Digital data

Besides turning on LEDs or sending a pulse to notify some other device, the digital pins can send data as binary numbers. Together, multiple signals can create several bits or digits of a binary number. Each digit of course is 2 times the place value of the previous one, 2**0, 2**1, 2**2, and so on. If 3 digital signals use to represent a binary number, and if all of the signals are at the high level, the 3 bit binary number is 111. This means that each signal gives three different values of 1 * 2**0, 1 * 2**1, and 1 * 2**3. That equals 1 + 2 + 4 in decimal which makes the number 7.

Parallel data

Since each pin can only signal one of two values at any particular moment, one bit of a digital number can be represented at a pin. To send larger digital numbers with more bits, you’ll need more pins. Each pin represents a differnet digit of the number. For a 3 digit number there are 3 bits. The first bit (right to left) is the 4’s bit, the second is the 2’s bit, and the third is the 1’s bit. Each digit on the left has a value of 2 times the digit on the right. The number 7 is equal to 4 + 2 + 1 so as a 3 bit binary number it would be 111. The number 3 is equal to 0 + 2 + 1 and its 3 bit binary number is 011.

To send the number 7 we could use 3 ||pins:digital write pin|| blocks using pins A1, A2, and A3 for each of the bits.

pins.A1.digitalWrite(true)
pins.A2.digitalWrite(true)
pins.A3.digitalWrite(true)

For the number 3 we would use the same blocks but with different logic values for some of the bits.

pins.A1.digitalWrite(true)
pins.A2.digitalWrite(true)
pins.A3.digitalWrite(false)

Maybe you noticed that for the number 3 we use A1 for the 1’s digit, A2 for the 2’s digit, and A3 for the 4’s digit. In the following diagram, signals are place on three pins to send a different 3 bit number every 2 microseconds. The red numbers at the bottom show what the three values on the pins together are in decimal.

Experiment: Send and receive parallel data

Setup:

1. Connect 3 alligator clip leads to pins A1, A2, and A3
2. Connect the other end of the A1 lead to pin A6.
3. Connect the other end of the A2 lead to pin A5.
4. Connect the other end of the A3 lead to pin A4.

3. Download the following code to the board:

let dataArray: number[] = []
let dataOut = 0
let dataIn = 0
dataArray = [7, 6, 5, 4, 3, 2, 1, 0]
forever(function () {
    dataIn = 0
    pause(300)
    if (pins.A4.digitalRead()) {
        dataIn += 4
    }
    if (pins.A5.digitalRead()) {
        dataIn += 2
    }
    if (pins.A6.digitalRead()) {
        dataIn += 1
    }
    light.setPixelColor(dataIn, 0xff0000)
    pause(500)
    light.clear()
    pause(500)
})
forever(function () {
    pause(2000)
    if (dataArray.length > 0) {
        dataOut = dataArray.shift()
        dataArray.push(dataOut)
        pins.A3.digitalWrite(false)
        pins.A2.digitalWrite(false)
        pins.A1.digitalWrite(false)
        if (dataOut > 3) {
            pins.A3.digitalWrite(true)
            dataOut = dataOut - 4
        }
        if (dataOut > 1 ) {
            pins.A2.digitalWrite(true)
            dataOut = dataOut - 2
        }
        if (dataOut > 0) {
            pins.A1.digitalWrite(true)
        }
    }
})

Test: Watch to see which LEDs are lighted while the program runs.

Result: The position of the each lighted LED should show that the digital number received at the input pins matches the order of the number sent at the write pins.

Clocked input and output

If you looked at the code from the experiment you might have noticed that the ||loops:forever|| loop that reads the input pins has to pause for a shorter amount of time than the loop that writes the data on the output pins. This is so that the input loop won’t miss reading a new value sent from the output loop. Instead of using an input loop we could use two more pins to send and receive a timing signal so input pins are read only when a new value is written.

This code change uses what is called clocked input and output. We can switch the input code in the experiment to use clocked input by replacing the ||loops:forever|| with a ||pins:on pin rise|| event and then removing the input delay ||loops:pause||.

pins.A7.onEvent(PinEvent.Rise, function () {} )

Now, once the ||loops:forever|| loop has written the new data values on the output pins, a quick trigger pulse is sent to pin A7 from A0 to clock the new input right after the data pins A1, A2, and A3 have their new output.

pins.A0.digitalWrite(true)
pins.A0.digitalWrite(false)

Experiment: Clocked parallel data transfer

Setup:

1. Connect 3 alligator clip leads to pins A0, A1, A2, and A3
2. Connect the other end of the A1 lead to pin A6.
3. Connect the other end of the A2 lead to pin A5.
4. Connect the other end of the A3 lead to pin A4.
5. Connect the other end of the A0 lead to pin A7.

