Now let’s look closely at the demo project that opened up when you started the IDE. It blinks the LED connected to pin 13 of your board. Many boards have a built-in LED on that pin, but let’s make it more clear by building a simple circuit:

Make sure you’ve uploaded the program to the board and the LED actually blinks.

Why does it blink?

The nodes

You see four nodes linked together in a chain to implement the blinking. Nodes are basic building blocks in XOD. Each of them handles a tiny portion of work and communicates with other nodes.

Let’s talk about each node one by one from bottom to top.

digital-output

This node represents a single physical output pin on the board. It can be either in a high (enabled) or low (disabled) state. We use it to switch our LED on and off.

The node has three inputs. They are PORT, SIG, and UPD.

The PORT defines what physical pin corresponds to the node. Select the node by clicking on it. You’ll see the Inspector sidebar with the properties of the selected node, i.e. our digital-output.

Note that the PORT value is set to 13th pin.

The value on the SIG input defines whether the digital output port should go into a high or low state. In the Inspector, you see its value is disabled and has the placeholder “linked”. That’s fine, because the value is defined by a linked upstream node. More on that later.

The UPD input listens for pulses, uses them to actually update the signal, and physically updates the physical pin according to the SIG value. In other words, updating the SIG value alone is not enough and wouldn’t lead to any visible results.

Note

Although it might seem excessive and strange at first, any update in a XOD program is accompanied by pulses. They’re like a heartbeat that delivers all updates to their destinations. No pulses, no observable effects.

Keeping actual values and pulses separate makes it easier to understand what may happen and when. It makes programs more explicit and reliable.

Finally, the digital-output node listens for pulses on its UPD pin. Once it get a pulse, it sets physical port 13 (PORT) to a state defined by the value on the SIG pin at the moment when the pulse was received.

flip-flop

This node is like a virtual light switch that can be turned on (SET), turned off (RST) or toggled (TGL).

In addition to its inputs, the flip-flop node has two outputs. They are MEM, which provides the current state (high or low), and CHNG, which sends a pulse when the value of MEM changes.

clock

The clock node emits a pulse on its TICK output at equal time intervals. The interval is defined by the value of the IVAL input.

Select the clock node and note the value set for IVAL in Inspector. The interval is expressed in seconds.

The second input, RST, accepts pulses. On each pulse, the clock accept a new interval value and starts counting from zero.

The clock node is a very common source of pulses. You’ll use it quite often to drive updates in your programs.

boot

The boot node is very simple. It sends a single pulse when the program starts, i.e. when the board gets powered on, reset, or flashed.

Hint

If pulses are heartbeats, then the clock is a heart. And the boot node is a defibrillator that starts the clock-heart.

The links

You see that nodes' inputs and outputs are connected together with lines. These lines are called links in XOD.

They make it possible for nodes to talk to each other. Upstream nodes produce values and downstream nodes consume that values.

What happens in our blink program? Take a look:

The boot node emits a pulse when the program starts.
The pulse goes to the clock node which start to tick at regular intervals
Each tick pulse goes to the flip-flop and toggles its state
The flip-flop provides its state value to the digital-output and asks it to update by sending a pulse each time the flip-flop changes state.

As a result, we see the LED blinking.

Tweaking the program

Try to change something.

Select the clock node and set a different IVAL value, e.g. 1.0 second, upload the updated program and observe the result.

That’s not too interesting. Let’s add another LED. Improve your circuit:

Place a new digital-output node. To do this, use the Project Browser sidebar. The digital-output node is available in the xod/core library. Hover the cursor over the item and click the (+).

You’ll see a new node appear in the main workspace. Drag it to the slot you want. The one next to the existing digital-output would be fine. In Inspector, set the PORT for the new node to 12, since it will control our new LED.

Now we need to provide the new node with data. Link its SIG and UPD pins to the flip-flop outputs:

Upload the updated program to the board. Whoa! Both LED’s are blinking.

Now let’s improve our program some more and make the lights blink alternately. To do this, we need to add a signal inversion into either the digital-output or SIG links. The not node under xod/core does exactly that. Delete the existing link, place a not node, and add new links so that the signal from our flip-flop to the digital-output goes through it:

Upload the new version to the board. See the result?

Disjoint graphs and independent tasks

In XOD, nodes do not have to be connected in a single circuit. You can build two or more disjoint clusters of nodes to perform several tasks simultaneously.

Try adding yet another LED with an absolutely independent blink interval and state:

Now we have three digital-output nodes. It can be hard to understand which node corresponds to each LED, so it would be better to give them clear labels. To set a custom label for a node, select it and provide the label via Inspector:

You can provide a custom label for any node. Now the program looks clearer:

What’s next

You’ve seen pins and links that carry values of different types. Some provide logical values and some transmit pulses. They are differentiated by colors. XOD has even more data types. Go to the Types and Conversions chapter to learn more on this topic.

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Nodes and Links