diff --git a/crates/fayalite/src/sim.rs b/crates/fayalite/src/sim.rs
index c2cbea4..4c20644 100644
--- a/crates/fayalite/src/sim.rs
+++ b/crates/fayalite/src/sim.rs
@@ -55,13 +55,17 @@ use petgraph::{
 use std::{
     any::Any,
     borrow::Cow,
+    cell::RefCell,
     collections::BTreeSet,
     fmt,
     future::{Future, IntoFuture},
     marker::PhantomData,
     mem,
     ops::IndexMut,
+    pin::Pin,
+    rc::Rc,
     sync::Arc,
+    task::Poll,
 };
 
 mod interpreter;
@@ -6755,10 +6759,35 @@ impl SimulationModuleState {
     }
 }
 
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+enum WaitTarget {
+    /// Settle is less than Instant
+    Settle,
+    Instant(SimInstant),
+}
+
+impl PartialOrd for WaitTarget {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for WaitTarget {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        match (self, other) {
+            (WaitTarget::Settle, WaitTarget::Settle) => std::cmp::Ordering::Equal,
+            (WaitTarget::Settle, WaitTarget::Instant(_)) => std::cmp::Ordering::Less,
+            (WaitTarget::Instant(_), WaitTarget::Settle) => std::cmp::Ordering::Greater,
+            (WaitTarget::Instant(l), WaitTarget::Instant(r)) => l.cmp(r),
+        }
+    }
+}
+
 struct SimulationExternModuleState {
     module_state: SimulationModuleState,
     sim: ExternModuleSimulation<Bundle>,
-    running_generator: Option<Box<dyn Future<Output = ()> + 'static>>,
+    running_generator: Option<Pin<Box<dyn Future<Output = ()> + 'static>>>,
+    wait_target: Option<WaitTarget>,
 }
 
 impl fmt::Debug for SimulationExternModuleState {
@@ -6767,6 +6796,7 @@ impl fmt::Debug for SimulationExternModuleState {
             module_state,
             sim,
             running_generator,
+            wait_target,
         } = self;
         f.debug_struct("SimulationExternModuleState")
             .field("module_state", module_state)
@@ -6775,6 +6805,7 @@ impl fmt::Debug for SimulationExternModuleState {
                 "running_generator",
                 &running_generator.as_ref().map(|_| DebugAsDisplay("...")),
             )
+            .field("wait_target", wait_target)
             .finish()
     }
 }
@@ -6782,7 +6813,7 @@ impl fmt::Debug for SimulationExternModuleState {
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 enum WhichModule {
     Main,
-    Extern { index: usize },
+    Extern { module_index: usize },
 }
 
 struct ReadBitFn {
@@ -6855,11 +6886,19 @@ impl MaybeNeedsSettleFn<&'_ mut interpreter::State> for ReadFn {
     }
 }
 
+struct GeneratorWaker;
+
+impl std::task::Wake for GeneratorWaker {
+    fn wake(self: Arc<Self>) {
+        panic!("can't await other kinds of futures in function passed to ExternalModuleSimulation");
+    }
+}
+
 struct SimulationImpl {
     state: interpreter::State,
     io: Expr<Bundle>,
     main_module: SimulationModuleState,
-    extern_modules: Vec<SimulationExternModuleState>,
+    extern_modules: Box<[SimulationExternModuleState]>,
     needs_settle: bool,
     trace_decls: TraceModule,
     traces: SimTraces<Box<[SimTrace<SimTraceKind, BitVec>]>>,
@@ -6868,6 +6907,7 @@ struct SimulationImpl {
     instant: SimInstant,
     clocks_triggered: Interned<[StatePartIndex<StatePartKindSmallSlots>]>,
     breakpoints: Option<BreakpointsSet>,
+    generator_waker: std::task::Waker,
 }
 
 impl fmt::Debug for SimulationImpl {
@@ -6891,6 +6931,7 @@ impl SimulationImpl {
             instant,
             clocks_triggered,
             breakpoints: _,
+            generator_waker: _,
         } = self;
         f.debug_struct("Simulation")
             .field("state", state)
@@ -6908,6 +6949,8 @@ impl SimulationImpl {
     }
     fn new(compiled: Compiled<Bundle>) -> Self {
         let io_target = Target::from(compiled.io);
+        // TODO: add extern_modules
+        let extern_modules: Box<[SimulationExternModuleState]> = Box::new([]);
         Self {
             state: State::new(compiled.insns),
             io: compiled.io.to_expr(),
@@ -6928,8 +6971,7 @@ impl SimulationImpl {
                         )
                     }),
             ),
-            // TODO: add extern_modules
-            extern_modules: vec![],
+            extern_modules,
             needs_settle: true,
             trace_decls: compiled.base_module.trace_decls,
             traces: SimTraces(Box::from_iter(compiled.traces.0.iter().map(
