Simulations with Instances

[rroohhh]

To make simulations of designs tightly integrated with fpga specific resources (like the PS7 on zynq) easier it would be nice to have some way to overwrite signals driven from / into Instance's.

[whitequark]

Yep, this is planned with the next round of simulator improvements. I'm not sure if replacing Instances with simulation models is the best way to address your specific problem (it could be), but it'll be possible.

[rroohhh]

For my specifc problem it would be helpful to drive Instance io's from the simulator (for example to initiate AXI reads / writes coming from the PS7) and being able substitue FHDL simulation models.

Furthermore it would be nice to be able to create clocks from the simulation models to make it posible to simulate things with PLL's.

[whitequark]

For my specifc problem it would be helpful to drive Instance io's from the simulator (for example to initiate AXI reads / writes coming from the PS7) and being able substitue FHDL simulation models.

Ah, so essentially you'd like the simulator to treat the ports of an unknown Instance as toplevel inputs and outputs, right?

Furthermore it would be nice to be able to create clocks from the simulation models to make it posible to simulate things with PLL's.

You can already do that!

[rroohhh]

Ah, so essentially you'd like the simulator to treat the ports of an unknown Instance as toplevel inputs and outputs, right?

Correct. (or disconnect them, so the signals connected to the ports can be driven directly)

Furthermore it would be nice to be able to create clocks from the simulation models to make it posible to simulate things with PLL's.

You can already do that!

How?

[whitequark]

(or disconnect them, so the signals connected to the ports can be driven directly)

That's the same thing. Toplevel ports currently are treated as disconnected.

[whitequark]

How?

Can you describe an example of what you want to do with a PLL?

[rroohhh]

Say I have a instance like this:

Instance("MMCME2_ADV",
    p_BANDWIDTH="OPTIMIZED",
    i_RST=mmcm_reset, o_LOCKED=mmcm_locked,

    p_REF_JITTER1=0.01, p_CLKIN1_PERIOD=10.0,
    p_CLKFBOUT_MULT_F=30.0, p_CLKFBOUT_PHASE=0.000, p_DIVCLK_DIVIDE=2,
    i_CLKIN1=ClockSignal("clk100"), i_CLKFBIN=mmcm_fb, o_CLKFBOUT=mmcm_fb,

    p_CLKOUT0_DIVIDE_F=10.0, p_CLKOUT0_PHASE=0.000, o_CLKOUT0=ClockSignal("mmcm_clk1"),
    p_CLKOUT1_DIVIDE=2, p_CLKOUT1_PHASE=0.000, o_CLKOUT1=ClockSignal("mmcm_clk2"))

I would like the mmcm_clk{1,2} domains to be work (without manually doing add_clock in my testbench).

[whitequark]

Are the derived clocks an integer fraction of the input clock?

[rroohhh]

No and they could even have phase shifts.

[whitequark]

OK, so the only way this could possibly work (other than implementing an actual simulated PLL) is if the implementation of the PLL instance in nmigen would be able to magically request the clock rate at its input port and then drive the output ports appropriately. There's currently no provision for this in nMigen and I'm not aware of other simulators providing similar capabilities.

That doesn't mean though that we can't implement this, but I'm not sure what the API would look like.

[rroohhh]

Yes thats what I meant by

Furthermore it would be nice to be able to create clocks from the simulation models to make it posible to simulate things with PLL's.

sorry that that wasn't clear.

API wise, mabye a special signal type could work, like this:

clock = SimulatedClock(frequency=1e6, phase=0)
m.d.comb += self.clkout0.eq(clock)

[whitequark]

I'm not really sure how that helps?

[rroohhh]

Then maybe I misunderstood this:

[...], but I'm not sure what the API would look like.

I am not sure what you have planned for the simulation models that would replace instance's in a simulation, but I was assuming they would just be normal nmigen modules, so you could write something like this:

class MMCMSimModel:
    def __init__(self, **params):
        self.clkout0 = Signal()
    def elaborate(self, plat):e
        clock = SimulatedClock(frequency=1e6, phase=0)
        m.d.comb += self.clkout0.eq(clock)

(Obviously a real simulation model for the MMCM would not be this simple)

[whitequark]

I was assuming they would just be normal nmigen modules, so you could write something like this:

Ah right so if your Instance can be replaced with a normal nMigen module then absolutely nothing special needs to be done because you can wrap it with a module that, in its elaborate method, would return Instance for a real platform and some simulation-only code otherwise. So there isn't anything to be done for that case.

I am not sure what you have planned for the simulation models that would replace instance's in a simulation

The plan was to make it possible to attach processes to instances that would be automatically added to the simulator whenever that instance is used. I.e. black boxes implemented in Python.

        clock = SimulatedClock(frequency=1e6, phase=0)
        m.d.comb += self.clkout0.eq(clock)

This wouldn't really work because you have to get the frequency of the input clock somehow.