.FIGURE 9-55 A decimal-to-BCD priority encoder.