@@ -6948,6 +6990,7 @@ impl SimulationImpl {
             instant: SimInstant::START,
             clocks_triggered: compiled.clocks_triggered,
             breakpoints: None,
+            generator_waker: Arc::new(GeneratorWaker).into(),
         }
     }
     fn write_traces<const ONLY_IF_CHANGED: bool>(
@@ -7086,10 +7129,11 @@ impl SimulationImpl {
         }
     }
     #[track_caller]
-    fn advance_time(&mut self, duration: SimDuration) {
-        self.settle();
-        self.instant += duration;
-        self.for_each_trace_writer_storing_error(|this, mut trace_writer_state| {
+    fn advance_time(this: &Rc<RefCell<Self>>, duration: SimDuration) {
+        Self::settle(this);
+        let mut this = this.borrow_mut();
+        this.instant += duration;
+        this.for_each_trace_writer_storing_error(|this, mut trace_writer_state| {
             match &mut trace_writer_state {
                 TraceWriterState::Decls(_) | TraceWriterState::Init(_) => unreachable!(),
                 TraceWriterState::Running(trace_writer) => {
@@ -7100,31 +7144,130 @@ impl SimulationImpl {
             Ok(trace_writer_state)
         });
     }
+    #[must_use]
+    fn yield_advance_time_or_settle(
+        this: Rc<RefCell<Self>>,
+        module_index: usize,
+        duration: Option<SimDuration>,
+    ) -> impl Future<Output = ()> + 'static {
+        struct MyGenerator {
+            sim: Rc<RefCell<SimulationImpl>>,
+            yielded_at_all: bool,
+            module_index: usize,
+            target: WaitTarget,
+        }
+        impl Future for MyGenerator {
+            type Output = ();
+
+            fn poll(
+                mut self: Pin<&mut Self>,
+                cx: &mut std::task::Context<'_>,
+            ) -> Poll<Self::Output> {
+                let this = &mut *self;
+                let yielded_at_all = mem::replace(&mut this.yielded_at_all, true);
+                let mut sim = this.sim.borrow_mut();
+                let sim = &mut *sim;
+                assert!(cx.waker().will_wake(&sim.generator_waker), "can't use ExternModuleSimulationState's methods outside of ExternModuleSimulation");
+                if let WaitTarget::Instant(target) = this.target {
+                    if target < sim.instant {
+                        this.target = WaitTarget::Settle;
+                    } else if yielded_at_all && target == sim.instant {
+                        this.target = WaitTarget::Settle;
+                    }
+                }
+                if let WaitTarget::Settle = this.target {
+                    if yielded_at_all {
+                        return Poll::Ready(());
+                    }
+                }
+                let wait_target = sim.extern_modules[this.module_index]
+                    .wait_target
+                    .get_or_insert(this.target);
+                *wait_target = (*wait_target).min(this.target);
+                Poll::Pending
+            }
+        }
+        let target = duration.map_or(WaitTarget::Settle, |duration| {
+            WaitTarget::Instant(this.borrow().instant + duration)
+        });
+        MyGenerator {
+            sim: this,
+            yielded_at_all: false,
+            module_index,
+            target,
+        }
+    }
+    fn get_ready_external_modules(&mut self, run_list: &mut Vec<usize>) -> Option<WaitTarget> {
+        run_list.clear();
+        let mut iter = self.extern_modules.iter_mut().enumerate().filter_map(
+            |(module_index, extern_module)| Some((module_index, extern_module.wait_target?)),
+        );
+        let (first_module_index, mut wait_target) = iter.next()?;
+        run_list.push(first_module_index);
+        for (next_module_index, next_wait_target) in iter {
+            match next_wait_target.cmp(&wait_target) {
+                std::cmp::Ordering::Less => run_list.clear(),
+                std::cmp::Ordering::Equal => {}
+                std::cmp::Ordering::Greater => continue,
+            }
+            run_list.push(next_module_index);
+            wait_target = next_wait_target;
+        }
+        Some(wait_target)
+    }
     #[track_caller]
-    fn settle(&mut self) {
+    fn settle(this_ref: &Rc<RefCell<Self>>) {
+        let mut this = this_ref.borrow_mut();
         assert!(
-            self.main_module.uninitialized_ios.is_empty(),
+            this.main_module.uninitialized_ios.is_empty(),
