Interrupt allocation

Overview

The ESP32 has two cores, with 32 interrupts each. Each interrupt has a certain priority level, most (but not all) interrupts are connected to the interrupt mux. Because there are more interrupt sources than interrupts, sometimes it makes sense to share an interrupt in multiple drivers. The esp_intr_alloc abstraction exists to hide all these implementation details.

A driver can allocate an interrupt for a certain peripheral by calling esp_intr_alloc (or esp_intr_alloc_sintrstatus). It can use the flags passed to this function to set the type of interrupt allocated, specifying a specific level or trigger method. The interrupt allocation code will then find an applicable interrupt, use the interrupt mux to hook it up to the peripheral, and install the given interrupt handler and ISR to it.

This code has two different types of interrupts it handles differently: Shared interrupts and non-shared interrupts. The simplest of the two are non-shared interrupts: a separate interrupt is allocated per esp_intr_alloc call and this interrupt is solely used for the peripheral attached to it, with only one ISR that will get called. Shared interrupts can have multiple peripherals triggering it, with multiple ISRs being called when one of the peripherals attached signals an interrupt. Thus, ISRs that are intended for shared interrupts should check the interrupt status of the peripheral they service in order to see if any action is required.

Non-shared interrupts can be either level- or edge-triggered. Shared interrupts can only be level interrupts (because of the chance of missed interrupts when edge interrupts are used.) (The logic behind this: DevA and DevB share an int. DevB signals an int. Int line goes high. ISR handler calls code for DevA -> does nothing. ISR handler calls code for DevB, but while doing that, DevA signals an int. ISR DevB is done, clears int for DevB, exits interrupt code. Now an interrupt for DevA is still pending, but because the int line never went low (DevA kept it high even when the int for DevB was cleared) the interrupt is never serviced.)

Multicore issues

Peripherals that can generate interrupts can be divided in two types:

  • External peripherals, within the ESP32 but outside the Xtensa cores themselves. Most ESP32 peripherals are of this type.
  • Internal peripherals, part of the Xtensa CPU cores themselves.

Interrupt handling differs slightly between these two types of peripherals.

Internal peripheral interrupts

Each Xtensa CPU core has its own set of six internal peripherals:

  • Three timer comparators
  • A performance monitor
  • Two software interrupts.

Internal interrupt sources are defined in esp_intr_alloc.h as ETS_INTERNAL_*_INTR_SOURCE.

These peripherals can only be configured from the core they are associated with. When generating an interrupt, the interrupt they generate is hard-wired to their associated core; it’s not possible to have e.g. an internal timer comparator of one core generate an interrupt on another core. That is why these sources can only be managed using a task running on that specific core. Internal interrupt sources are still allocatable using esp_intr_alloc as normal, but they cannot be shared and will always have a fixed interrupt level (namely, the one associated in hardware with the peripheral).

External Peripheral Interrupts

The remaining interrupt sources are from external peripherals. These are defined in soc/soc.h as ETS_*_INTR_SOURCE.

Non-internal interrupt slots in both CPU cores are wired to an interrupt multiplexer, which can be used to route any external interrupt source to any of these interrupt slots.

  • Allocating an external interrupt will always allocate it on the core that does the allocation.
  • Freeing an external interrupt must always happen on the same core it was allocated on.
  • Disabling and enabling external interrupts from another core is allowed.
  • Multiple external interrupt sources can share an interrupt slot by passing ESP_INTR_FLAG_SHARED as a flag to esp_intr_alloc().

Care should be taken when calling esp_intr_alloc() from a task which is not pinned to a core. During task switching, these tasks can migrate between cores. Therefore it is impossible to tell which CPU the interrupt is allocated on, which makes it difficult to free the interrupt handle and may also cause debugging difficulties. It is advised to use xTaskCreatePinnedToCore() with a specific CoreID argument to create tasks that will allocate interrupts. In the case of internal interrupt sources, this is required.

IRAM-Safe Interrupt Handlers

The ESP_INTR_FLAG_IRAM flag registers an interrupt handler that always runs from IRAM (and reads all its data from DRAM), and therefore does not need to be disabled during flash erase and write operations.

This is useful for interrupts which need a guaranteed minimum execution latency, as flash write and erase operations can be slow (erases can take tens or hundreds of milliseconds to complete).

It can also be useful to keep an interrupt handler in IRAM if it is called very frequently, to avoid flash cache misses.

Refer to the SPI flash API documentation for more details.

Multiple Handlers Sharing A Source

Several handlers can be assigned to a same source, given that all handlers are allocated using the ESP_INTR_FLAG_SHARED flag. They’ll be all allocated to the interrupt, which the source is attached to, and called sequentially when the source is active. The handlers can be disabled and freed individually. The source is attached to the interrupt (enabled), if one or more handlers are enabled, otherwise detached. A handler will never be called when disabled, while its source may still be triggered if any one of its handler enabled.

Sources attached to non-shared interrupt do not support this feature.

