Interrupt Allocation

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Overview

The ESP32 has two cores, with 32 interrupts each. Each interrupt has a fixed priority, most (but not all) interrupts are connected to the interrupt matrix.

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_intrstatus()). It can use the flags passed to this function to specify the type, priority, and trigger method of the interrupt to allocate. The interrupt allocation code will then find an applicable interrupt, use the interrupt matrix to hook it up to the peripheral, and install the given interrupt handler and ISR to it.

The interrupt allocator presents two different types of interrupts, namely shared interrupts and non-shared interrupts, both of which require different handling. Non-shared interrupts will allocate a separate interrupt for every esp_intr_alloc() call, and this interrupt is use solely for the peripheral attached to it, with only one ISR that will get called. Shared interrupts can have multiple peripherals triggering them, 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 check if any action is required.

Non-shared interrupts can be either level- or edge-triggered. Shared interrupts can only be level interrupts due to the chance of missed interrupts when edge interrupts are used.

To illustrate why shared interrupts can only be level-triggered, take the scenario where peripheral A and peripheral B share the same edge-triggered interrupt. Peripheral B triggers an interrupt and sets its interrupt signal high, causing a low-to-high edge, which in turn latches the CPU's interrupt bit and triggers the ISR. The ISR executes, checks that peripheral A did not trigger an interrupt, and proceeds to handle and clear peripheral B's interrupt signal. Before the ISR returns, the CPU clears its interrupt bit latch. Thus, during the entire interrupt handling process, if peripheral A triggers an interrupt, it will be missed due the CPU clearing the interrupt bit latch.

  • 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 allocate interrupts. In the case of internal interrupt sources, this is required.

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 is not possible to have, for example, 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.

IRAM-Safe Interrupt Handlers

When performing write and erase operations on SPI flash, ESP32 will disable the cache, making SPI flash and SPIRAM inaccessible for interrupt handlers. This is why there are two types of interrupt handlers in ESP-IDF, which have their advantages and disadvantages:

IRAM-safe interrupt handlers - only access code and data in internal memory (IRAM for code, DRAM for data).

  • + Latency: They execute relatively fast and with low latency, since they are not blocked by slow flash write and erase operations (erases can take tens or hundreds of milliseconds to complete). This is useful for interrupts which need a guaranteed minimum execution latency.

  • - Internal memory use: They consume precious internal memory that could otherwise be used for something else.

  • + Cache misses: They do not rely on the cache with potential cache misses since the code and data are in internal memory already.

  • Usage: To register such an interrupt via the interrupt allocator API, use the ESP_INTR_FLAG_IRAM flag.

Non-IRAM-safe interrupt handlers - may access code and (read-only) data in flash.

  • - Latency: In case of flash operations, these interrupt handlers are postponed, which makes their average latency longer and less predictable.

  • + Internal memory use: They do not use any or not as much memory in internal RAM as IRAM-safe interrupts.

  • Usage: To register such an interrupt via the interrupt allocator API, do not use the ESP_INTR_FLAG_IRAM flag.

Note that there is nothing that explicitly marks an interrupt handler as IRAM-safe. An interrupt handler is IRAM-safe implicitly if and only if the code and data it may access are placed in internal memory. The term "IRAM-safe" is actually a bit misleading, since there are more requirements than just placing the handler's code in IRAM memory. Examples of interrupt handlers that are not IRAM-safe include:

  • A handler that has some of its code placed in flash memory.

  • A handler that is placed in IRAM but calls functions placed in flash memory.

  • A handler that accesses a read-only variable placed in flash, even though the handler's code is actually placed in IRAM.

For details on placing code and data in IRAM or DRAM, see How to Place Code in IRAM.

For more details about SPI flash operations and their interactions with interrupt handlers, see the SPI flash API documentation.

Note

Never register an interrupt handler with ESP_INTR_FLAG_IRAM flag if you are not 100% sure that all the code and data that the interrupt ever accesses are in IRAM (code) or DRAM (data). Disregarding this will lead to (sometimes spurious) cache errors. This must also be true for code and data accessed indirectly through function calls.

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 will all be 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.