             "didn't initialize all inputs",
         );
+        let mut run_list = Vec::new();
+        let generator_waker = this.generator_waker.clone();
         for _ in 0..100000 {
-            if !self.needs_settle {
+            if !this.needs_settle {
+                if let Some(wait_target) = this.get_ready_external_modules(&mut run_list) {
+                    if wait_target > WaitTarget::Instant(this.instant) {
+                        return;
+                    }
+                    for module_index in run_list.drain(..) {
+                        this.extern_modules[module_index].wait_target = None;
+                        let Some(mut generator) =
+                            this.extern_modules[module_index].running_generator.take()
+                        else {
+                            continue;
+                        };
+                        drop(this);
+                        let generator = match generator
+                            .as_mut()
+                            .poll(&mut std::task::Context::from_waker(&generator_waker))
+                        {
+                            Poll::Ready(()) => None,
+                            Poll::Pending => Some(generator),
+                        };
+                        this = this_ref.borrow_mut();
+                        this.extern_modules[module_index].running_generator = generator;
+                    }
+                    continue;
+                }
                 return;
             }
-            self.state.setup_call(0);
-            if self.breakpoints.is_some() {
+            this.state.setup_call(0);
+            if this.breakpoints.is_some() {
                 loop {
-                    match self
+                    let this = &mut *this;
+                    match this
                         .state
-                        .run(self.breakpoints.as_mut().expect("just checked"))
+                        .run(this.breakpoints.as_mut().expect("just checked"))
                     {
                         RunResult::Break(break_action) => {
                             println!(
                                 "hit breakpoint at:\n{:?}",
-                                self.state.debug_insn_at(self.state.pc),
+                                this.state.debug_insn_at(this.state.pc),
                             );
                             match break_action {
                                 BreakAction::DumpStateAndContinue => {
-                                    println!("{self:#?}");
+                                    println!("{this:#?}");
                                 }
                                 BreakAction::Continue => {}
                             }
@@ -7133,21 +7276,21 @@ impl SimulationImpl {
                     }
                 }
             } else {
-                let RunResult::Return(()) = self.state.run(());
+                let RunResult::Return(()) = this.state.run(());
             }
-            if self.main_module.did_initial_settle {
-                self.read_traces::<false>();
+            if this.main_module.did_initial_settle {
+                this.read_traces::<false>();
             } else {
-                self.read_traces::<true>();
+                this.read_traces::<true>();
             }
-            self.state.memory_write_log.sort_unstable();
-            self.state.memory_write_log.dedup();
-            self.main_module.did_initial_settle = true;
-            self.needs_settle = self
+            this.state.memory_write_log.sort_unstable();
+            this.state.memory_write_log.dedup();
+            this.main_module.did_initial_settle = true;
+            this.needs_settle = this
                 .clocks_triggered
                 .iter()
-                .any(|i| self.state.small_slots[*i] != 0);
-            self.for_each_trace_writer_storing_error(|this, trace_writer_state| {
+                .any(|i| this.state.small_slots[*i] != 0);
+            this.for_each_trace_writer_storing_error(|this, trace_writer_state| {
                 Ok(match trace_writer_state {
                     TraceWriterState::Decls(trace_writer_decls) => TraceWriterState::Running(
                         this.init_trace_writer(trace_writer_decls.write_decls(
@@ -7165,20 +7308,22 @@ impl SimulationImpl {
                     TraceWriterState::Errored(e) => TraceWriterState::Errored(e),
