# Using Extended OTP on HS devices¶

K3 family of devices have a set of one-time programmable(OTP) efuses to carry root of trust keys and other information used during device boot. The purpose of these efuses is fixed. K3 family of devices have another set of OTP efuses for general purpose use by the customer. We refer to these efuses as extended OTP. In this document, we describe how the extended OTP can be accessed through TISCI API. This document must be read along side

Note

The API in this document deal only with the extended OTP area. They do not access the predefined key area containing the root of trust keys.

Note

This document is only applicable to HS devices.

## Hardware Configuration¶

The extended OTP area can have a maximum of 1024 bits. The extended OTP area is organized into rows of bits. The width of each row can vary based on the device. Please refer to the Chapter 5: SoC Family Specific Documentation for the below details.

• number of bits available in the OTP area
• Width of the OTP rows

On device boot, the data in the OTP area is read into a set of 32 bit memory mapped registers(MMRs). There are 32 such registers to accommodate the maximum number of OTP bits. The width of these registers is always 32 bits irrespective of width of the underlying hardware OTP rows. These registers are only refreshed on a POR of the device and not on a warm reset.

The hardware allows each individual OTP row (not MMR) to be locked to prevent modification. Once the OTP row is locked, any bit in the row can never be modified.

## Programming the extended OTP¶

TI provides two pieces of software to program the extended OTP area.

1. Key Writer - Key Writer is used in the factory for programming the customer root of trust keys, key revision and key count on a HS Field Securable(FS) Device. It can be used to program the extended OTP area at the same time. Once the key revision and key count are programmed a HS FS device becomes HS Security Enforced(SE) device. Key Writer does not run on a HS SE device.
2. System Firmware - System Firmware provides API that can be used to access and modify the extended OTP area on a HS SE device. System Firmware is not a standalone executable and must be used along with software running on another core to access or modify the extended OTP area.

In this document, we describe how to use System Firmware to access/modify the extended OTP area.

Note

Key Writer and System Firmware are independent software deliveries. Please contact TI Support for information on the Key Writer.

The System Firmware TISCI messages for OTP access are in two categories due to the granularity at which they work.

1. Read - Works on MMR granularity (32 bit)
2. Write/Lock - Works on row granularity (hardware dependent)

The reasons for the different granularity are

1. System Firmware does not maintain state across device reboots. Any write protection performed must be tied to the hardware. Hardware provides write protection at row granularity. Therefore the write/lock API must operate at a row granularity.
2. We expect the OTP read operation to be performed more often compared to the OTP write operation. We expect most data stored in the OTP area to be a multiple of 32 bit width. If it is smaller, it could be easily extracted from 32 bit data with bit operations. So System Firmware offers a read API with 32 bit granularity instead of the underlying row width.

System Firmware allows specifying access permissions for each 32 bit MMR through the security board configuration. Each row can be assigned a host ID as an owner and be specified as secure or non-secure. See Extended OTP Row Entry.

Secure vs. non-secure - Marking a row as non-secure implies that System Firmware can provide the value of the 32-bit MMR as a response to the TISCI read message. If a row is marked secure, the 32-bit MMR value will not be returned in the TISCI response message. The MMR value can only be used to setup encryption or decryption contexts for processing data.

If a row is assigned to a single host, the MMR can be read only by that host. To allow multiple hosts to read the MMR, the host ID must be set to 128. If the MMR must not be accessible to any host, the host ID must be set to 0.

If a row is marked as non-secure, the owning host can read the MMR by using the TISCI_MSG_READ_OTP_MMR message.

## Extended OTP Write API¶

System Firmware allows for only one host in the system to have write access to the extended OTP region. This host is specified in the security board configuration Extended OTP array configuration using the write_host field.

The write host can write to any row that has not been locked using the TISCI_MSG_WRITE_OTP_ROW message. The API takes a bit mask in addition to the value to be written. This allows single bits to modified if necessary.

The write host can perform a soft write lock using the TISCI_MSG_SOFT_LOCK_OTP_WRITE_GLOBAL message to prevent further writes to OTP until a warm/cold reset of the device. A soft write lock prevents even the write host from performing further writes until the next reset.

The write host can also lock a row permanently using the TISCI_MSG_LOCK_OTP_ROW message. This prevents any further modifications to the row even after a device cold/warm reset.

The lock status of a row can be queried using the TISCI_MSG_GET_OTP_ROW_LOCK_STATUS message.