Programmable logic

Logic gates are formed by connecting transistors together on a semiconductor material to make an integrated circuit. The wafers, or chips, of semiconductor contain lots of logic gates that make up different types of devices which work together to read, store, calculate, and transmit digital information.

Most integrated circuits contain a specific arrangement logic gates at the time they are manufactured. Because of their physical and chemical properties, some semiconductors can let you change connections between the gates after the device is manufactured. By applying special voltages at programming pins, a custom arrangement of gates can be “programmed” into the integrated circuit. These types of semiconductors are part of a category of electronics called Programmable Logic Devices (PLD). There are many different kinds. Some of them you program only once and others you can erase the original gate arrangement and program in new ones multiple times.

We can use the Adafruit Circuit Playground Express to create our own PLD. The digital pins are the inputs for the logic circuits. The logic gates we program are logical expressions in code that combine the digital inputs we read from the pins. The result of the expression is written to a digital output pin.

Board PLD

The physical idea of using your board as a PLD looks like this:

The logic inputs for A and B are connected to digital input pins. The resulting output Q is connected to a digital output pin. We can make a general representation of your board as a PLD by selecting some digital pins to use as inputs and outputs for our programmable logic.

By “connecting” the pins together with code, we can program virtual logic gates and make the board act like a PLD. With multiple pins and some more code, we can even create a combined logic circuit.

Logic observer

As easy way to see what the outputs of our PLD are, we’ll use the A6 pin as a logic observer input and display green pixels when an output of the PLD is true and yellow pixels when false.

forever(function () {
    if (pins.A6.digitalRead()) {
        light.setAll(0x00ff00)
    } else {
        light.setAll(0xffff00)
    }
    pause(100)
})

Programmable NOT gate

The NOT gate takes the logic value of the input and inverts it at the output. This is a single input gate using just the A4 pin for input.

The NOT gate is wired using alligator test clips as shown in the following diagram. The output pin A1 is connected to the observer at pin A6.

The script to program the NOT gate is simply a logical inverse of ||pins:digital read pin|| inside a ||pins:digital write pin||.

pins.A1.digitalWrite((!pins.A4.digitalRead()))

Programmable OR gate

The OR gate takes two inputs and makes the output true if any input is true. The A4 and A7 pins are the inputs.

The OR gate is wired using alligator test clips as shown in this diagram. The output pin A2 is connected to the observer at pin A6.

The script to program an OR gate is two ||pins:digital read pin|| blocks, connected with an ||logic:or||, inside a ||pins:digital write pin||.

pins.A2.digitalWrite(pins.A4.digitalRead() || pins.A7.digitalRead())

Programmable AND gate

The AND gate takes two inputs and makes the output true if both inputs are true. The A4 and A7 pins are the inputs.

The AND gate is wired using alligator test clips as shown in the next diagram. The output pin A3 is connected to the observer at pin A6.

The script for an AND gate is two ||pins:digital read pin|| blocks, connected with an ||logic:and||, inside a ||pins:digital write pin||.

pins.A3.digitalWrite(pins.A4.digitalRead() && pins.A7.digitalRead())

Combined logic

You can program your board to have multiple logic gates that operate on the two inputs. Just combine the three logic gate scripts from above into one ||loops:forever|| loop.

forever(function () {
    pins.A1.digitalWrite((!pins.A4.digitalRead()))
    pins.A2.digitalWrite(pins.A4.digitalRead() || pins.A7.digitalRead())
    pins.A3.digitalWrite(pins.A4.digitalRead() && pins.A7.digitalRead())
    pause(100)
})

Input tests

You can test different input combinations by connecting the other ends of alligator clip leads on pins A4 and A7 to either GND or 3.3V. The GND pin will make a false input value and 3.3V will make a true input value. Move the other end alligator clip lead connected to the observer pin A6 to each of the outputs A1, A2, and A3 to see the result of the logic operation programmed for those pins.

As an example, here’s the truth table with pin voltages for the NOT operation:

Do test connections for the inputs and check the results for the OR and AND outputs.

OR truth table

AND truth table

XOR device

As we learned earlier, the XOR gate operation is made up from several other gates. The result Q was made from this expression in code:

let A = false
let B = false
let Q = (!A && B) || (A && !B)

We’ll make an XOR gate by programming a combined logic device for it. This time let’s say that the whole Adafruit Circuit Playground Express is a programmed XOR gate.

Let’s use the same wiring diagram as we did for the OR gate using A4 and A7 as input pins with A2 connected to the observer pin A6.

Our logic gate script is a bit different this time. To simplify forming the expression for XOR, we’ll assign variables to the input and output values.

let A = false
let B = false
let Q = false
forever(function () {
    A = pins.A4.digitalRead()
    B = pins.A7.digitalRead()
    Q = !(A) && B || A && !(B)
    pins.A2.digitalWrite(Q)
    pause(100)
})

Connect the inputs for A4 and A7 according to the XOR truth table and see if the outputs in the table match your results.