Though the framework support this feature, you have to use it very carefully. There usually exist 2 ways to stop a interrupt from being triggered: disable the sourse or mask peripheral interrupt status. IDF only handles the enabling and disabling of the source itself, leaving status and mask bits to be handled by users. Status bits should always be masked before the handler responsible for it is disabled, or the status should be handled in other enabled interrupt properly. You may leave some status bits unhandled if you just disable one of all the handlers without mask the status bits, which causes the interrupt being triggered infinitely, and finally a system crash.

API Reference

Functions

esp_err_t esp_intr_mark_shared(int intno, int cpu, bool is_in_iram)

Mark an interrupt as a shared interrupt.

This will mark a certain interrupt on the specified CPU as an interrupt that can be used to hook shared interrupt handlers to.

Return
ESP_ERR_INVALID_ARG if cpu or intno is invalid ESP_OK otherwise
Parameters
  • intno: The number of the interrupt (0-31)
  • cpu: CPU on which the interrupt should be marked as shared (0 or 1)
  • is_in_iram: Shared interrupt is for handlers that reside in IRAM and the int can be left enabled while the flash cache is disabled.

esp_err_t esp_intr_reserve(int intno, int cpu)

Reserve an interrupt to be used outside of this framework.

This will mark a certain interrupt on the specified CPU as reserved, not to be allocated for any reason.

Return
ESP_ERR_INVALID_ARG if cpu or intno is invalid ESP_OK otherwise
Parameters
  • intno: The number of the interrupt (0-31)
  • cpu: CPU on which the interrupt should be marked as shared (0 or 1)

esp_err_t esp_intr_alloc(int source, int flags, intr_handler_t handler, void *arg, intr_handle_t *ret_handle)

Allocate an interrupt with the given parameters.

This finds an interrupt that matches the restrictions as given in the flags parameter, maps the given interrupt source to it and hooks up the given interrupt handler (with optional argument) as well. If needed, it can return a handle for the interrupt as well.

The interrupt will always be allocated on the core that runs this function.

If ESP_INTR_FLAG_IRAM flag is used, and handler address is not in IRAM or RTC_FAST_MEM, then ESP_ERR_INVALID_ARG is returned.

Return
ESP_ERR_INVALID_ARG if the combination of arguments is invalid. ESP_ERR_NOT_FOUND No free interrupt found with the specified flags ESP_OK otherwise
Parameters
  • source: The interrupt source. One of the ETS_*_INTR_SOURCE interrupt mux sources, as defined in soc/soc.h, or one of the internal ETS_INTERNAL_*_INTR_SOURCE sources as defined in this header.
  • flags: An ORred mask of the ESP_INTR_FLAG_* defines. These restrict the choice of interrupts that this routine can choose from. If this value is 0, it will default to allocating a non-shared interrupt of level 1, 2 or 3. If this is ESP_INTR_FLAG_SHARED, it will allocate a shared interrupt of level 1. Setting ESP_INTR_FLAG_INTRDISABLED will return from this function with the interrupt disabled.
  • handler: The interrupt handler. Must be NULL when an interrupt of level >3 is requested, because these types of interrupts aren’t C-callable.
  • arg: Optional argument for passed to the interrupt handler
  • ret_handle: Pointer to an intr_handle_t to store a handle that can later be used to request details or free the interrupt. Can be NULL if no handle is required.

esp_err_t esp_intr_alloc_intrstatus(int source, int flags, uint32_t intrstatusreg, uint32_t intrstatusmask, intr_handler_t handler, void *arg, intr_handle_t *ret_handle)

Allocate an interrupt with the given parameters.

This essentially does the same as esp_intr_alloc, but allows specifying a register and mask combo. For shared interrupts, the handler is only called if a read from the specified register, ANDed with the mask, returns non-zero. By passing an interrupt status register address and a fitting mask, this can be used to accelerate interrupt handling in the case a shared interrupt is triggered; by checking the interrupt statuses first, the code can decide which ISRs can be skipped

Return
ESP_ERR_INVALID_ARG if the combination of arguments is invalid. ESP_ERR_NOT_FOUND No free interrupt found with the specified flags ESP_OK otherwise
Parameters
  • source: The interrupt source. One of the ETS_*_INTR_SOURCE interrupt mux sources, as defined in soc/soc.h, or one of the internal ETS_INTERNAL_*_INTR_SOURCE sources as defined in this header.
  • flags: An ORred mask of the ESP_INTR_FLAG_* defines. These restrict the choice of interrupts that this routine can choose from. If this value is 0, it will default to allocating a non-shared interrupt of level 1, 2 or 3. If this is ESP_INTR_FLAG_SHARED, it will allocate a shared interrupt of level 1. Setting ESP_INTR_FLAG_INTRDISABLED will return from this function with the interrupt disabled.
  • intrstatusreg: The address of an interrupt status register
  • intrstatusmask: A mask. If a read of address intrstatusreg has any of the bits that are 1 in the mask set, the ISR will be called. If not, it will be skipped.
  • handler: The interrupt handler. Must be NULL when an interrupt of level >3 is requested, because these types of interrupts aren’t C-callable.
  • arg: Optional argument for passed to the interrupt handler
  • ret_handle: Pointer to an intr_handle_t to store a handle that can later be used to request details or free the interrupt. Can be NULL if no handle is required.

esp_err_t esp_intr_free(intr_handle_t handle)

Disable and free an interrupt.

