Inject vs Put

[quekyj]

When is a suitable time to use inject() and put()? Any rules of thumb that can be followed? It's seems to be that put() alone is enough for testing.

[mkorbel1]

First, to clarify the difference between the two:

• put places a value on a signal immediately
• inject is equivalent to calling put inside of a Simulator.injectAction call

A put on a signal will propagate changes to any combinational logic "instantly" and synchronously, without any involvement from the Simulator. An inject will only register the put to occur at the next opportunity at the current Simulator.time while the Simulator is actively running. The implication is that anything relying on events from the Simulator to behave properly will behave differently on put vs inject.

One key place this is different is on any sequential logic (Sequential, FlipFlop, etc.). The Simulator must be involved to properly understand how to sample transitions that happen before vs. after a clock edge. Modifying a clock into sequential logic with put will not trigger an edge, but modifying a clock during a running simulation will trigger a clock edge. If you're modifying a clock or input to sequential logic during a simulation, you probably want to use inject.

Another case is in testbench or model logic interacting with hardware. There's often code like await clk.nextPosedge in these cases, and you often want to drive some change at the current timestep during simulation upon those events. In these cases, inject makes sense.

A third example is the WaveDumper, which relies on signals changed event, which is calculated using the Simulator phases. If you just call put, depending on when it's called, the WaveDumper may not record the signal change in the proper timestamp in the waves.

I think a good rule of thumb is:

• If you're modelling combinational logic (e.g. custom gate) or interacting with combinational logic, then put is convenient and you don't need to use the Simulator at all.
• If you're running the Simulator and want to interact with a design over simulation time, you probably want to use inject. Especially if you're collecting waves, have sequential logic, or are modelling stateful behavior. Testbenches, BFMs, models, etc. that run over time in a simulation usually should use inject.