When is Ground Ref Required?

SIMPLIS digital devices do not require a reference to the global ground node, which is node 0 in the main circuit and all subcircuits; as a result, SIMPLIS logic can appear anywhere in a power system - referenced to ground or to a power rail, such as -48V or left floating. With the exception of the digital constant, all digital devices allow you to use a ground-reference pin. Two simple rules define when a ground-reference pin is required:

  • The use of a ground-reference pin is optional whenever a SIMPLIS digital device interfaces only with other SIMPLIS digital devices.
  • A ground-reference pin is required for any SIMPLIS digital device that interfaces with an analog device or a SIMPLIS classic digital device.

The following circuit examples are intended to help you better understand these rules.

Mixing SIMPLIS Digital and Analog components

In the following schematic example:

  • U1, U2, and U3 do not require a ground-reference pin because these devices are connected only to other SIMPLIS digital devices.
  • The output of U4 is connected to the SIMPLIS primitive switch S1. Because all SIMPLIS primitive switches are analog components, U4 requires a ground-reference pin.

This example circuit can be downloaded here: simplis_009_when_is_ground_ref_required_example_1.sxsch

Mixing SIMPLIS Digital and SIMPLIS Classic Digital components

The SIMPLIS classic digital logic family is the original logic family in SIMPLIS. These devices can be identified by the lack of additional lines on the lower left corner of the symbol. All devices in this family are treated as analog components and, if a digital device interfaces with a SIMPLIS classic digital device, the SIMPLIS digital device requires a ground-reference pin.

  • U1, U2, and U3 do not require a ground-reference pin. These devices are connected only to other SIMPLIS digital devices.
  • The output of U4 is connected to the SIMPLIS classic digital gate U5. All SIMPLIS classic digital devices are considered analog components; therefore U4 requires a ground-reference pin.

This example circuit can be downloaded here: simplis_009_when_is_ground_ref_required_example_2.sxsch

Interface Levels

When a SIMPLIS digital device is connected to an analog component, the interfacing input or output voltage range is controlled by the edit dialog box entries on the Interface tab. The Input parameters and Output parameters boxes for the D-Type Flip-Flop are shown below.

Note: If SIMPLIS digital devices are connected to other SIMPLIS digital devices, the I/O voltages will be a logical 1 or 0.

Common Errors

If a required ground reference for a SIMPLIS digital device is missing, an error message is generated and the simulation stops. If the ground reference for U4 is removed from the first example, the error message would be as follows:

*** ERRORS REPORTED BY SIMPLIS ***
****************************************
<<<<<<<< Error Message ID: 5045 >>>>>>>>
  
The device
    X$U4.A_BUF
has at least one analog output node
but it does not have a gnd ref pin.
  
*** END SIMPLIS ERROR REPORT ***

If you see this error message in your circuit, follow these steps to correct the errors:

  1. Find the devices identified in the error messages on your schematic; in this example X$U4 is the device with reference U4.
  2. For each device listed in the error message:
    1. Double click on the symbol to bring up the edit dialog.
    2. Select Y for the Ground Ref parameter.
      Result: The symbol will be redrawn using the Ground Ref pin.
    3. Add a ground symbol to the Ground Ref pin.