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The M54/74HC is a high speed CMOS 10 TO 4 . CPD is defined as the value of the IC’s internal equivalent capacitance which is calculated from the. Buy IC 74HC, TTL compatible, High Speed CMOS Logic to-4 Line Priority Encoder, DIP16 TEXAS INSTRUMENTS for € through Vikiwat online store. IC’s – Integrated Circuits 74LS – 10 to 4 Priority Encoder / 74HC 74LS – 10 to 4 Priority The 74LS/74HC is priority encoders. It provide.

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The Web This site. Digital Electronics Module 1 Number Systems described a number of different binary codes that are used to perform a range of functions in digital circuits. Mathematics, graphics, data manipulation and physical control systems are among many of the functions that are carried out using binary data, and each of these uses may require binary data arranged in various forms of binary codes.

For example text may be represented by an ASCII code American standard Code for Information Interchangein which each letter, number or symbol is represented by a 7-bit binary code. Binary Encoders generally have a number of inputs that must be mutually exclusive, i. The encoder then produces a binary code on the output pins, which changes in response to 74hc417 input that has been activated.

For example, if 6 and 7 ix pressed together the BCD output will indicate 7. Depending on the encoding purpose, each each different IC has its own particular method for solving encoding problems.

For example, a simple decimal to BCD or to-4 line encoder would be expected to 74c147 ten input pins, but in fact the 74HC has only 9. The tenth condition zero is assumed to be present because when none of the 1 to 9 input pins is active, this must indicate zero. The input pins may be used to connect to switches on a decimal keypad, and the encoder would output a 4-bit BCD code, 2 to 2 depending on which key has been pressed, or simply to identify which one of ten input lines in a circuit is active, by outputting an appropriate number in four bit BCD code.

Chip Enable Inputs Some other encoder ICs also feature extra inputs and outputs that allow several ICs to be connected together to achieve more flexibility in the numbers of input and output lines available. These include ENABLE inputs, typically labelled Ewhich may consist uc one or more input pins that need to have a particular logic level applied usually logic 0 in order to activate the encoding action. One problem with combinational logic circuits is that 74hc1477 changes in output data can occur during the times when the outputs of the IC are changing.

For example two logic signals that change simultaneously at two circuit inputs may take different routes through the circuit before being applied to some common gate later in the circuit.

However, if one 7hc147 passes through six gates for example, while the other signal passes through seven gates, each of the signals will have encountered a different total propagation delay due to the different number of gates they encountered.

Therefore they will each arrive at the common gate at slightly different times, and so for a very short time an unexpected logic level may occur at that gate output. In using combinational logic ICs such as an encoder, problems like switch bounce and race hazards must be allowed for, and one though 774hc147 necessarily the best solution can be to temporarily uc the ENABLE pin high during times when data is likely to change.

This disables the encoder for a short time until the signal data has settled at its new state, so that there is no chance of errors kc the output during changes of input signals. The 74HC also uses priority encoding and features eight active low inputs and a three-bit active low binary Octal output. The internal logic of the 74HC is shown in Fig. The IC is enabled by an active low Enable Input EIand an active low Enable output EO is provided so that several ICs can be connected in cascade, allowing the encoding of more inputs, for example a toline encoder using two 8-to-3 encoders.

The operation of the 74HC can be seen from its truth table shown in Table 4. Notice from Table 4. As the output 16 to FFFF 16 will now ci 4 bits. The GS Group Select pin, which changes to its low 74hc1447 state when any input on the most significant IC is active, is used to create the fourth output bit, 2 3 for any output value above 7.

In this simulation, available from Module 4. Where encoders are needed 74gc147 non-standard applications, they can also be implemented using a diode matrix, such as the decimal-to-BCD encoder shown in Fig 4. Any diode that has its anode connected to that horizontal line and its cathode connected to a vertical line that is held at zero volts by a resistor connected to Gnd will conduct.

This particular diode matrix will therefore give an output in BCD code from to for closure of switches 0 to 9. Many other output sequences are possible therefore, by using different arrangements of the diode positions.


