High Priority Interrupts

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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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