C501_1 Datasheet, PDF (96/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

OTP Memory Operation
C501
Note that the EA/V_PP pin must not be allowed to go above the maximum specified V_PP level for any
amount of time. Even a narrow glitch above that voltage can cause permanent damage to the
device. The V_PP source should be well regulated and free of glitches and overshoots.
9.3 Encryption Table
The encryption table feature of the C501-1E is a feature that protects the program code in the OTP
memory from being easily read by anyone other than the programmer. The encryption table is
32 byte of code that is exclusive NORed with the OTP memory data as it is read out. The first byte
is XNORed with the first location read, the second with the second read, etc. through the 32nd byte
read. The 33rd read byte is XNORED with the first byte of the encryption table, the 34rd with the
second, etc. and so on in 32-byte groups.
After the encryption table has been programmed, the user has to know its contents in order to
correctly decode the program code stored in the OTP memory. The encryption table itself cannot be
read out.
For programming of the encryption table, the 25 pulse programming sequence must be repeated for
addresses 0 through 1FH, using the “Program encryption table“ levels. After the encryption table is
programmed, verification cycles will produce only encrypted data.
9.4 Security Bits
There are two security bits on the C501-1E that, when set, prevent the OTP program memory from
being read out or programmed further. For programming of the security bits, the 25 pulse
programming sequence must be repeated using the “Program security bit“ levels as specified in
table 9-11. After the first security bit is programmed, further programming of the OTP memory and
the encryption table is disabled. However, the other security bit can still be programmed. With only
security bit one programmed, the OTP memory can still be read out for program verification. After
the second security bit is programmed, it is no longer possible to read out (verify) the OTP memory
content.
Semiconductor Group
9-3