What is IAP in microcontroller?

 In microcontroller context, IAP almost always means In-Application Programming.

1. Basic idea

In-Application Programming (IAP) =
The MCU can re-program its own Flash while your application code is running, without needing an external programmer (ST-Link, J-Link, etc.) or going into a separate factory programming mode.

So instead of:

PC → programmer → MCU (ISP / SWD / JTAG)

you can do:

PC / other device → your firmware → MCU’s Flash (via IAP)

Your code acts as its own flash programmer.

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2. IAP vs ISP vs bootloader

• ISP (In-System Programming)

  • Usually means programming via a debug/programmer tool (JTAG/SWD, UART, etc.)

  • Often uses a bootloader mode that runs instead of your application.

• Bootloader

  • Small piece of code that runs first after reset.

  • It may:

    • Check for a new firmware image,

    • Receive it over UART/USB/CAN/etc.,

    • Program it into Flash (using IAP services),

    • Then jump to the main application.

• IAP

  • The mechanism by which code already running on the MCU calls special routines (ROM functions or library calls) to erase/program Flash.

  • Often used inside a bootloader or the main application.

In practice:

• The ROM (or vendor library) provides low-level IAP functions (erase sector, program word/page).

• Your bootloader or app calls these IAP routines to update firmware or store configuration.

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3. Typical uses of IAP

1. Firmware update in the field

   • Device receives new firmware via:

     • USB, UART, CAN, Ethernet, BLE, etc.

   • Stores received image in a buffer area / separate flash bank.

   • Uses IAP to:

     • Erase old firmware area.

     • Program new image.

   • Optionally verifies checksum/CRC.

   • Jumps to the new firmware.

2. Dual-bank / fail-safe updates

   • Bank A = running firmware

   • Bank B = new firmware image

   • IAP swaps which bank is “active” or copies new image over.

   • If update fails, device can still boot from the old image.

3. Storing parameters / logs in Flash

   • Using part of Flash as simple NVM.

   • Your code uses IAP to erase/program parameter blocks, calibration data, usage counters, etc.

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4. How it looks in code (conceptually)

Exact API depends on vendor, but the pattern is typically:

// 1) Prepare IAP interface (unlock flash, etc.)
flash_unlock();

// 2) Erase one or more pages/sectors
flash_erase_sector(SECTOR_5);

// 3) Program data (word/page at a time)
for (uint32_t i = 0; i < size; i += 4) {
    flash_program_word(DEST_ADDR + i, *(uint32_t *)(src + i));
}

// 4) Lock flash again
flash_lock();

Important details:

• You CANNOT execute from Flash while erasing/programming that same region.
  Common solutions:

  • Run IAP routines from ROM (provided by vendor).

  • Or copy a small flash-programming routine to RAM and execute from there.

• Interrupts may need to be disabled or moved to RAM-based vectors to avoid executing from Flash mid-operation.

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5. Key safety considerations

When using IAP for firmware updates:

• Never erase the only copy of your working code before verifying the new image.

• Use CRC/Checksum on the received firmware to ensure integrity.

• Ideally implement a:

  • Bootloader partition (never overwritten in normal updates).

  • Application partition (the part you update via IAP).

• Have a rollback or recovery strategy:

  • E.g., if update fails, bootloader detects invalid app image and stays in update mode.

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6. One-sentence summary

IAP (In-Application Programming) is the ability of a microcontroller to rewrite its own Flash memory under control of the running firmware, typically used for self-updates and storing non-volatile data, without needing an external programmer.