OTP (One-Time Programmable)

OTP (One-Time Programmable)

OTP (One-Time Programmable) refers to memory bits in an NFC chip that can be irreversibly changed from 0 to 1 but never written back to 0. Once an OTP bit is set, it remains set permanently — there is no erase operation. This irreversibility makes OTP bits valuable for permanent configuration locks, monotonic counters, and write-once data fields where tampering must be detectable.

How OTP Works at the Silicon Level

In NFC chip EEPROMEEPROMNon-volatile memory technology retaining data without powerView full →, OTP behavior is implemented by configuring specific memory cells to accept OR-write operations only. When a WRITE command targets an OTP byte:

1. The chip reads the current byte value.
2. It performs a bitwise OR between the current value and the new value.
3. The result is stored back to EEPROM.

This means individual bits can only transition from 0 to 1. For example, writing 0x0F to a byte containing 0x30 produces 0x3F (0x30 OR 0x0F). Writing 0x00 to any OTP byte has no effect — bits already set to 1 remain at 1.

OTP in NTAG Chips

In the NTAG 21x family (NTAG 213, NTAG 215, NTAG 216), page 3 contains the capability container and is an OTP area. Additionally, these chips provide:

• Static lock bitslock bitsControl bits making memory blocks permanently read-onlyView full → (page 2, bytes 2-3): OTP bits that permanently write-protect pages 3 through 15. See lock bits.
• Dynamic lock bits: OTP bits that protect the remaining user memory pages.
• Counter bytes: NTAG 213/215/216 include a 24-bit NFC counter that increments on every successful read. This counter is OTP in spirit — it only increases, never decreases.

Use Cases

OTP memory serves several engineering purposes:

• Permanent configuration: Setting lock bits via OTP prevents subsequent modification of critical data like the NDEF message or capability container.
• Monotonic counters: The OTP mechanism ensures counters cannot be rolled back, which is useful for loyalty programs (counting taps), product usage tracking, and tamper evidence.
• Write-once data fields: Manufacturing date codes, calibration values, or product serial numbers that must be set once and never altered.

Limitations

The irreversibility of OTP is both its strength and its risk. A misconfigured OTP write cannot be undone — if you accidentally set lock bits on a tag, that tag's protected pages are permanently read-only. Always verify OTP write operations on test tags before deploying to production inventory.