TSC80251G2D
8.2.1 Lock Bit System
The TSC87251G2D products implement 3 levels of security for User’s program as described in Table 35. The
TSC83251G2D products implement only the first level of security.
Level 0 is the level of an erased part and does not enable any security features.
Level 1 locks the programming of the User’s internal Code Memory, the Configuration Bytes and the Encryption
Array.
Level 2 locks the verifying of the User’s internal Code Memory. It is always possible to verify the Configuration
Bytes and the Lock Bits. It is never possible to verify the Encryption Array.
Level 3 locks the external execution.
Table 35. Lock Bits Programming
External
Lock bits
LB[2:0]
Internal
External
Level
Verification
Programming
PROM read
(MOVC)
Execution
Execution
(1)
(2)
0
1
2
3
000
001
Enable
Enable
Enable
Enable
Enable
Enable
Enable
Disable
Enable
Enable
Disable
Disable
Disable
Enable
(1)
Enable
Disable
Disable
Disable
(3)
01x
Disable
Disable
(3)
1xx
Notes:
1. Returns encrypted data if Encryption Array is programmed.
2. Returns non encrypted data.
3. x means don’t care. Level 2 always enables level 1, and level 3 always enables levels 1 and 2.
The security level may be verified according to Table 36.
Table 36. Lock Bits Verifying
Level
Lock bits Data(1)
0
1
2
3
xxxxx000
xxxxx001
xxxxx01x
xxxxx1xx
Note:
1. x means don’t care.
8.2.2 Encryption Array
The TSC83251G2D and TSC87251G2D products include a 128-byte Encryption Array located in non-volatile
memory outside the memory address space. During verification of the on-chip code memory, the seven low-order
address bits also address the Encryption Array. As the byte of the code memory is read, it is exclusive-NOR’ed
(XNOR) with the key byte from the Encryption Array. If the Encryption Array is not programmed (still all 1s),
the user program code is placed on the data bus in its original, unencrypted form. If the Encryption Array is
programmed with key bytes, the user program code is encrypted and cannot be used without knowledge of the
key byte sequence.
To preserve the secrecy of the encryption key byte sequence, the Encryption Array can not be verified.
Cautions:
1. When a MOVC instruction is executed, the content of the ROM is not encrypted. In order to fully protect the user program code, the lock
bit level 1 (see Table 35) must always be set when encryption is used.
2. If the encryption feature is implemented, the portion of the on-chip code memory that does not contain program code should be filled with
“random” byte values to prevent the encryption key sequence from being revealed.
29
Rev. A - May 7, 1999
TEMIC [ TEMIC SEMICONDUCTORS ]
