Digital Electronics: Principles, Devices and Applications

Troubleshooting Digital Circuits and Test Equipment 6615. Pin 6 of IC-2 is externally or internally shorted to GND.A continuity check can be used to rule out one by one the first, second, third and fifth causes.Remember that, although pin 6 is not used in the circuit, if it is shorted to GND it will force the Qoutput permanently to go to the logic ‘1’ state owing to the cross-coupling arrangement in the internalstructure of the flip-flop. Incidentally, pin 6 in IC-2 happens to be very close to the GND pin, whichis pin 7. Even a solder bridge between pin 6 and pin 7 could lead to this. What is important to note isthat a troubleshooter may tend to ignore IC pins that are not used, but even those unused pins in theIC can cause faulty conditions. Once the continuity check rules all these possibilities out, the IC canbe replaced.Example 16.4Figure 16.5 shows a cascaded arrangement of three D flip-flops belonging to the TTL family of digitalICs. The circuit shown here is only a small part of a complex digital circuit. Each of the three flip-flopshas a clock input-to-output propagation delay of 15 ns. The expected and observed outputs of theflip-flops for the first few clock cycles are listed in Table 16.2. Although the circuit shown here is thatof a three-bit shift counter, the observed outputs are nowhere near to what they should have been inthe case of a three-bit shift counter. Identify the possible cause for the observed outputs being differentfrom the expected ones. All flip-flops are observed to be in the logic ‘0’ state just before application ofFF2D 2Clk2Q 2D 1 Q 1D 1 Q 0FF1FF0Clk1Clk0Q 2 Q 1Q 0Figure 16.5 Three-bit shift counter (example 16.4).Table 16.2 Example 16.4.Clock pulse Expected output Actual outputQ 2 Q 1 Q 0 Q 2 Q 1 Q 00 0 0 0 0 0 01 1 0 0 1 1 12 1 1 0 0 0 03 1 1 1 1 1 14 0 1 1 0 0 05 0 0 1 1 1 16 0 0 0 0 0 07 1 0 0 1 1 1