Use an interrupt handle to disable the interrupt and release the resources associated with it.

Note
When the handler shares its source with other handlers, the interrupt status bits it’s responsible for should be managed properly before freeing it. see esp_intr_disable for more details.
Return
ESP_ERR_INVALID_ARG if handle is invalid, or esp_intr_free runs on another core than where the interrupt is allocated on. ESP_OK otherwise
Parameters
  • handle: The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

int esp_intr_get_cpu(intr_handle_t handle)

Get CPU number an interrupt is tied to.

Return
The core number where the interrupt is allocated
Parameters
  • handle: The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

int esp_intr_get_intno(intr_handle_t handle)

Get the allocated interrupt for a certain handle.

Return
The interrupt number
Parameters
  • handle: The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

esp_err_t esp_intr_disable(intr_handle_t handle)

Disable the interrupt associated with the handle.

Note
  1. For local interrupts (ESP_INTERNAL_* sources), this function has to be called on the CPU the interrupt is allocated on. Other interrupts have no such restriction.
  2. When several handlers sharing a same interrupt source, interrupt status bits, which are handled in the handler to be disabled, should be masked before the disabling, or handled in other enabled interrupts properly. Miss of interrupt status handling will cause infinite interrupt calls and finally system crash.
Return
ESP_ERR_INVALID_ARG if the combination of arguments is invalid. ESP_OK otherwise
Parameters
  • handle: The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

esp_err_t esp_intr_enable(intr_handle_t handle)

Enable the interrupt associated with the handle.

Note
For local interrupts (ESP_INTERNAL_* sources), this function has to be called on the CPU the interrupt is allocated on. Other interrupts have no such restriction.
Return
ESP_ERR_INVALID_ARG if the combination of arguments is invalid. ESP_OK otherwise
Parameters
  • handle: The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

esp_err_t esp_intr_set_in_iram(intr_handle_t handle, bool is_in_iram)

Set the “in IRAM” status of the handler.

Note
Does not work on shared interrupts.
Return
ESP_ERR_INVALID_ARG if the combination of arguments is invalid. ESP_OK otherwise
Parameters
  • handle: The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus
  • is_in_iram: Whether the handler associated with this handle resides in IRAM. Handlers residing in IRAM can be called when cache is disabled.

void esp_intr_noniram_disable()

Disable interrupts that aren’t specifically marked as running from IRAM.

void esp_intr_noniram_enable()

Re-enable interrupts disabled by esp_intr_noniram_disable.

Macros

ESP_INTR_FLAG_LEVEL1

Interrupt allocation flags.

These flags can be used to specify which interrupt qualities the code calling esp_intr_alloc* needs.Accept a Level 1 interrupt vector (lowest priority)

ESP_INTR_FLAG_LEVEL2

Accept a Level 2 interrupt vector.

ESP_INTR_FLAG_LEVEL3

Accept a Level 3 interrupt vector.

ESP_INTR_FLAG_LEVEL4

Accept a Level 4 interrupt vector.

ESP_INTR_FLAG_LEVEL5

Accept a Level 5 interrupt vector.

ESP_INTR_FLAG_LEVEL6

Accept a Level 6 interrupt vector.

ESP_INTR_FLAG_NMI

Accept a Level 7 interrupt vector (highest priority)

ESP_INTR_FLAG_SHARED

Interrupt can be shared between ISRs.

ESP_INTR_FLAG_EDGE

Edge-triggered interrupt.

ESP_INTR_FLAG_IRAM

ISR can be called if cache is disabled.

ESP_INTR_FLAG_INTRDISABLED

Return with this interrupt disabled.

ESP_INTR_FLAG_LOWMED

Low and medium prio interrupts. These can be handled in C.

ESP_INTR_FLAG_HIGH

High level interrupts. Need to be handled in assembly.

ESP_INTR_FLAG_LEVELMASK

Mask for all level flags.

ETS_INTERNAL_TIMER0_INTR_SOURCE

Xtensa timer 0 interrupt source.

The esp_intr_alloc* functions can allocate an int for all ETS_*_INTR_SOURCE interrupt sources that are routed through the interrupt mux. Apart from these sources, each core also has some internal sources that do not pass through the interrupt mux. To allocate an interrupt for these sources, pass these pseudo-sources to the functions.

ETS_INTERNAL_TIMER1_INTR_SOURCE

Xtensa timer 1 interrupt source.

ETS_INTERNAL_TIMER2_INTR_SOURCE

Xtensa timer 2 interrupt source.

ETS_INTERNAL_SW0_INTR_SOURCE

Software int source 1.

ETS_INTERNAL_SW1_INTR_SOURCE

Software int source 2.

ETS_INTERNAL_PROFILING_INTR_SOURCE

Int source for profiling.

ETS_INTERNAL_INTR_SOURCE_OFF

Type Definitions

typedef void (*intr_handler_t)(void *arg)
typedef struct intr_handle_data_t intr_handle_data_t
typedef intr_handle_data_t *intr_handle_t