esp-idf/docs/en/api-reference/peripherals/spi_flash/auto_suspend.inc

.. _auto-suspend:

Flash Auto Suspend Feature
--------------------------

.. important::

    1. The flash chip you are using should have a suspend/resume feature.
    2. The MSPI hardware should support the auto-suspend feature (hardware can send suspend command automatically).

    If you use suspend feature on an unsupported chip, it may cause a severe crash. Therefore, we strongly suggest you reading the flash chip datasheets first. Ensure the flash chip satisfies the following conditions at minimum.

    1. SUS bit in status registers should in SR2 bit7 (or SR bit15)(This is caused by the restriction of out software implementation).

    2. Suspend command is 75H, resume command is 7AH(This is caused by the restriction of out software implementation).

    3. When the flash is successfully suspended, all address of the flash, except from the section/block being erased, can be read correctly. And resume can be sent immediately at this state.

    4. When the flash is successfully resumed, another suspend can be sent immediately at this state.


When :ref:`CONFIG_SPI_FLASH_AUTO_SUSPEND` is enabled, the caches will be kept enabled (they would be disabled if :ref:`CONFIG_SPI_FLASH_AUTO_SUSPEND` is disabled). The hardware handles the arbitration between SPI0 and SPI1. If SPI1 operation is short (like reading operation), the CPU and the cache will wait until the SPI1 operation is done. However, if it is erasing, page programming or status register writing (e.g. `SE`, `PP` and `WRSR`), an auto suspend will happen, interrupting the ongoing flash operation, making the CPU able to read from cache and flash in limited time.

This way some code/variables can be put into the flash/psram instead of IRAM/DRAM, while still able to be executed during flash erasing. This reduces the some usage of IRAM/DRAM.

Please note this feature has the overhead of the flash suspend/resume. The flash erasing can be extremely long if the erasing is interrupted too often. Use FreeRTOS task priorities to ensure that only real-time critical tasks are executed at higher priority than flash erase, to allow the flash erase to complete in reasonable time.


In other words, there are three kinds of code:

1. Critical code: inside IRAM/DRAM. This kind of code usually has high performance requirements, related to cache/flash/psram, or called very often.

2. Cached code: inside flash/psram. This kind of code has lower performance requirements or called less often. They will execute during erasing, with some overhead.

3. Low priority code: inside flash/psram and disabled during erasing. This kind of code should be forbidden from executed to avoid affecting the flash erasing, by setting a lower task priority than the erasing task.

Regarding the flash suspend feature usage, and corresponding response time delay, please also see this example :example:`system/flash_suspend` .

.. note::

    The flash auto suspend feature relies heavily on strict timing. You can see it as a kind of optimisation plan, which means that you cannot use it in every situation, like high requirement of real-time system or triggering interrupt very frequently (e.g. lcd flush, bluetooth, wifi, etc.). You should take following steps to evaluate what kind of ISR can be used together with flash suspend.

    .. wavedrom:: /../_static/diagrams/spi_flash/flash_auto_suspend_timing.json

    As you can see from the diagram. Two key values should be noted.

    1. ISR time: The ISR time cannot be very long, at least not larger than the value you set in `IWDT`. But it will significantly lengthen the erase/write completion times.
    2. ISR interval: The ISR cannot be triggered very often. The most important time is the `ISR interval minus ISR time`(from point b to point c in the diagram). During this time, SPI1 will send resume to restart the operation, but it needs a time `tsus` for preparation, and the typical value of `tsus` is about **40us**. If SPI1 cannot resume the operation but another suspend comes, it will cause CPU starve and `TWDT` may be triggered.

    Furthermore, the flash suspend might be delayed. If CPU and the cache operation accesses to flash via SPI0 very frequently and SPI1 sending suspend command is also very frequently, it will influence the efficiency of MSPI data transfer. So, we have a "lock" inside to prevent this. When SPI1 send suspend command, then SPI0 will take over memory SPI bus and take the "lock". After SPI0 finishes sending data, SPI0 will still take the memory SPI bus until the "lock" delay period time finishes. During this "lock" delay period, if there is any other SPI0 transaction, then start SPI0 transaction and "lock" delay period start again. Otherwise, SPI0 will release the memory bus and start SPI0/1 arbitration.