                 })
             });
-            self.state.memory_write_log.clear();
+            this.state.memory_write_log.clear();
         }
         panic!("settle(): took too many steps");
     }
     fn get_module(&self, which_module: WhichModule) -> &SimulationModuleState {
         match which_module {
             WhichModule::Main => &self.main_module,
-            WhichModule::Extern { index } => &self.extern_modules[index].module_state,
+            WhichModule::Extern { module_index } => &self.extern_modules[module_index].module_state,
         }
     }
     fn get_module_mut(&mut self, which_module: WhichModule) -> &mut SimulationModuleState {
         match which_module {
             WhichModule::Main => &mut self.main_module,
-            WhichModule::Extern { index } => &mut self.extern_modules[index].module_state,
+            WhichModule::Extern { module_index } => {
+                &mut self.extern_modules[module_index].module_state
+            }
         }
     }
     #[track_caller]
@@ -7374,14 +7519,31 @@ impl SimulationImpl {
         );
     }
     #[track_caller]
-    fn settle_if_needed<F, O>(&mut self, v: MaybeNeedsSettle<F, O>) -> O
+    fn settle_if_needed<F, O>(this_ref: &Rc<RefCell<Self>>, v: MaybeNeedsSettle<F, O>) -> O
     where
         for<'a> F: MaybeNeedsSettleFn<&'a mut interpreter::State, Output = O>,
     {
         match v {
             MaybeNeedsSettle::NeedsSettle(v) => {
-                self.settle();
-                v.call(&mut self.state)
+                Self::settle(this_ref);
+                v.call(&mut this_ref.borrow_mut().state)
+            }
+            MaybeNeedsSettle::NoSettleNeeded(v) => v,
+        }
+    }
+    #[track_caller]
+    async fn yield_settle_if_needed<F, O>(
+        this_ref: &Rc<RefCell<Self>>,
+        module_index: usize,
+        v: MaybeNeedsSettle<F, O>,
+    ) -> O
+    where
+        for<'a> F: MaybeNeedsSettleFn<&'a mut interpreter::State, Output = O>,
+    {
+        match v {
+            MaybeNeedsSettle::NeedsSettle(v) => {
+                Self::yield_advance_time_or_settle(this_ref.clone(), module_index, None).await;
+                v.call(&mut this_ref.borrow_mut().state)
             }
             MaybeNeedsSettle::NoSettleNeeded(v) => v,
         }
@@ -7455,17 +7617,17 @@ impl SimulationImpl {
         self.trace_writers = trace_writers;
         retval
     }
-    fn close(mut self) -> std::io::Result<()> {
-        if self.main_module.did_initial_settle {
-            self.settle();
+    fn close(this: Rc<RefCell<Self>>) -> std::io::Result<()> {
+        if this.borrow().main_module.did_initial_settle {
+            Self::settle(&this);
         }
-        self.close_all_trace_writers()
+        this.borrow_mut().close_all_trace_writers()
     }
-    fn flush_traces(&mut self) -> std::io::Result<()> {
-        if self.main_module.did_initial_settle {
-            self.settle();
+    fn flush_traces(this_ref: &Rc<RefCell<Self>>) -> std::io::Result<()> {
+        if this_ref.borrow().main_module.did_initial_settle {
+            Self::settle(this_ref);
         }
-        self.for_each_trace_writer_getting_error(
+        this_ref.borrow_mut().for_each_trace_writer_getting_error(
             |this, trace_writer: TraceWriterState<DynTraceWriterDecls>| match trace_writer {
                 TraceWriterState::Decls(v) => {
                     let mut v = v.write_decls(
@@ -7499,7 +7661,7 @@ impl Drop for SimulationImpl {
 }
 
 pub struct Simulation<T: BundleType> {
-    sim_impl: SimulationImpl,
+    sim_impl: Rc<RefCell<SimulationImpl>>,
     io: Expr<T>,
 }
 
@@ -7546,24 +7708,113 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for SortedMapDebug<'_, K, V> {
 impl<T: BundleType> fmt::Debug for Simulation<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let Self { sim_impl, io } = self;
-        sim_impl.debug_fmt(Some(io), f)
+        sim_impl.borrow().debug_fmt(Some(io), f)
     }
 }
 
+macro_rules! impl_simulation_methods {
+    (async_await = ($($async:tt, $await:tt)?)) => {
+        #[track_caller]
+        pub $($async)? fn read_bool_or_int<I: BoolOrIntType>(&mut self, io: Expr<I>) -> I::Value {
+            let retval = self
+                .sim_impl
+                .borrow_mut()
+                .read_bool_or_int(io, WhichModule::Main);
+            self.settle_if_needed(retval)$(.$await)?
