Note
NOTE:
Any read of the CSM password would yield 0x0000 until the device is unlocked. These
reads are labeled "dummy read" or a "fake read." The application reads the password locations, but
will always get 0's no matter what the actual value is. What is important is the actual value of the
password. If the actual value is all 0xFFFF, then doing this "dummy read" will unlock the device. If the
actual value is all 0x0000, then no matter what the application code does, one will never be able to
unlock the device. If the actual value is something other than all 0xFFFF or 0x0000, then when the
dummy read is performed, the actual value must match the password the user provided.
1.2.3.3 Unsecuring Considerations for Devices With/Without Code Security
Case 1 and Case 2 provide unsecuring considerations for devices with and without code security.
Case 1: Device With Code Security
A device with code security should have a predetermined password stored in the password locations (0x3F 7FF8
- 0x3F 7FFF in memory). In addition, locations 0x3F 7F80 - 0x3F 7FF5 should be programmed with all 0x0000
and not used for program and/or data storage. The following are steps to unsecure this device:
1. Perform a dummy read of the password locations. The CSM blocks the OTP reads to password location.
Hence the dummy reads to password location can be done only from RAM (secure/unsecure) or Flash.
2. Write the password into the KEY registers (locations 0x00 0AE0 - 0x00 0AE7 in memory).
3. If the password is correct, the device becomes unsecure; otherwise, it stays secure.
Case 2: Device Without Code Security
A device without code security should have 0x FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF (128 bits of all
ones) stored in the password locations. The following are steps to use this device:
1. At reset, the CSM will lock memory regions protected by the CSM.
2. Perform a dummy read of the password locations. The CSM blocks the OTP reads to password location.
Hence the dummy reads to password location can be done only from RAM (secure/unsecure) or Flash.
3. Since the password is all ones, this alone will unlock all memory regions. Secure memory is fully accessible
immediately after this operation is completed.
Note
Even if a device is not protected with a password (all password locations all ones), the CSM will lock
at reset. Thus, a dummy read operation must still be performed on these devices prior to reading,
writing, or programming secure memory if the code performing the access is executing from outside of
the CSM protected memory region. The Boot ROM code does this dummy read for convenience.
System Control and Interrupts
58
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Summary of Contents for TMS320 2806 Series
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