Power Connections

Power Connections:


ISIS supports a powerful scheme for making power connections implicitly, thus vastly reducing the number of wires on the schematic. There are three main concepts involved that we will discuss in turn below.

Hidden Power Pins

Almost all relevant parts in ISIS have their power pins hidden (not visible on the schematic). The crucial point to remember is that in such cases is that by default the name of the pin denotes the net to which it will connect.

 Form more information please visit Hidden Power Pins

Power Rail Configuration

The easiest way to manage power nets and connectivity is through the Power Rail Configuration dialogue form, which is invoked from the Design Menu. If we do this now we can see that we have three pre-defined power supplies, namely GND, VCC/VDD and VEE. Changing the combo box at the top we should also see that the GND net is connected to the GND supply and that both the VCC and the VDD nets are connected to the VCC/VDD supply. It’s worth examining how this happened more closely:

  • The GND net is created by connections to an unlabelled ground terminal.
  • The VDD net is created by connections to the power terminal labeled VDD.
  • The I2C memory has two hidden power pins, VCC and GND, which are assigned to nets bearing their names.

Managing power nets and power supplies is a very important concept so we’ll experiment a little to re-inforce the point. Exit the Power Rail Configuration dialogue form, right click on the I2C memory part and select Edit Properties from the resulting dialogue form. Next, click on the Hidden Pins button at the right hand side to view the hidden pins and their net assignments.

Editing hidden power pins
Editing hidden power pins

Let’s change the power pin to be on net VDD, instead of the default of VCC. If we then exit the dialog and re-invoke the Power Rail Configuration dialogue and switch to the VDD/VCC power supply we should see that the VCC net is no longer present (this was the only connection on our schematic so far).

While we are here and for completeness change the Voltage of the power supply to be 3.3V, which is actually what we will be using. In PCB Design this is useful only as a reference to ourselves but actually has significance in the software for simulation purposes.

This exercise, while useful in explaining how to view/change the power nets for individual components, had no effect on design connectivity. The software is clever enough to assign both the VCC and the VDD nets to the same supply and the Power Rail Configuration dialogue allows us to configure our power connectivity at design level.

Returning to the schematic now, edit the VDD terminal (right click, edit properties) and change the terminal label to ‘MY_POWER_NET’. If you go back to the Power Rail Configuration dialogue you should see that this is now an unconnected power net (a power net which is not associated with a power supply).

Signing unconnected nets in the Power Rail Configuration tool
Signing unconnected nets in the Power Rail Configuration tool

If we were doing this for real, we would now have to select the VSS/VDD power supply and then click the assign button to map our power net onto the correct supply.

If we had simply removed the label from the power terminal (instead of renaming it), it would have been designated to the VCC net and again everything would be done for us.

A final point worth noting is that new power supplies are created automatically when you label a terminal with a voltage. For example, labeling a power terminal +12V would create a +12V supply and assign the +12V net to the supply automatically. This means again that no action is required by the user.

In general usage therefore, the software will handle the assignments of power nets to power supplies and will create new supplies automatically. However, if you need more flexibility (for example, an analogue ground and a digital ground) then using terminal labels and the Power Rail Configuration dialogue gives you the control you need. Please see the reference manual for more information if required.

Let’s change our terminal label back to VDD for consistency before we move on.

 Form more information please visit Power Rail Configuration

#collected from the proteus>>help>> tutorial page

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