+        }
+        #[track_caller]
+        pub $($async)? fn write_bool_or_int<I: BoolOrIntType>(
+            &mut self,
+            io: Expr<I>,
+            value: impl ToExpr<Type = I>,
+        ) {
+            let value = value.to_expr();
+            assert_eq!(Expr::ty(io), Expr::ty(value), "type mismatch");
+            let value = value
+                .to_literal_bits()
+                .expect("the value that is being written to an input must be a literal");
+            self.sim_impl.borrow_mut().write_bool_or_int(
+                io,
+                I::bits_to_value(Cow::Borrowed(&value)),
+                WhichModule::Main,
+            );
+        }
+        #[track_caller]
+        pub $($async)? fn write_clock(&mut self, io: Expr<Clock>, value: bool) {
+            self.sim_impl
+                .borrow_mut()
+                .write_bit(Expr::canonical(io), value, WhichModule::Main);
+        }
+        #[track_caller]
+        pub $($async)? fn read_clock(&mut self, io: Expr<Clock>) -> bool {
+            let retval = self
+                .sim_impl
+                .borrow_mut()
+                .read_bit(Expr::canonical(io), WhichModule::Main);
+            self.settle_if_needed(retval)$(.$await)?
+        }
+        #[track_caller]
+        pub $($async)? fn write_bool(&mut self, io: Expr<Bool>, value: bool) {
+            self.sim_impl
+                .borrow_mut()
+                .write_bit(Expr::canonical(io), value, WhichModule::Main);
+        }
+        #[track_caller]
+        pub $($async)? fn read_bool(&mut self, io: Expr<Bool>) -> bool {
+            let retval = self
+                .sim_impl
+                .borrow_mut()
+                .read_bit(Expr::canonical(io), WhichModule::Main);
+            self.settle_if_needed(retval)$(.$await)?
+        }
+        #[track_caller]
+        pub $($async)? fn write_reset<R: ResetType>(&mut self, io: Expr<R>, value: bool) {
+            self.sim_impl
+                .borrow_mut()
+                .write_bit(Expr::canonical(io), value, WhichModule::Main);
+        }
+        #[track_caller]
+        pub $($async)? fn read_reset<R: ResetType>(&mut self, io: Expr<R>) -> bool {
+            let retval = self
+                .sim_impl
+                .borrow_mut()
+                .read_bit(Expr::canonical(io), WhichModule::Main);
+            self.settle_if_needed(retval)$(.$await)?
+        }
+        #[track_caller]
+        pub $($async)? fn read<IO: Type>(&mut self, io: Expr<IO>) -> SimValue<IO> {
+            let retval = self
+                .sim_impl
+                .borrow_mut()
+                .read(Expr::canonical(io), WhichModule::Main);
+            SimValue::from_canonical(self.settle_if_needed(retval)$(.$await)?)
