Components and nets

In the last section we created a design with a single component. This section will show you how define components starting from scratch, and connect their pins with nets.

We typically use components to model discrete devices (e.g. resistors, capacitors, op-amps, FPGAs,etc ..). In practice, each component definition captures a whole family of devices. For example, every fuse in this datasheet would be captured by a single component model.

We create a component using the pcb-component statement and add pins to it using the pin and port statements. Here are definitions for three components needed for our design, a photodiode, an op-amp, and a pin-header.

pcb-component photodiode :
  port a 
  port c

pcb-component op-amp :
  port in+ 
  port in- 
  port out 
  port v+  
  port v-  

pcb-component three-pin-header :
 port p : pin[3]

This code snippet:

• Defines a new component named photodiode with two pins named a and c
• Defines a new component named op-amp with five pins named in+, in-, out, v+, and v-
• Defines a new component named three-pin-header with a port named p, that has three pins. The pins in this port have the names p[0] p[1] and p[2].

We can add these components to our design using the inst statement (short for instantiate). Let's create a small circuit connecting the photodiode to the op-amp, and pins associated with power to the connector.

pcb-module my-design :
  inst d : photodiode
  inst opa : op-amp
  inst connector : three-pin-header

  ; Set up power connections
  net GND (opa.v- connector.p[0])
  public net VDD (opa.v+ connector.p[1])

  ; Connect op-amp
  net (d.c opa.in-)
  net (GND opa.in+ d.a)

To break this down, we first instantiate our components:

  inst d : photodiode
  inst opa : op-amp
  inst connector : three-pin-header

• One instance of op-amp named opa
• One instance of photodiode named d
• One instance of three-pin-header named connector

Then we create a net named GND that connects the negative supply on the op-amp, and the first pin on the pin-header :

  net GND (opa.v- connector.p[0])

This statement creates a new net named GND. Every pin inside the parentheses will be added to this GND net. We named the instance of our connector connector and we can access its pins using the . operator, so the name of the first pin of the connector is j.p[0].

Similarly this statement creates the VDD net, and connects the positive op-amp supply pin and the second pin on the connector :

  public net VDD (opa.v+ connector.p[1])

Then we connect the cathode of the photodiode to the inverting input of the op-amp :

  net (d.c opa.in-)

We could give that net a name but it's optional. Nets that we don't name get auto-generated names later.

Finally, we connect the non-inverting op-amp input and the anode of the photodiode to ground :

  net (GND opa.in+ d.a)

Notice that we are using the name GND to add these other pins to the GND net.

Example code for this design is here. Because we didn't add symbols to our components (yet) we can't generate a schematic to see what our code is doing. Instead, we used some introspection to print out the members of the VDD net (which we made public for that purpose).

This section showed you how to create new components, instantiate them in a design, and connect them up using the net statement. However we're still missing some information in the components. There's no symbol or land pattern or sourcing information (the reference has a complete list of component statements). And so even though this is a valid design in JITX, we wouldn't be able to create a schematic (because there are no symbols), or a layout (because there are no land patterns). We will see how to add that information in the next section.