By default, when ESP_INTR_FLAG_SHARED flag is specified, the interrupt allocator will allocate only priority level 1 interrupts. Use ESP_INTR_FLAG_SHARED | ESP_INTR_FLAG_LOWMED to also allow allocating shared interrupts at priority levels 2 and 3.

Though the framework supports this feature, you have to use it very carefully. There usually exist two ways to stop an interrupt from being triggered: disable the source or mask peripheral interrupt status. ESP-IDF only handles enabling and disabling of the source itself, leaving status and mask bits to be handled by users.

Status bits shall either be masked before the handler responsible for it is disabled, or be masked and then properly handled in another enabled interrupt.

Note

Leaving some status bits unhandled without masking them, while disabling the handlers for them, will cause the interrupt(s) to be triggered indefinitely, resulting therefore in a system crash.

Troubleshooting Interrupt Allocation

On most Espressif SoCs, CPU interrupts are a limited resource. Therefore it is possible for a program to run out of CPU interrupts, for example by initializing several peripheral drivers. Typically, this will result in the driver initialization function returning ESP_ERR_NOT_FOUND error code.

If this happens, you can use esp_intr_dump() function to print the list of interrupts along with their status. The output of this function typically looks like this:

CPU 0 interrupt status:
Int  Level  Type   Status
0     1    Level  Reserved
1     1    Level  Reserved
2     1    Level  Used: RTC_CORE
3     1    Level  Used: TG0_LACT_LEVEL
...

The columns of the output have the following meaning:

  • Int: CPU interrupt input number. This is typically not used in software directly, and is provided for reference only.

  • Level: Interrupt priority (1-7) of the CPU interrupt. This priority is fixed in hardware, and cannot be changed.

  • Type: Interrupt type (Level or Edge) of the CPU interrupt. This type is fixed in hardware, and cannot be changed.

  • Status: One of the possible statuses of the interrupt:

    • Reserved: The interrupt is reserved either at hardware level, or by one of the parts of ESP-IDF. It can not be allocated using esp_intr_alloc().

    • Used: <source>: The interrupt is allocated and connected to a single peripheral.

    • Shared: <source1> <source2> ...: The interrupt is allocated and connected to multiple peripherals. See Multiple Handlers Sharing A Source above.

    • Free: The interrupt is not allocated and can be used by esp_intr_alloc().

  • Free (not general-use): The interrupt is not allocated, but is either a high-priority interrupt (priority 4-7) or an edge-triggered interrupt. High-priority interrupts can be allocated using esp_intr_alloc() but requires the handlers to be written in Assembly, see High Priority Interrupts. Edge-triggered low- and medium-priority interrupts can also be allocated using esp_intr_alloc(), but are not used often since most peripheral interrupts are level-triggered.

If you have confirmed that the application is indeed running out of interrupts, a combination of the following suggestions can help resolve the issue:

  • On multi-core targets, try initializing some of the peripheral drivers from a task pinned to the second core. Interrupts are typically allocated on the same core where the peripheral driver initialization function runs. Therefore by running the initialization function on the second core, more interrupt inputs can be used.

  • Determine the interrupts which can tolerate higher latency, and allocate them using ESP_INTR_FLAG_SHARED flag (optionally ORed with ESP_INTR_FLAG_LOWMED). Using this flag for two or more peripherals will let them use a single interrupt input, and therefore save interrupt inputs for other peripherals. See Multiple Handlers Sharing A Source above.

  • Some peripheral driver may default to allocating interrupts with ESP_INTR_FLAG_LEVEL1 flag, so priority 2 and 3 interrupts do not get used by default. If esp_intr_dump() shows that some priority 2 or 3 interrupts are available, try changing the interrupt allocation flags when initializing the driver to ESP_INTR_FLAG_LEVEL2 or ESP_INTR_FLAG_LEVEL3.

  • Check if some of the peripheral drivers do not need to be used all the time, and initialize or deinitialize them on demand. This can reduce the number of simultaneously allocated interrupts.

API Reference

Header File

Macros

ESP_INTR_CPU_AFFINITY_TO_CORE_ID(cpu_affinity)

Convert esp_intr_cpu_affinity_t to CPU core ID.

