Note
· Michele Forese
OTP memory
One-time programmable fuses — what they are, what they are for, and why you do not use them as EEPROM.
OTP means one-time programmable. The bits are fuses (or antifuses). You write them, typically by a programming pulse from the MCU or a factory tool. You do not erase them. A blown fuse stays blown for the life of the part.
This is not flash and not EEPROM.
What it is for
OTP holds values that must survive firmware updates, mass erase, and attackers who can rewrite NOR:
- Boot and security configuration (which port boots, TrustZone options)
- Keys and hashes for secure boot
- Calibration or board identity
- On some MCUs, including the STM32N6 series, an area reserved for Ethernet MAC addresses (often unprogrammed in the factory, reading as zero until you write it)
ST exposes this on STM32N6 through BSEC (boot and security control), with 8 KB of OTP.
What it is not for
Do not store logs, frame counters, or anything you might want to change twice. There is no wear levelling and no undo. A wrong write on a security word can brick a boot path.
For mutable data use NOR flash, EEPROM, or battery-backed SRAM (a few kilobytes in VBAT on many STM32s — that is erasable, and it dies with the backup supply).
Programming
On STM32, OTP is written with STM32CubeProgrammer or a HAL/BSEC driver, in a controlled sequence. Some words are reserved by the silicon vendor. Read the reference manual before you pick an address.
Once a bit is one, it stays one. Design the map so that unused fields stay at the unprogrammed state until you need them.