High-Level Interrupts

The Xtensa architecture has support for 32 interrupts, divided over 7 levels (levels 1 to 7, with 7 being an NMI), plus an assortment of exceptions. On the ESP32-S2, the interrupt mux allows most interrupt sources to be routed to these interrupts using the interrupt allocator. Normally, interrupts will be written in C, but ESP-IDF allows high-level interrupts to be written in assembly as well, resulting in very low interrupt latencies.

Interrupt Levels

Level	Symbol	Remark
1	N/A	Exception and level 0 interrupts. Handled by ESP-IDF
2-3	N/A	Medium level interrupts. Handled by ESP-IDF
4	xt_highint4	Normally used by ESP-IDF debug logic
5	xt_highint5	Free to use
NMI	xt_nmi	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 (suffix .S) and defining the named symbols, like this:

    .section .iram1,"ax"
    .global     xt_highint5
    .type       xt_highint5,@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-level 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 will 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 will happen 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 should cause the linker to always include the file defining ``ld_include_my_isr_file``, causing the ISR to always be linked in.
  

• High-level interrupts can be routed and handled using esp_intr_alloc() and associated functions. The handler and handler arguments to esp_intr_alloc() must be NULL, however.

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

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