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القسم الهندسة الكهربائية
المرحلة 4
أستاذ المادة ايهاب عبد الرزاق حسين محمد
16/12/2016 18:37:31
DIGITAL COUNTER AND APPLICATIONS A digital counter is a device that generates binary numbers in a specified count sequence. The counter progresses through the specified sequence of numbers when triggered by an incoming clock waveform, and it advances from one number to the next only on a clock pulse. The counter cycles through the same sequence of numbers continuously so long as there is an incoming clock pulse. The binary number sequence generated by the digital counter can be used in logic systems to count up or down, to generate truth table input variable sequences for logic circuits, to cycle through addresses of memories in microprocessor applications, to generate waveforms of specific patterns and frequencies, and to activate other logic circuits in a complex process. Two common types of counters are decade counters and binary counters. A decade counter counts a sequence of ten numbers, ranging from 0 to 9. The counter generates four output bits whose logic levels correspond to the number in the count sequence. Figure (1) shows the output waveforms of a decade counter. Figure (1): Decade Counter Output Waveforms. A binary counter counts a sequence of binary numbers. A binary counter with four output bits counts 24 or 16 numbers in its sequence, ranging from 0 to 15. Figure (2) showsthe output waveforms of a 4bit binary counter.
Figure (2): Binary Counter Output Waveforms. EXAMPLE (1): Decade Counter. Problem: Determine the 4bit decade counter output that corresponds to the waveforms shown in Figure (1). Determine the time required for the counter to generate the entire count sequence if the input clock has a period of 1 ms. Solution: The count sequence generated by the decade counter whose waveforms are shown in Figure (1) is listed here: The time necessary to generate the decade count sequence is 10 ms if the input clock has a 1 ms period. D C B A 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 0 0 0 1 0 0 1
EXAMPLE (2): Four Bit Binary Counter. Problem: Determine the 4 bit output that corresponds to the output waveforms shown in Figure (2). Solution: The 4bit output generated by the waveforms in Figure (2) is listed here. D C B A 0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 EXAMPLE (3): Four Bit Binary Counter. Problem: Determine the period and frequency of each output waveform shown in Figure (2) if the input clock has a period of 1 ms. Solution: A output: Period = 2 ms, frequency = 500 Hz. B output: Period = 4 ms, frequency = 250 Hz. C output: Period = 8 ms, frequency = 125 Hz. D output: Period = 16 ms, frequency = 62.5 Hz.
Section SelfTest 1. What is a decade counter? 2. What is a binary counter? 3. What relationship is there between the frequency outputs from each bit of a binary counter? ANSWERS: (Not necessarily in the order of the questions). • The frequency of the least significant bit is one half the clock frequencies. The frequency of each bit is one half the frequency of the previous output. • Counts binary numbers in a sequence, either up or down. For a binary counter with n bits, there are 2n count values in the sequence. • Counts a sequence of ten values. COUNTER CLASSIFICATION Digital logic counters can be classified according to their operational characteristics. The key characteristics that must be determined through analyzing the counter circuit are the count modulus, counter stages, output bits, frequency division, asynchronous operation, synchronous operation, and trigger characteristics, as described in sections below. 1. Count Modulus The count modulus is the total number of states or values generated by the counter as it progresses through its specified sequence. The modulus, or MOD, is one of the most important characteristics to specify when classifying a counter since it identifies the number of values in the count sequence and determines the frequency division capabilities of the counter. Decade counters always have a modulus of 10 and are sometimes referred to as MOD10 counters. Binary counters of n bits have a modulus of 2n. Figures (1 and 2) show that the modulus of a counter can be determined by a waveform generated by the counter. A count modulus can also be designated by a
state transition diagram. The states of a counter are the output numbers generated by the counter. A state transition diagram shows the progression of the number generated from one clock pulse to the next. The total number of states in the diagram is the modulus of the counter. Figure (3) shows the state transition diagrams for the decade counter and binary counter whose waveforms were shown in Figures (1 & 2). Figure (3): Counter State Transition Diagrams.
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