3. Download the following code to the board:

let dataArray: number[] = []
let dataOut = 0
let dataIn = 0
dataArray = [7, 6, 5, 4, 3, 2, 1, 0]
pins.A7.onEvent(PinEvent.Rise, function () {
    dataIn = 0
    if (pins.A4.digitalRead()) {
        dataIn += 4
    }
    if (pins.A5.digitalRead()) {
        dataIn += 2
    }
    if (pins.A6.digitalRead()) {
        dataIn += 1
    }
    light.setPixelColor(dataIn, 0xff0000)
    pause(500)
    light.clear()
    pause(500)
})
forever(function () {
    pause(2000)
    if (dataArray.length > 0) {
        dataOut = dataArray.shift()
        dataArray.push(dataOut)
        pins.A3.digitalWrite(false)
        pins.A2.digitalWrite(false)
        pins.A1.digitalWrite(false)
        if (dataOut > 3) {
            pins.A3.digitalWrite(true)
            dataOut = dataOut - 4
        }
        if (dataOut > 1 ) {
            pins.A2.digitalWrite(true)
            dataOut = dataOut - 2
        }
        if (dataOut > 0) {
            pins.A1.digitalWrite(true)
        }
        pins.A0.digitalWrite(true)
        pins.A0.digitalWrite(false)
    }
})

Test: Watch to see which LEDs are lighted while the program runs.

Result: The position of the each lighted LED should show that the digital number received at the input pins matches the order of the number sent at the write pins.

Serial data

It’s not necessary to use a separate pin to transfer each bit of a digital number. Each bit of a number can be written out on one pin in a sequence. Once all of the bits of one number are written, the bits of the next number are written in a sequence too. The receiving device needs to properly assemble the bits to form a value. It also needs to know how many bits each number will have so it can start to assemble the next number. This method of sending and receiving data is call serial data transfer.

In the following diagram several 3 bit numbers are written in series on the A2 pin. The red numbers show what the binary digital numbers are in dedimal.

Each bit of the 3 bit number is a digit. Each digit has a place value that is 2 times that of the next digit. The code to write a single number out on a pin as bits will have to factor out each place value to know whether to write a 1 or 0 for that bit. Here’s some code that will convert a decimal number to bits and write them to the A3 pin:

let digitalNum = 0
let digitValue = 2**3
for (let i = 0; i < 3; i++) {
    if (digitalNum >= digitValue) {
        pins.A3.digitalWrite(true)
        digitalNum += -digitValue
    } else {
        pins.A3.digitalWrite(false)   
    }
    digitValue = digitValue / 2
    pause(10)
}

Reading a number at an input pin will require something that does the opposite. Each time a new bit is read on the input pin, the code which reads the bit will need multiply the current value of the digital number by 2 and then add in this next bit. To read one number at pin A6, the code might look like this:

let bits = 0
let digitalNum = 0
for (let i = 0; i < 3; i++) {
    digitalNum = digitalNum * 2
    if (pins.A6.digitalRead()) {
        digitalNum += 1
    }
    bits += 1
    pause(10)
}

Experiment: Read and write serial data

Setup:

1. Connect on end of an alligator clip lead to the A1 pin.
2. Connect the other end of that lead to the A6 pin.
3. Get another alligator clip lead and connect one end to the A2 pin.
4. Connect the other end of the second lead to the A5 pin.

5. Copy the following code into the editor.

let digitValue = 0
let bits = 0
let dataOut = 0
let digitalNum = 0
let dataArray: number[] = []
let dataIn = 0
pins.A5.onEvent(PinEvent.Rise, function () {
    if (bits > 2) {
        light.setPixelColor(digitalNum, 0xff0000)
        digitalNum = 0
        bits = 0
    } else {
        light.clear()
    }
    digitalNum = digitalNum * 2
    if (pins.A6.digitalRead()) {
        digitalNum += 1
    }
    bits += 1
})
dataIn = 0
dataArray = [7, 6, 5, 4, 3, 2, 1, 0]
forever(function () {
    if (dataArray.length > 0) {
        dataOut = dataArray.shift()
        digitValue = 2 ** 3
        for (let i = 0; i < 3; i++) {
            if (dataOut >= digitValue) {
                pins.A1.digitalWrite(true)
                digitalNum += 0 - digitValue
            } else {
                pins.A1.digitalWrite(false)
            }
            pins.A2.digitalWrite(true)
            pins.A2.digitalWrite(false)
            digitValue = digitValue / 2
            pause(300)
        }
        dataArray.push(dataOut)
    }
})

Test: Watch to see which LEDs are lighted while the program runs.

Result: The position of the each lighted LED should show that the digital number received at the input pins matches the order of the number sent at the write pins.