A technician receives a trouble ticket for a circuit board that says the variable frequency divider operates

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A technician receives a “trouble ticket” for a circuit board that says the variable frequency divider operates “sometimes.” Sounds like a dreaded intermittent fault problem—often the hardest problems to find! His first thought is to send it back with the note “Use only when operating correctly!” but he decides to investigate further since he feels up to a good challenge today. The schematic for the circuit block is shown in Figure 7-80. The desired divide-by factor is applied to input f[7..0] in binary. The eightbit counter counts down from this number until it reaches zero and then asynchronously loads in f[ ] again, making zero a transient state. The resulting modulus will be equal to the value on f[ ]. The output frequency signal is obtained by decoding state 00000001, making the frequency of out equal to the frequency of in divided by the binary value f[ ]. In the application, the frequency of in is 100 kHz. Change f[ ] and a new frequency will be output.


Figure 7-80

17.0 fo f1 12 13 In LDN A R 74191 17.01 8888 INPUT Vcc QA QB B  D GN MXMN DNUP RCON CLK COUNTER qo q1 02 q3

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Digital Systems Principles And Application

ISBN: 9780134220130

12th Edition

Authors: Ronald Tocci, Neal Widmer, Gregory Moss

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