Type Definitions

typedef void (*intr_handler_t)(void *arg)

Function prototype for interrupt handler function

typedef struct intr_handle_data_t *intr_handle_t

Handle to an interrupt handler

Enumerations

enum esp_intr_cpu_affinity_t

Interrupt CPU core affinity.

This type specify the CPU core that the peripheral interrupt is connected to.

Values:

enumerator ESP_INTR_CPU_AFFINITY_AUTO

Install the peripheral interrupt to ANY CPU core, decided by on which CPU the interrupt allocator is running.

enumerator ESP_INTR_CPU_AFFINITY_0

Install the peripheral interrupt to CPU core 0.

enumerator ESP_INTR_CPU_AFFINITY_1

Install the peripheral interrupt to CPU core 1.

Header File

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.

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.

Returns

ESP_ERR_INVALID_ARG if cpu or intno is invalid ESP_OK otherwise

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.

Parameters

  • intno -- The number of the interrupt (0-31)

  • cpu -- CPU on which the interrupt should be marked as shared (0 or 1)

Returns

ESP_ERR_INVALID_ARG if cpu or intno is invalid ESP_OK otherwise

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.

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.

Returns

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

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

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.

Returns

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

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. If the current core is not the core that registered this interrupt, this routine will be assigned to the core that allocated this interrupt, blocking and waiting until the resource is successfully released.

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. Please do not call this function in esp_ipc_call_blocking.

Parameters

handle -- The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

Returns

ESP_ERR_INVALID_ARG the handle is NULL ESP_FAIL failed to release this handle ESP_OK otherwise

int esp_intr_get_cpu(intr_handle_t handle)

Get CPU number an interrupt is tied to.

Parameters

handle -- The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

Returns

The core number where the interrupt is allocated

int esp_intr_get_intno(intr_handle_t handle)

Get the allocated interrupt for a certain handle.

Parameters

handle -- The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

Returns

The interrupt number

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.

Parameters

handle -- The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

Returns

ESP_ERR_INVALID_ARG if the combination of arguments is invalid. ESP_OK otherwise

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.

Parameters

handle -- The handle, as obtained by esp_intr_alloc or esp_intr_alloc_intrstatus

Returns

ESP_ERR_INVALID_ARG if the combination of arguments is invalid. ESP_OK otherwise

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.

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.

Returns

ESP_ERR_INVALID_ARG if the combination of arguments is invalid. ESP_OK otherwise

void esp_intr_noniram_disable(void)

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

void esp_intr_noniram_enable(void)

Re-enable interrupts disabled by esp_intr_noniram_disable.

void esp_intr_enable_source(int inum)

enable the interrupt source based on its number

Parameters

inum -- interrupt number from 0 to 31

void esp_intr_disable_source(int inum)

disable the interrupt source based on its number

Parameters

inum -- interrupt number from 0 to 31

static inline int esp_intr_flags_to_level(int flags)

Get the lowest interrupt level from the flags.

Parameters

flags -- The same flags that pass to esp_intr_alloc_intrstatus API

static inline int esp_intr_level_to_flags(int level)

Get the interrupt flags from the supplied level (priority)

Parameters

level -- The interrupt priority level

esp_err_t esp_intr_dump(FILE *stream)

Dump the status of allocated interrupts.

Parameters

stream -- The stream to dump to, if NULL then stdout is used

Returns

ESP_OK on success

bool esp_intr_ptr_in_isr_region(void *ptr)

Check if the given pointer is in the safe ISR area. In other words, make sure that the pointer's content is accessible at any time, regardless of the cache status.

Parameters

ptr -- Pointer to check

Returns

true if ptr points to ISR area, false else

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

Platform 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

Platform timer 1 interrupt source.

ETS_INTERNAL_TIMER2_INTR_SOURCE

Platform 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_UNUSED_INTR_SOURCE

Interrupt is not assigned to any source.

ETS_INTERNAL_INTR_SOURCE_OFF

Provides SystemView with positive IRQ IDs, otherwise scheduler events are not shown properly

ESP_INTR_ENABLE(inum)

Enable interrupt by interrupt number

ESP_INTR_DISABLE(inum)

Disable interrupt by interrupt number

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