What is" one time programmable" mean?

One-time programmable (OTP) memory is a type of non-volatile memory that can be programmed exactly once after 

manufacturing or outside the manufacturing factory, and cannot be erased or reprogrammed thereafter.

 OTP memory can be programmed once by the user using a programmable read-only memory (PROM) programmer.

 Unlike erasable programmable read-only memory (EPROM), 

which can be erased and reprogrammed multiple times using ultraviolet light through a built-in window,

 OTP memory is typically packaged in plastic without an erasure window,

 making it non-erasable and permanently retaining its programmed state. 

3 Electrically erasable programmable read-only memory (EEPROM) and flash memory differ from OTP in that they can be erased 

and reprogrammed multiple times, 

with EEPROM allowing byte-level erasure and flash memory enabling block-level erasure. 

The fundamental concept of OTP technology is that it allows for permanent programming of data or configuration after manufacturing. 

Some OTP devices use anti-fuse or fuse-based mechanisms, which create permanent connections or break links to store information. 

OTP devices are relevant in hardware design, embedded systems, and security applications, such as storing cryptographic keys or unique device signatures, 

where data integrity and persistence are critical. 8 9 10 OTP versions of EPROM are packaged without a quartz window to reduce cost and prevent erasure,

 and have been largely supplanted by flash memory in modern systems due to flash’s reprogrammability and speed. 2 3

One-time programmable memory implementations include fuse-based, antifuse-based, and mask-programmed architectures. Fuse-based OTP memory, 

such as programmable read-only memory (PROM), utilizes bipolar transistors with fusible links; 

programming is achieved by burning out fuses in the cells using high voltage or current pulses, 

which stores a “0” in the cell when the link is broken and a “1” when the link is intact. 1 11 In antifuse-based OTP memory, 

programming is performed by applying high voltage to the gate of a transistor,

causing gate oxide breakdown and forming a conductive path; the split channel one-transistor antifuse structure offers advantages in cell area, access speed, and resistance to reverse engineering. 

12 Antifuse-based OTP memory is used in field-programmable gate array (FPGA) devices, where the technology provides instant-on availability, nonvolatility, and resistance to reverse engineering. 

Mask read-only memory (ROM) is manufactured with data content permanently etched into the microchip during processor fabrication and cannot be modified later.

Circuit architectures differ among OTP types. Fuse-based PROMs are programmed outside the manufacturing factory using a ROM burner 11, 

while antifuse-based FPGAs are programmed off-line with a special device programmer. 14 Antifuse-based FPGAs are nonvolatile,

 provide instant-on availability, and do not require external configuration devices, 

which saves board space and cost; they are also more radiation hardened than other technologies, 

which makes them of particular interest for aerospace-type applications. 13 Mask ROMs are a cheaper solution.

Packaging impacts cost. OTP EPROMs are packaged in plastic without a quartz window, making them cheaper and allowing only one-time programming,

 whereas erasable EPROMs require ceramic packaging with a quartz window for ultraviolet light erasure, which raises their price and reduces their flexibility. 

Compared to other programmable memories, EPROMs can be erased and reprogrammed multiple times using ultraviolet light, and EEPROMs allow electrical erasure and reprogramming. 

Flash memory can be written and erased in blocks or sectors, making it a cheaper and faster variation of EEPROM. 1

One-time programmable memories are a type of read-only memory that can be integrated on-chip and programmed outside the manufacturing factory using a PROM programmer, 

after which their contents cannot be modified.

 In secure boot processes, the public key of the first image, such as the bootloader, is stored directly or indirectly in a one-time-programmable (fuse) memory of the device,

 ensuring that the initial boot process cannot be altered and supporting system integrity and trustworthiness by preventing unauthorized firmware from running. 

OTP memory is also used for device identification and anti-counterfeiting,

 where a key is internally stored in a one-time-programmable non-volatile memory, 

and intrinsic random variations in programmed resistances are exploited to create unique integrated circuit (IC)-specific signatures for authentication. 

10 In cryptographic applications, passwords for firmware-over-the-air (FoTA) updates are derived from random values stored in OTP memory and are unique for each device, 

enhancing security by restricting access and making the system more secure in the field. 

One-time-programmable fuses are used to store cryptographic keys and hashes in hardware, providing protection against tampering and unauthorized modification by making it extremely difficult

 for attackers to overwrite the stored values after manufacturing. 

 During the manufacturing process, the public key of the initial boot image is stored in OTP memory, and the boot code itself is placed in read-only memory (ROM), 

ensuring that the initial boot process cannot be altered.

 8 OTP fuses are integral to secure boot implementations, where the boot process proceeds only if the public key hash stored in the OTP fuse matches the key used for verification, 

thereby preventing attackers from substituting their own root certificate authority (CA) public keys at the start of the boot process. 

This mechanism helps eliminate threats from unauthorized firmware modification, loss of functionality,

 and protection of intellectual property rights.

Mitigation strategies include secure boot implementations that rely on OTP memory for public key storage. 

 The challenge remains in balancing programmability constraints, security robustness, and cost, 

as OTP fuses are programmed only once and cannot be updated, requiring careful planning during device provisioning. 16 17 Secure key management practices, 

such as using distinct keys for different functions and provisioning keys at manufacturing time, are essential for maintaining security in OTP-based systems.

One-time programmable devices are programmed by applying pulses of relatively high voltage and current to the device's inputs, which selectively remove undesired fuses in fusible-link technologies;

 this process is commonly referred to as blowing the fuses or burning the device, and once a fuse has been blown, it cannot be replaced or reversed. 

 OTP read-only memory can be programmed outside the manufacturing factory using a ROM burner, which burns out fuses of cells to program them to “1” using high voltage/current pulses. 

11 For antifuse-based OTP devices, programming is performed using single-site or multi-site device programmers, which are used to program a device before it is mounted on the system board, 

either before soldering or before placing it into a socket; no programming hardware is required on the system board for this method. 

20 Actel supplies programming software for both single-site and multiple-site programmers, enabling users to select the device, programming files, program, and verify the device.

The constraint imposed by one-time programmability is that the configuration of the device cannot be changed after it has been programmed,

 necessitating thorough validation and testing of the design before programming,

 as any errors require discarding the device and configuring a new one. 1

One-time-programmable technology is a type of read-only memory that can be programmed only once, typically outside the manufacturing factory using a PROM programmer, 

and is often integrated on-chip for cost-effective solutions. 

1 OTP versions of erasable programmable read-only memory are packaged in plastic without a window, 

making them programmable only once and never erasable, which reduces cost compared to windowed EPROMs. 

 OTP memories are useful for mass production after a project has been fully developed, as they allow for the creation of many copies of memory 

chips at a lower cost than reprogrammable EPROMs. 

In security-critical applications, OTP technology remains relevant for storing cryptographic keys and sensitive information, as it provides a non-volatile and tamper-resistant solution. 

 OTP memory is used to store public keys in secure boot processes, ensuring that the initial boot code cannot be modified and protecting against unauthorized firmware execution. 

Hardware-intrinsic keys stored in OTP non-volatile memory are used to create unique integrated circuit-specific signatures for authentication, enhancing device security.