Although the encoder circuits described in this module may be used in a number of useful encoding situations, they have some features that limit their use for realistic keyboard encoding. To overcome common problems such as these, a 47hc147 complex circuit or IC is required. These will typically have features such as key bounce elimination, built in data memory, timing control using a clock oscillator circuit and some ability to differentiate between two or more keys pressed at the same time.

Another important feature is the ability to signal to the system that the keyboard is 744hc147, when a key has 74bc147 pressed and new data needs to be read. For small keypads having less than 20 keys the processing has typically been carried out by an ASIC Application Specific Integrated Circuit such as the 74nc147 Keyboard Oc although this IC is now being listed as obsolete by some manufacturers, as many modern circuits, especially those with more keys, use a dedicated microprocessor or micro-controller MCU to carry out keyboard decoding.

In a complete digital system therefore it is often necessary to convert one code to another, or to convert a binary code to drive some user interface such as a LED display.

A decoder is a combinational logic circuit that takes a binary input, usually in a coded form, and produces a iv output, on each of a 7h4c147 of output lines. The logic state 1 or 0 on any of the output lines depends on a particular code appearing on the input lines.

Module 4.4

For example, a 2-toline decoder is shown in Fig. Resulting from this input, and provided that the active high Enable input is set to logic 1, the output line corresponding to the binary value at inputs A and B changes to logic 1.

The other output lines remain at logic 0. When the binary value at inputs A and B changes, the logic 1 on the output changes to a different line as appropriate.

If the enable input is set to logic 0, all the 74yc147 remain at logic 0 whatever values appear at inputs A and B. To obtain a logic 1 at any of the four outputs, the appropriate 74nc147 input AND gate must have all of its inputs at logic 1.

Provided that the Enable input is at logic 1, the output is controlled by using NOT gates to invert the logic applied from inputs A and B as required.

Encoders and Decoders

For example if inputs A and B are both at logic 0, the NOT gates at the inputs to the top 00 AND gate, invert both 0 inputs to logic 1, and therefore logic 1 appears at the 00 output.

The 01 and 10 AND gates each have one input directly connected to the A or B input, whilst the other input is inverted. The 11 gate has both A and B inputs directly connected to the AND gate so that applied to A and B results in logic 1 at the 11 output. Notice the similarity between Fig 4.

The circuit operation of Fig. One difference, commonly used from the basic example shown in Fig. This provides a greater drive capability than would be available if logic 1 was at its high voltage, and sourcing current.

Also, decoder ICs are very often used to activate the Enable or Chip Select CS inputs of other ICs, which are usually active low, so having a decoder with an active low output saves using extra inverter gates. Another feature found in 74 series ICs is the common presence of buffer gates which may be inverting or non-inverting at the IC inputs and outputs to give improved input and output capabilities Clamp diodes and current limiting resistors are also often incuded at the inputs and outputs to give improved protection from high electrostatic external voltages.

The input is in 4-bit BCD format, and each of the ten outputs, labelled Y0 to Y9 produce a logic 0 for an appropriate BCD input of to Any input value greater than results in all of the output pins remaining at their high level, as shown in pale blue in Table 4.

Note that the truth table Table 4.

On most data sheets for ICs the levels are shown as H the higher voltage and L 7h4c147 lower voltage to avoid confusion in cases where negative logic is used.

BCD to decimal decoders were originally used for driving cold cathode numerical displays Nixie tubeswhich are neon filled glass plug-in tubes 74ch147 ten anodes in the shape of numbers 0 to 9 that glow when activated by a high voltage.

However, decimal decoders are also useful for a variety of other uses. Remember that decoders are often also called demultiplexers, as they can be used for many demultiplexing tasks and for driving devices such as lamps, motors and relays in control systems.

Because cold cathode displays require a high voltage drive, they have mostly been replaced by low voltage LED or LCD displays using 7 segment displays, therefore the BCD-tosegment decoder has become one of the most commonly available decoders.

As shown in block diagram format in Fig. The eighth LED 7hc147 dp or sometimes h will normally be controlled by some extra logic outside the decoder. When illuminated by the correct logic levels, the seven-segment display will show all the decimal numbers from 0 to 9. Depending on the logic design of the IC, some decoders will automatically blank the display for any value greater than 9, while others display a unique non-numeric pattern 74hd147 each value from 10 to 15 as shown in Fig.


For displaying Hexadecimal numbers, the letters A b C d E and F are used to avoid confusion between capital B and 8, and capital D and 0. This IC uses the font illustrated in Fig. The blanking input pin BI can be used to turn off the display to reduce iv consumption, or it can be driven with a variable width pulse waveform to rapidly switch the display on 74hc17 off thereby varying the apparent brightness of the display. As a BCD to 7 Segment decoder is designed to 74gc147 a single 7 segment display, each digit of a numeric display is driven by a separate decoder, so where multiple digits are required, a technique called Ripple Blanking is used, this allows the blanking inputs of several ICs to be connected in cascade.

The Ripple Blanking Output RBO of the first decoder IC controlling the most significant digit is fed to the blanking input pin of the next most significant digit decoder and so on. When Logic 0 is applied to the ripple blanking input RBI of a decoder, it blanks the display only when the BCD input to that particular decoder is A logic 0 input will therefore blank any display digit that is 0. This allows for the suppression of any leading or trailing zeros in numbers such as or 7.

Note that although the simulation works in a similar manner to a real decoder such as the 74LS48, because the BI input and RBO output on the real chip share a common pin, this creates problems for the simulator. 74bc147 the logic has been changed by 74hc14 two tri-state buffers to separate the input and output signals. The simulation illustrated in 74hhc147.

The necessary isolation was achieved by using two simple 774hc147 buffers, shown in Fig 4. The tri-state buffer a in Fig. This input, when held at logic 1 enables the buffer, so whatever logic level appears at its input also appears at its output.

When logic 0 is applied to the Ctrl input however, the buffer is disabled and its output assumes a high 74h147 state. That is, it will take up whatever logic level occurs on the line connected to its output, no matter what logic level is on its input.

It is effectively open circuit, just as though oc the enable input low had opened a switch between its input and 74yc147. Tri-state buffers are also available with an active low 74hx147 input, that are enabled by logic 0 band as inverting buffers, that invert the output when Ctrl is activated c. There are whole ranges of devices that have 3-state outputs.

Devices such as microprocessors and memory chips, intended for use in bus systems, where many inputs and outputs share a common connection e. In these smaller scale ICs, 74gc147 such as open collector logic are more suitable. Discrete 3-state logic components are more often used for connections between, rather than within ICs.

This is a one nibble memory for the 4 bit BCD input controlled by a Latch Enable LE pin, which allows the decoder to store the 4 bit input present, when LE is logic 0 so that only the stored data is displayed. It is also common on later ranges of decoders that any input values greater than BCD 9 10 are automatically blanked. Decoders may also be used in computer systems for address decoding.

This common connection means that each of the memory chips will have the same address range as all the other memory ICs, and therefore any address within the range 16 to 16 10 put out by the microprocessor will contact the same address in all 74hc1477 memory ICs.

This 74hf147 creates a problem; each memory chip should have its own range of addresses with the 8 ICs forming a continuous address sequence in blocks of 10 locations.

IC 74HC High Speed CMOS Logic to-4 Line Priority

This is where the address decoder is used. Notice that, in Fig. Therefore, provided that the three Enable inputs E1E2 and E3 of the decoder are fed with the appropriate logic levels to enable the decoder, each of the Y0 to Y7 pins of the decoder will output a logic 0 for one of the 8 possible combinations of the three bit value on the address lines A 13 to A Since this three bit value will only change when the bit value on the address bus changes by 10 16 the memory chips will be selected using their chip select CS inputs, every 8 Kbytes.

The eight memory ICs will therefore provide a sequential set of memory locations covering the whole 64K of 74nc147, addressable by the microprocessor.