+        }
+        #[track_caller]
+        pub $($async)? fn write<IO: Type, V: ToSimValue<IO>>(&mut self, io: Expr<IO>, value: V) {
+            self.sim_impl.borrow_mut().write(
+                Expr::canonical(io),
+                value.into_sim_value(Expr::ty(io)).into_canonical(),
+                WhichModule::Main,
+            );
+        }
+    };
+}
+
 impl<T: BundleType> Simulation<T> {
     pub fn new(module: Interned<Module<T>>) -> Self {
         Self::from_compiled(Compiled::new(module))
     }
     pub fn add_trace_writer<W: TraceWriterDecls>(&mut self, writer: W) {
         self.sim_impl
+            .borrow_mut()
             .trace_writers
             .push(TraceWriterState::Decls(DynTraceWriterDecls::new(writer)));
     }
     pub fn flush_traces(&mut self) -> std::io::Result<()> {
-        self.sim_impl.flush_traces()
+        SimulationImpl::flush_traces(&self.sim_impl)
     }
     pub fn close(self) -> std::io::Result<()> {
-        self.sim_impl.close()
+        SimulationImpl::close(self.sim_impl)
     }
     pub fn canonical(self) -> Simulation<Bundle> {
         let Self { sim_impl, io } = self;
@@ -7586,92 +7837,29 @@ impl<T: BundleType> Simulation<T> {
         let sim_impl = SimulationImpl::new(compiled.canonical());
         Self {
             io: Expr::from_bundle(sim_impl.io),
-            sim_impl,
+            sim_impl: Rc::new(RefCell::new(sim_impl)),
         }
     }
     #[track_caller]
     pub fn settle(&mut self) {
-        self.sim_impl.settle();
+        SimulationImpl::settle(&self.sim_impl);
     }
     #[track_caller]
     pub fn advance_time(&mut self, duration: SimDuration) {
-        self.sim_impl.advance_time(duration);
+        SimulationImpl::advance_time(&self.sim_impl, duration);
     }
     #[track_caller]
-    pub fn read_bool_or_int<I: BoolOrIntType>(&mut self, io: Expr<I>) -> I::Value {
-        let retval = self.sim_impl.read_bool_or_int(io, WhichModule::Main);
-        self.sim_impl.settle_if_needed(retval)
-    }
-    #[track_caller]
-    pub fn write_bool_or_int<I: BoolOrIntType>(
-        &mut self,
-        io: Expr<I>,
-        value: impl ToExpr<Type = I>,
-    ) {
-        let value = value.to_expr();
-        assert_eq!(Expr::ty(io), Expr::ty(value), "type mismatch");
-        let value = value
-            .to_literal_bits()
-            .expect("the value that is being written to an input must be a literal");
-        self.sim_impl.write_bool_or_int(
-            io,
-            I::bits_to_value(Cow::Borrowed(&value)),
-            WhichModule::Main,
-        );
-    }
-    #[track_caller]
-    pub fn write_clock(&mut self, io: Expr<Clock>, value: bool) {
-        self.sim_impl
-            .write_bit(Expr::canonical(io), value, WhichModule::Main);
-    }
-    #[track_caller]
-    pub fn read_clock(&mut self, io: Expr<Clock>) -> bool {
-        let retval = self
-            .sim_impl
-            .read_bit(Expr::canonical(io), WhichModule::Main);
-        self.sim_impl.settle_if_needed(retval)
-    }
-    #[track_caller]
-    pub fn write_bool(&mut self, io: Expr<Bool>, value: bool) {
-        self.sim_impl
-            .write_bit(Expr::canonical(io), value, WhichModule::Main);
-    }
-    #[track_caller]
-    pub fn read_bool(&mut self, io: Expr<Bool>) -> bool {
-        let retval = self
-            .sim_impl
-            .read_bit(Expr::canonical(io), WhichModule::Main);
-        self.sim_impl.settle_if_needed(retval)
-    }
-    #[track_caller]
-    pub fn write_reset<R: ResetType>(&mut self, io: Expr<R>, value: bool) {
-        self.sim_impl
-            .write_bit(Expr::canonical(io), value, WhichModule::Main);
-    }
-    #[track_caller]
-    pub fn read_reset<R: ResetType>(&mut self, io: Expr<R>) -> bool {
-        let retval = self
