pxt-calliope/docs/projects/telegraph/code.md

Code

Let's build the code that, when the user presses the button A on a @boardname@, will send an impulse over a wire to the receiving @boardname@ and turn on an LED there.

Make sure that the sending and receiving wires run "symmetrically" between the two boards. That is: pin 1 on one @boardname@ is connected to pin 2 on the other, and vice versa. Just like it's shown in the pictures in the make section. This way we can use the same code on both @boardname@s .

Step 1

We start with a block that digitally writes high value (a digital 1) to P1 which sends the value to @boardname@'s pin 1. This block is found in Pins drawer of the Advanced section of the Toolbox.

pins.digitalWritePin(DigitalPin.P1, 1)

Step 2

To show that we are sending the 1, we add a block to turn on an LED in the center of the LED display (2, 2) using ||led:plot x y||:

pins.digitalWritePin(DigitalPin.P1, 1)
led.plot(2, 2)

Step 3

Now that we know how to send the signal, we only want to do it while the button A is pressed. Pick an ||logic:if then else|| block from the Logic drawer (you'll leave the ||logic:else|| part empty for now). Add a check for when button A is pressed. Get an ||input:on button pressed|| from the Input drawer and move the blocks from the previous step into ||logic:then|| part of the ||logic:if then else||:

if (input.buttonIsPressed(Button.A)) {
    pins.digitalWritePin(DigitalPin.P1, 1)
    led.plot(2, 2)
} else { }

Step 4

For the ||logic:else|| section (while button A is not pressed) we want to do the opposite of what we did in the |logic:then|| section. Which is, make the value of pin P1 go to low (digital 0) and unplot the corresponding LED on the sending @boardname@:

if (input.buttonIsPressed(Button.A)) {
    pins.digitalWritePin(DigitalPin.P1, 1)
    led.plot(2, 2)
} else {
    pins.digitalWritePin(DigitalPin.P1, 0)
    led.unplot(2, 2)
}

Step 5

Let's wrap it all in a forever loop so this code is running in the background always checking button A and sending the appropriate signal to the receiver. Modify your code to add the blocks below. Download the code onto one of the @boardname@s, press and release button A a few times.

basic.forever(() => {
    if (input.buttonIsPressed(Button.A)) {
        pins.digitalWritePin(DigitalPin.P1, 1)
        led.plot(2, 2)
    } else {
        pins.digitalWritePin(DigitalPin.P1, 0)
        led.unplot(2, 2)
    }
})

The sending part is done, so now we'll add the receiving part.

Step 6

The receiver needs to digitally read from the pin where the other @boardname@ sends its value to pin 2 across the wire. Let's start by going to the Pins drawer, adding a ||pins:digital read pin|| and change the pin value to P2.

Now, we want to examine the value read from P2 and check whether it's high (1) or low (0). Go to the Logic drawer and pick an ||logic:if then else|| block, then come back for the comparison operator ||logic:0 = 0||. Plug in our ||pins:digital read pin|| block as one operand and the value 1 as the other.

We'll turn the LED in the bottom right corner (4, 4) on to show that we received a high value and turn it off in not.

Make sure your code looks like this:

basic.forever(() => {
    if (input.buttonIsPressed(Button.A)) {
        pins.digitalWritePin(DigitalPin.P1, 1);
        led.plot(2, 2);
    } else {
        pins.digitalWritePin(DigitalPin.P1, 0);
        basic.clearScreen();
    }
    if (pins.digitalReadPin(DigitalPin.P2) == 1) {
        led.plot(4, 4);
    } else {
        led.unplot(4, 4);
    }
});

Your telegraph is ready!

Step 7

Ok, let's try it out:

1. Connect the first @boardname@ to your computer using your USB cable and download the telegraph code to it.
2. Disconnect the first @boardname@.
3. Connect the second @boardname@ to your computer using your USB cable and download the telegraph code to it.
4. Disconnect the second @boardname@.
5. Connect the battery holder to one of the @boardname@s.
6. The first person, and then second person, can take turns pressing button A on their own @boardname@s to play the telegraph game!