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, 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

Free to use (See 1)

5

xt_highint5

Normally used by ESP-IDF debug logic (See 1)

NMI

xt_nmi

Free to use

dbg

xt_debugexception

Debug exception. Called on e.g. a BREAK instruction. (See 2)

The following notes give more information about the items in the tables above.

  1. ESP-IDF debug logic can be configured to run on xt_highint4 or xt_highint5 in CONFIG_ESP_SYSTEM_CHECK_INT_LEVEL. Bluetooth’s interrupt can be configured to run on level 4 by enabling CONFIG_BTDM_CTRL_HLI. If CONFIG_BTDM_CTRL_HLI is enabled, ESP-IDF debug logic must be running on level 5 interrupt.

  2. If CONFIG_BTDM_CTRL_HLI is enabled, xt_debugexception is used to fix live lock issue in ESP32 ECO3.

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/esp32/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.

    When CONFIG_BTDM_CTRL_HLI is enabled, C code is also called from a high-level interrupt, this is possible thanks to some additional protection added to it.

  • 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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