-            .sim_impl
-            .read_bit(Expr::canonical(io), WhichModule::Main);
-        self.sim_impl.settle_if_needed(retval)
-    }
-    #[track_caller]
-    pub fn read<IO: Type>(&mut self, io: Expr<IO>) -> SimValue<IO> {
-        let retval = self.sim_impl.read(Expr::canonical(io), WhichModule::Main);
-        SimValue::from_canonical(self.sim_impl.settle_if_needed(retval))
-    }
-    #[track_caller]
-    pub fn write<IO: Type, V: ToSimValue<IO>>(&mut self, io: Expr<IO>, value: V) {
-        self.sim_impl.write(
-            Expr::canonical(io),
-            value.into_sim_value(Expr::ty(io)).into_canonical(),
-            WhichModule::Main,
-        );
+    fn settle_if_needed<F, O>(&mut self, v: MaybeNeedsSettle<F, O>) -> O
+    where
+        for<'a> F: MaybeNeedsSettleFn<&'a mut interpreter::State, Output = O>,
+    {
+        SimulationImpl::settle_if_needed(&self.sim_impl, v)
     }
+    impl_simulation_methods!(async_await = ());
     #[doc(hidden)]
     /// This is explicitly unstable and may be changed/removed at any time
     pub fn set_breakpoints_unstable(&mut self, pcs: HashSet<usize>, trace: bool) {
-        self.sim_impl.breakpoints = Some(BreakpointsSet {
+        self.sim_impl.borrow_mut().breakpoints = Some(BreakpointsSet {
             last_was_break: false,
             set: pcs,
             trace,
@@ -7679,22 +7867,74 @@ impl<T: BundleType> Simulation<T> {
     }
 }
 
-#[derive(Debug)]
 pub struct ExternModuleSimulationState<T: BundleType> {
+    sim_impl: Rc<RefCell<SimulationImpl>>,
+    module_index: usize,
     io_ty: T,
 }
 
+impl<T: BundleType> fmt::Debug for ExternModuleSimulationState<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self {
+            sim_impl: _,
+            module_index,
+            io_ty,
+        } = self;
+        f.debug_struct("ExternModuleSimulationState")
+            .field("sim_impl", &DebugAsDisplay("..."))
+            .field("module_index", module_index)
+            .field("io_ty", io_ty)
+            .finish()
+    }
+}
+
 impl<T: BundleType> ExternModuleSimulationState<T> {
     pub fn canonical(self) -> ExternModuleSimulationState<Bundle> {
+        let Self {
+            sim_impl,
+            module_index,
+            io_ty,
+        } = self;
         ExternModuleSimulationState {
-            io_ty: Bundle::from_canonical(self.io_ty.canonical()),
+            sim_impl,
+            module_index,
+            io_ty: Bundle::from_canonical(io_ty.canonical()),
         }
     }
     pub fn from_canonical(sim: ExternModuleSimulationState<Bundle>) -> Self {
+        let ExternModuleSimulationState {
+            sim_impl,
+            module_index,
+            io_ty,
+        } = sim;
         Self {
-            io_ty: T::from_canonical(sim.io_ty.canonical()),
+            sim_impl,
+            module_index,
+            io_ty: T::from_canonical(io_ty.canonical()),
         }
     }
+    #[track_caller]
+    pub async fn settle(&mut self) {
+        SimulationImpl::yield_advance_time_or_settle(self.sim_impl.clone(), self.module_index, None)
+            .await
+    }
+    #[track_caller]
+    pub async fn advance_time(&mut self, duration: SimDuration) {
+        SimulationImpl::yield_advance_time_or_settle(
+            self.sim_impl.clone(),
+            self.module_index,
+            Some(duration),
+        )
+        .await
+    }
+    #[track_caller]
+    async fn settle_if_needed<F, O>(&mut self, v: MaybeNeedsSettle<F, O>) -> O
+    where
+        for<'a> F: MaybeNeedsSettleFn<&'a mut interpreter::State, Output = O>,
+    {
+        SimulationImpl::yield_settle_if_needed(&self.sim_impl, self.module_index, v).await
+    }
+    impl_simulation_methods!(async_await = (async, await));
 }
 
 pub trait ExternModuleSimGenerator: