What is an OTP chip?
OTP (One-Time Programmable) is a type of one-time programmable non-volatile memory, a key storage technology in chip design, primarily used in microcontrollers, embedded systems, and industrial electronics. Its core characteristic is that data written to it cannot be irreversibly modified. Permanent storage is achieved through physical structures such as eFuse or anti-fuse, and it is commonly used to store fixed data such as hardware version numbers, encryption keys, and calibration parameters.
This memory uses registers for data storage, supports a one-way programming mechanism (1 to 0), and is equipped with a LOCK register to lock write permissions for enhanced security. Compared to FLASH and MTP memories, OTP offers lower cost and higher reliability, but there is a risk of chip failure due to programming errors. In industrial applications, OTP is widely used for camera module parameter fixing, voice chip programming, device authentication, and secure boot key storage, while also supporting die programming technology to shorten production cycles.
With the increasing prevalence of embedded applications, product security is becoming increasingly important. On the one hand, this is to protect the hardware design; on the other hand, it’s also for the security of the product itself, preventing hacking. In embedded systems, all code and system data are stored inside the FLASH chip. FLASH chips are characterized by being rewritable and erasable multiple times, and data is not lost when power is off. To protect the data in the FLASH, more and more FLASH manufacturers provide a special register inside the FLASH: the OTP register. OTP stands for One-Time Programmable, meaning this register can only be programmed once and cannot be modified afterward. The OTP register is similar to the FLASH data area; a 1 can be changed to a 0, but a 0 can never be changed to a 1. If there is a 32-bit OTP register with a default value of 0xFFFFFFFF, and the user programs it to 0xFFFFFFFE, then the OTP register can never be changed back to 0xFFFFFFFF; of course, the value of this OTP register can also be changed to 0xFFFFFFFC or other values.
To provide protection for the OTP register, these types of FLASH chips generally also provide a LOCK register. The LOCK register (also part of the OTP register) can only be programmed once. Each bit of the LOCK register corresponds to an OTP register. If the bit in the LOCK register corresponding to the OTP register is written from 1 to 0, it means that the locked OTP register can no longer be written to. Even if the current value of the OTP is 0xFFFFFFFF, its value can never be rewritten. Through the OTP and LOCK registers, users can store specific information in the OTP, such as software version number, hardware version number, and keys. Additionally, these FLASH chips also have another internal OTP register, which is programmed by the manufacturer during production, written with a specific serial number, and locked. If needed, the FLASH manufacturer can also write specific serial numbers according to customer requirements. This ensures that each chip has a unique ID number, preventing duplication.
Besides FLASH chips, many CPU manufacturers also provide similar OTP registers within their CPUs for user use, facilitating the development of more secure applications. Larger manufacturers, such as TI and Freescale, often do not publicly release datasheets for certain chips. To obtain their datasheets, you need to place a large order and sign a confidentiality agreement.
OTP itself cannot provide absolutely secure applications. However, OTP helps developers create and deploy more secure applications, protecting their own and their customers’ interests. Many large manufacturers, such as Nokia, Motorola, Samsung, and Sony Ericsson, need to develop phones that can only be used on their own networks, based on customer (operator) requirements. Much of the software and hardware protection in these phones is based on OTP.
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