Now let’s look closely on the demo project which opened up on IDE start. It blinks the LED connected to pin 13 of your board. Although many boards have a built-in LED on that pin, let’s make it more visible 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 make this possible. Nodes are basic building blocks in XOD. Each of them make a tiny portion of work and communicate to others.

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

digital-output

This node represents a single physical pin on the board that is used as an output, and can be either in 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 Inspector sidebar with properties related to the selected node, i.e. our digital-output.

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

Value on SIG input defines wether the digital output port should go high or low state. In Inspector you see its value disabled with placeholder “linked”. That’s fine because the value is defined by an upstream node it is linked to. More on that later.

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

Note

Although it could look excessive and strange at first, but any update in the XOD program is accompanied by pulses. They’re like heartbeat which delivers all updates to their destinations. No pulses, no observable effects.

Splitting actual values and pulses helps to understand what and when could ever happen. That makes programs more explicit and reliable.

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

flip-flop

This node is like a virtual light switch that could be turned on (SET), turned off (RST) or inverted to an opposite state (TGL).

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

clock

The clock node emits a pulse on TICK output over equal periods of time. The period is defined by the value of IVAL input.

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

The second input RST accepts pulses. On pulse the clock accept new interval value and start counting from scratch.

Clock is a very usual source of pulses. You’d use it quite often to drive updates in your programs.

boot

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

Hint

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

The links

You see that inputs and outputs of the nodes 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 on program start
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 actually update by sending a pulse on each state update.

As a final result we see the LED blinking.

Tweaking the program

Try to change something.

Select the clock node and set another IVAL value. Say, set it to 1.0 seconds, upload the updated program and observe the result.

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

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

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

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

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

Now let’s improve our program another bit and make the lights opposite. To do this we should cut signal inversion into either of digital-output SIG links. Node not under xod/core does exactly that. Delete existing link, place not node and add new links so that signal from our flip-flop to digital-output goes through it:

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

Disjoint graphs and independent tasks

In XOD nodes are not required to be connected in a single network. You can build two or more disjoint clusters of nodes to perform several tasks simultaneously.

Try to add yet another LED with absolutely independent blink interval and state:

Now we have three digital-output’s. It could be hard to understand which one corresponds to what 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 could look more clear:

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. There are more data types in XOD. Follow to Types and Conversions chapter to learn more on this topic.

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Nodes and Links