In this tutorial, we will review the SIPO and PISO shift registers (which stands for Series In Parallel Out, and Parallel In Series Out, respectively) and will dive more deeply into how they work. For the sample project, we will wire up a 74HC165 (PISO) to synthesize a low-frequency sine wave. An understanding of some bitwise Arduino manipulation will prove helpful for this project. You can check some of that out here. There is also a very similar article on shift registers showing how to use both SIPO and PISO here.

Understanding How Shift Registers Work

The heart or ‘guts’ of a shift register is based on a flip flop. A flip flop is a memory cell or bistable that can exist in two states, either a 1 or a 0. Once you set it to a certain state, it retains it as that until a further clock pulse sets it into a different state. A shift register is made up of several flip flops connected together in different ways depending on the purpose.

How Shift Registers Work - Bistable
Figure 1. Bistable

Figure 1 shows two transistors that are cross wired so that if a low is sent to the base of Q1 via the set switch, it turns off, causing its collector to go high which forces Q2 into an ‘on’ state. As the collector of Q2 becomes low, it will reinforce the low on Q1 base. This state will remain static until a second ground pulse arrives at the base of Q2 “flipping” it off again and the Q1 on, hence called flip flop. Of course, there should be additional steering logic to make this functional. This is also known as a bistable multivibrator. For the early shift registers mentioned above, the two transistors would have been triode valves.

How Shift Registers Work - D-Type Flip Flop
Figure 2. D-Type Flip Flop

In Figure 2, we see a real logic gate flip-flop that has a data pin where it reads the data as either a 0 or a 1, and a clock pin which will clock that data state into the flip flop causing it to become available on the output pin (usually called Q).

This D-type flip flop is a commonly used type of flip flop (4042, 4013, 7473). Every time a clock pulse arrives, it will copy the state of whatever is on pin D to pin Q.

Figure 3. SIPO shift register

Figure 3 shows the basis of a Series In Parallel Out shift register. It is a chain of D-type flip flops, thus the output of each feeds the input of the next. Because they are all clocked together at the same time, the state of the input at D1 will clock or move along until it emerges at the last Q output. While this is going on, a parallel version of this data is being presented at all the Q outputs as a nibble (a nibble is half a byte).

Figure 4. PISO shift register

Figure 4 shows the opposite of this. Here, a parallel nibble is loaded via a preset enable pulse into the flip flops. Again, the clock signal consistently marches them out to the end resulting in a parallel-to-series conversion. Both a PISO and a SIPO would make the basis for a communications channel if the serial in and the serial out were connected to each other. As each flip flop needs two clock half cycles to change it, it is effectively dividing by two (remember when we discussed shift right and shift left >> << in the bitwise tutorial). So in the figure above, as each flip flop divides by two and we have 4, then this circuit will divide by 16.

Figure 5.1 Real Shift Register (a 74HCT165 (PISO))
Figure 5.2 Real Shift Register (a 74HCT595 (SIPO))

Sine Wave Generator

There are other examples of shift register projects in the Arduino section.

Creating sine, triangle and square wave oscillators is usually done with oscillators using an R and C as the timing elements. This works well at audio and higher frequencies. When you want something a lot lower, i.e. to design a sea wave generator for your pool and you wanted a frequency of less than 1Hz maybe even 1/10 of a Hz, you would end up with huge values in the RC circuit (either tens of Megs or thousands of microFarads).

The circuit below is very stable as it is not dependent on any RC values and the frequency is only proportional to the clock frequency. In the example, this was derived from an Arduino so it is accurate and it is usually better than 1 part in 106.

Figure 6. Sine Wave Generator

So how does it work? A low bit is clocked through the 8-bit shift register. As each Q output goes low, it draws current through the resistor on that pin forming a potential divider and producing a voltage which is dependent on the value of the resistor. If these are calculated correctly, they form a point on a sine wave curve. Since there are only eight (8) points in the illustration below (see figure 7.1), it looks a bit rough. But if we add a capacitor (or better still a proper low pass filter), the square harmonics are removed creating a plausible low frequency sine wave. Here is a website showing the formula for calculating the resistors if you wanted to use all 16 bits.

Figure 7.1 Wave Forms of the Shift Register Output
Figure 7.2 Wave Forms of the Shift Register Output

Wiring Up the PISO and Arduino

This wiring is pretty much the same as the wiring in the Arduino shift register section that uses LEDs (get that here). But this time, the LEDs are replaced with resistors. The funny looking thing next to the resistors is just a header to join the ends together.

Application and Testing

This set up was able to generate frequency stable sine waves at below 1Hz. If you want to go further, use all 16 bits of the 74HC595 and a decent two-stage low pass filter. This creates a sine wave with 1/16 steps. The raw wave form has only 6% distortion but with the filter, it will be very much better at 1%. In this circuit with eight (8) steps and a simple capacitor, the distortion was only 6%.