What Shift Registers Do

You are able to control 8 LEDs with just 4 pins!

Lets look at the code. At the top, you will see:

Pb_outputs shregs(datapin, clkpin, latchpin, numreg);

This is a way to tell the Arduino that you have connected shift registers to it on those pins (data, clock, and latch). The fourth argument numreg is telling the Arduino the number of shift registers being connected, which is right now set to 1.

The shift register we use is one of those black chips in the breadboard with 16 legs. The legs have identities:

Note that it has 8 output pins called QA through QH. It is possible to write any digital value you want to those eight pins using the data, clock and latch pins. The pin named QH’ can be used to connect more shift registers to it. We will explore that in a few moments.

Each of the pins QA through QH will receive one bit of data. A bit is a single value of data, which can be 0 or 1. We need to tell the shift register which bits to write to which pin.

In the code, we have defined a new variable byte serdata[1]. This stores 1 byte, or 8 bits of digital information. Inside setup(), we assign it values by typing:

serdata[0] = 0b00000000;

This sets all 8 bits to 0. Then, we can shift these values to the shift register (which we have named shregs) by typing:

shregs.update(serdata); // Shifting out the array

This will set all the LEDs to not light up at the start.

• Try pressing the reset button and see what happens to the red LEDs for a moment.
• Try changing some of those zeroes in 0b00000000 to ones, upload, and see what reset does.

CHECKPOINT!

The rest of the program should be pretty easy to understand. We already now how to use stopwatches, and we have defined two functions. The function void changeflag() just changes the variable flag from 1 to 0 or 0 to 1.

The function void changeserdata(int ff) has a new variable inside the brackets, which we haven’t seen before. It simply means that this function has to be used with an int type variable or number supplied to it. For example, inside loop(), it is being called like this:

So it is being supplied with the value of flag. Insidechangeserdata(int ff), it captures this value into its own private variable called ff. ff can only be used inside this function. Right now, it is being used to change the LED pattern conditionally depending on the value of flag:

if (ff == 0) { serdata[0] = 0b00111100; } else if (ff == 1) { serdata[0] = 0b11000011; }

Do you see how these ones and zeros correspond to the LEDs that are glowing? Try changing them and see how the LEDs change.

CHECKPOINT!

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