# High Priority Interrupts

[中文]

The Xtensa architecture supports 32 interrupts, divided over 7 priority levels from level 1 to 7, with level 7 being an non-maskable interrupt (NMI), plus an assortment of exceptions. On the ESP32-S2, the Interrupt Allocation can route most interrupt sources to these interrupts via the interrupt mux. Normally, interrupts are written in C, but ESP-IDF allows high-priority interrupts to be written in assembly as well, resulting in very low interrupt latencies.

## Interrupt Priorities

Priority Level | Symbol | Remark
--- | --- | ---
1 | N/A | Exception and low priority interrupts, handled by ESP-IDF.
2-3 | N/A | Medium priority interrupts, handled by ESP-IDF.
4 | xt_highint4 | Normally used by ESP-IDF debug logic.
5 | xt_highint5 | High priority interrupts, free to use.
NMI | xt_nmi | Non-maskable interrupt, free to use.
dbg | xt_debugexception | Debug exception. Called on e.g., a BREAK instruction.

Using these symbols is done by creating an assembly file with suffix `.S` and defining the named symbols, like this:

        .section .iram1,"ax"
        .global     xt_highint4
        .type       xt_highint4,@function
        .align      4
    xt_highint5:
        ... your code here
        rsr     a0, EXCSAVE_5
        rfi     5

For a real-life example, see the esp_system/port/soc/esp32s2/highint_hdl.S file; the panic handler interrupt is implemented there.

## Notes

- Do not call C code from a high-priority interrupt; as these interrupts are run from a critical section, this can cause the target to crash. Note that although the panic handler interrupt does call normal C code, this exception is allowed due to the fact that this handler never returns (i.e., the application does not continue to run after the panic handler), so breaking C code execution flow is not a problem.

- Make sure your assembly code gets linked in. Indeed, as the free-to-use symbols are declared as weak, the linker may discard the file containing the symbol. This happens if the only symbol defined, or used from the user file is the `xt_*` free-to-use symbol. To avoid this, in the assembly file containing the `xt_*` symbol, define another symbol, like:

    .global ld_include_my_isr_file
    ld_include_my_isr_file:

Here it is called `ld_include_my_isr_file` but can have any name, as long as it is not defined anywhere else in the project.

Then, in the component `CMakeLists.txt`, add this name as an unresolved symbol to the ld command line arguments:

    target_link_libraries(${COMPONENT_TARGET} "-u ld_include_my_isr_file")

This will ensure the linker to always includes the file defining `ld_include_my_isr_file`, so that the ISR is always linked.

- High-priority interrupts can be routed and handled using `esp_intr_alloc()` and associated functions. However, the handler and handler arguments to `esp_intr_alloc()` must be NULL.

- In theory, medium priority interrupts could also be handled in this way. ESP-IDF does not support this yet.

- To check Xtensa instruction set architecture (ISA), please refer to Xtensa ISA Summary.

See system/nmi_isr for an example of how to implement a custom NMI handler on Xtensa-based targets.

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