Start Your Engines: Modeling Current-Based Port Connections between Electrical and Real Number Modeling Modules

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In a mixed-signal simulation, the electrical signal modules and real number modeling (RNM) modules mostly have voltage-based ports. However, there may be still a few ports that need to model the current flow. Given that the fundamental properties for an electrical signal are very different  from that of a real signal, modeling current needs special attention.

Let’s look at a simple example to get a better understanding. Consider that there are two blocks, one of them being a driver and the other a receiver, and a current-based port connected to them. Irrespective of whether you use a RNM or electrical module on either the driver or the receiver side, the goal of the mixed-signal boundary modeling is to make sure that it always represents the same circuit. There can be four combination scenarios as shown below:

Figure 1: Using Real Number Modeling and Electrical Modules with a Current-based Port

If all of these four scenarios are to model the same electrical circuit, the electrical and RNM can be switched without any issues.

Now we need to assess whether to use a positive real number to represent the forward current flow or a negative real number to represent the forward current. The reason that we can’t use the electrical way to define the sign of the signal, like flow-in is positive and flow-out is negative, is that RNM doesn’t follow Kirchhoff’s Current Law (KCL). So, a positive number from a driver will be received as a positive number at the receiver. Therefore, the flow-in/flow-out definition doesn’t work with RNM—only the forward/backward works.

Let’s review what happens if we use a positive real number signal to represent the forward current flow. The following image shows the sign of each signal in the example discussed above:

Figure 2: Using a positive real number to represent the forward current flow

Let me explain the series of events that take place in this flow:

• In the beginning, both the blocks use electrical modules. As the current flows forward, we can see a negative sign electrical current on the output port of the driver and a positive sign on the input port of the receiver.
• Then, we switch the driver to use the RNM module. An R2E connect module is inserted, which converts the positive real signal into a negative electrical signal. So, the negative electrical signal represents the forward current flowing to the receiver.
• Next, we switch both blocks to RNM. The positive real signal is assigned to a positive real signal at the receiver.
• At last, we use the electrical block for the driver and RNM for the receiver. An E2R connect module is inserted, which converts a positive electrical signal (because current flows into the connect module) to a positive real signal and then assigns it to the RNM receiver. Now, all four scenarios are compatible and represent the same Silicon circuit.

We can also use a positive real number signal to represent the backward current flow. The analysis is similar:

Figure 3 Using positive real number to represent backward current flow

In either of the cases shown above, we can see one of the E2R/R2E will need to reverse the sign of the converted signal so that the entire modeling process is self-consistent.

These two examples show how to model two current-based ports connection. You may think what happens if there are three or more ports? After a simple analysis, the conclusion is we can’t model all scenarios self-consistently because of the lack of KCL in RNM.

For a current-based connection with multiple ports, we have to follow KCL:

Two ports’ connection can be considered as a special case as we can use reversal of signs to mimic KCL:

However, with three or more ports, the reverse methodology would not work:

Another problem with multiple ports is that there may be multiple receivers. The current flow needs to distribute between them in the current-based modeling instead of all receivers getting the same value in the voltage-based modeling.

To conclude, here are some guidelines to model the current with a real signal (wreal/SV-real):

• Modeling a current-based design block in wreal/SV-real depends on the relationship between the current flow direction and port direction definition.
• If defining a positive real signal value as representing the forward current flow direction, keep the following in mind while performing RNM modeling for the current-based port:
  • For the input port, the flow-in is positive and flow-out is negative
• • For the output port, flow-in is negative and flow-out is positive
• If defining a positive real signal value as representing the backward current flow direction, then a reversed conclusion is also self-consistent:
  • For the input port, the flow-in is negative and flow-out is positive
  • For the output port, the flow-in is positive and flow-out is negative

~Qingyu Lin

Related Resources

• One-Stop Knowledge Resource for Mixed-Signal Verification
• Spectre AMS Designer Product Page
• Spectre AMS Designer and Xcelium Simulator Mixed-Signal User Guide

For more information on Cadence circuit design products and services, visit www.cadence.com. 

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