embassy/docs/modules/ROOT/pages/faq.adoc

= Frequently Asked Questions

These are a list of unsorted, commonly asked questions and answers.

Please feel free to add items to link:https://github.com/embassy-rs/embassy/edit/main/docs/modules/ROOT/pages/faq.adoc[this page], especially if someone in the chat answered a question for you!

== How to deploy to RP2040 without a debugging probe.

Install link:https://github.com/JoNil/elf2uf2-rs[elf2uf2-rs] for converting the generated elf binary into a uf2 file.

Configure the runner to use this tool, add this to `.cargo/config.toml`:
[source,toml]
----
[target.'cfg(all(target_arch = "arm", target_os = "none"))']
runner = "elf2uf2-rs --deploy --serial --verbose"
----

The command-line parameters `--deploy` will detect your device and upload the binary, `--serial` starts a serial connection. See the documentation for more info.

== Missing main macro

If you see an error like this:

[source,rust]
----
#[embassy_executor::main]
|                   ^^^^ could not find `main` in `embassy_executor`
----

You are likely missing some features of the `embassy-executor` crate.

For Cortex-M targets, consider making sure that ALL of the following features are active in your `Cargo.toml` for the `embassy-executor` crate:

* `arch-cortex-m`
* `executor-thread`
* `nightly`

For Xtensa ESP32, consider using the executors and `#[main]` macro provided by your appropriate link:https://crates.io/crates/esp-hal-common[HAL crate].

== Why is my binary so big?

The first step to managing your binary size is to set up your link:https://doc.rust-lang.org/cargo/reference/profiles.html[profiles].

[source,toml]
----
[profile.release]
debug = false
lto = true
opt-level = "s"
incremental = true
----

All of these flags are elaborated on in the Rust Book page linked above.

=== My binary is still big... filled with `std::fmt` stuff!

This means your code is sufficiently complex that `panic!` invocation's formatting requirements could not be optimized out, despite your usage of `panic-halt` or `panic-reset`.

You can remedy this by adding the following to your `.cargo/config.toml`:

[source,toml]
----
[unstable]
build-std = ["core"]
build-std-features = ["panic_immediate_abort"]
----

This replaces all panics with a `UDF` (undefined) instruction.

Depending on your chipset, this will exhibit different behavior.

Refer to the spec for your chipset, but for `thumbv6m`, it results in a hardfault. Which can be configured like so:

[source,rust]
----
#[exception]
unsafe fn HardFault(_frame: &ExceptionFrame) -> ! {
    SCB::sys_reset() // <- you could do something other than reset
}
----

Refer to cortex-m's link:https://docs.rs/cortex-m-rt/latest/cortex_m_rt/attr.exception.html[exception handling] for more info.

== `embassy-time` throws linker errors

If you see linker error like this:

[source,text]
----
  = note: rust-lld: error: undefined symbol: _embassy_time_now
          >>> referenced by driver.rs:127 (src/driver.rs:127)
          >>>               embassy_time-846f66f1620ad42c.embassy_time.4f6a638abb75dd4c-cgu.0.rcgu.o:(embassy_time::driver::now::hefb1f99d6e069842) in archive Devel/Embedded/pogodyna/target/thumbv7em-none-eabihf/debug/deps/libembassy_time-846f66f1620ad42c.rlib

          rust-lld: error: undefined symbol: _embassy_time_allocate_alarm
          >>> referenced by driver.rs:134 (src/driver.rs:134)
          >>>               embassy_time-846f66f1620ad42c.embassy_time.4f6a638abb75dd4c-cgu.0.rcgu.o:(embassy_time::driver::allocate_alarm::hf5145b6bd46706b2) in archive Devel/Embedded/pogodyna/target/thumbv7em-none-eabihf/debug/deps/libembassy_time-846f66f1620ad42c.rlib

          rust-lld: error: undefined symbol: _embassy_time_set_alarm_callback
          >>> referenced by driver.rs:139 (src/driver.rs:139)
          >>>               embassy_time-846f66f1620ad42c.embassy_time.4f6a638abb75dd4c-cgu.0.rcgu.o:(embassy_time::driver::set_alarm_callback::h24f92388d96eafd2) in archive Devel/Embedded/pogodyna/target/thumbv7em-none-eabihf/debug/deps/libembassy_time-846f66f1620ad42c.rlib

          rust-lld: error: undefined symbol: _embassy_time_set_alarm
          >>> referenced by driver.rs:144 (src/driver.rs:144)
          >>>               embassy_time-846f66f1620ad42c.embassy_time.4f6a638abb75dd4c-cgu.0.rcgu.o:(embassy_time::driver::set_alarm::h530a5b1f444a6d5b) in archive Devel/Embedded/pogodyna/target/thumbv7em-none-eabihf/debug/deps/libembassy_time-846f66f1620ad42c.rlib
----

You probably need to enable a time driver for your HAL (not in `embassy-time`!). For example with `embassy-stm32`, you might need to enable `time-driver-any`:

[source,toml]
----
[dependencies.embassy-stm32]
version = "0.1.0"
features = [
    # ...
    "time-driver-any", # Add this line!
    # ...
]
----