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. Problems can also occur due to ‘race hazards’ where different paths that digital signals take through a logic circuit may have different numbers of gates. This can be due to problems such as switch contacts ‘bouncing’ as they close, creating rapid and unpredictable changes in logic levels for a very short time, however logic IC operate at high speed and will respond to these very fast changes. One problem with combinational logic circuits is that unintended changes in output data can occur during the times when the outputs of the IC are changing. In the absence of a correct ENABLE signal the output pins of the IC will remain in their inactive state. These include ENABLE inputs, (typically labelled E), which may consist of one or more input pins that need to have a particular logic level applied (usually logic 0) in order to activate the encoding action. 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. The input pins may be used to connect to switches on a decimal keypad, and the encoder would output a 4-bit BCD code, (0000 2 to 1001 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. 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. For example, a simple decimal to BCD (or 10-to-4 line) encoder would be expected to have ten input pins, but in fact the 74HC147 has only 9. 4.4.1 74HC147 10-to-4-Line Priority Encoderĭepending on the encoding purpose, each each different IC has its own particular method for solving encoding problems. The Pinout diagram for the 74HC147 10-to-4-line priority encoder from NXP (Philips Semiconductor), is illustrated in Fig.4.4.1.įig. For example, if 6 and 7 are pressed together the BCD output will indicate 7.
DECIMAL TO BCD PRIORITY ENCODER TRUTH TABLE CODE
The encoder then produces a binary code on the output pins, which changes in response to the input that has been activated.īecause it is always possible when using input switches that more than one input may be active at a single time, most encoders of this type feature ‘priority encoding’ where, if more than one input is made active at the same time, the output will select only the most significant active input. only one of the inputs can be active at any one time. Priority Encodersīinary Encoders generally have a number of inputs that must be mutually exclusive, i.e. Notice that the word ‘code’ appears in each of these titles, and a binary code differs from normal binary because it is arranged in a particular way to suit a given purpose. Decimal numbers in a calculator may be sent to a numeric display using BCD (Binary Coded Decimal). For example text may be represented by an ASCII code (American standard Code for Information Interchange), in which each letter, number or symbol is represented by a 7-bit binary code. 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. Simulate circuit operation using software.ĭigital Electronics Module 1 (Number Systems) described a number of different binary codes that are used to perform a range of functions in digital circuits.Understand common features on Decoders/Demultiplexers.Understand the operation of Decoders/Demultiplexers.Understand the operation of Binary Encoders.
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Recognise the need for Code Converters.
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After studying this section, you should be able to:.