Shift Registers

An analog-to-digital converter is a circuit that inputs an analog signal and outputs a multiple-bit binary number equivalent to that signal’s amplitude:

A free-running analog-to-digital converter is one that updates its digital output as often as it can, not waiting for any prompting from another device. If we were to connect a free-running ADC to a computer (microprocessor or microcontroller), we would need some way to sample the ADC’s output at times specified by the computer, and hold that binary number long enough for the computer to register it. Otherwise, the ADC may update its output in the middle of one of the computer’s “input” cycles, possibly resulting in corrupted data.

We could build such a sample-and-hold circuit out of flip-flops. What type of flip-flop would we use for this purpose, and how many would we need for the ADC circuit shown above? This circuit we would build is also known as a shift register. What kind of shift register inputs multiple bits of data all at once, and transfers that data to its output lines all at once, at the command of a clock pulse?

This shift register circuit energizes one LED at a time (beginning with the bottom LED at power-up), in a rotating pattern that moves at the pace of the clock:

A technician decides to reverse the direction of pattern motion, and alters the circuit as such:

Unfortunately, this does not work as planned. Now, the bottom LED blinks once upon power-up, then all LEDs remain off. What did the technician do that was incorrect? What needs to be done to fix the problem?

This shift register circuit produces a sequential light pattern reminiscent of the old Mercury Cougar tail-lights: first one LED energizes, then two LEDs energize, and then all three LEDs energize before all de-energizing and repeating the sequence. The 74HC194 shift register circuit is set to always operate in the “shift right” mode with the shift-right serial input (DSR) tied high, the master reset ([MR]) input used to set all output lines to a low state at the end of each cycle:

The sequential light pattern is supposed to begin whenever the “Trigger” input momentarily goes high. Unfortunately, something has failed in this circuit which is preventing any of the LEDs to come on. No blinking light sequence ensues, no matter what the state of the “Trigger” input.

Identify some likely failures in this circuit that could cause this to happen, other than a lack of power supply voltage. Explain why each of your proposed faults would cause the problem, and also identify how you would isolate each fault using test equipment.