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

Transcribed Image Text:

17.0 fo f1 12 13 In LDN A R 74191 17.01 8888 INPUT Vcc QA QB B с D GN MXMN DNUP RCON CLK COUNTER qo q1 02 q3 qo q1 q2 q3 q5 q7 14 15 16 17 BANDS LDN A B с D GN 17.01 74191 QA QB 8889 QD MXMN DNUP RCON CLK COUNTER OUTPUT JOUTPUT Q4 q5 q6 q7 NAND2 out 07.0