C508_01 Datasheet, PDF (52/267 Pages) Infineon Technologies AG – 8-Bit CMOS Microcontroller

C508
External Bus Interface
4.7
ROM/OTP Protection for the C508-4R / C508-4E
The C508-4R allows protection of the contents of the internal ROM against unauthorized
read out. The type of ROM protection is fixed with the ROM mask. Therefore, users of
the C508-4R version must define whether ROM protection has to be selected or not.
The C508-4E OTP version also allows program memory protection at several levels (see
Chapter 10.6). The program memory protection for the C508-4E can be activated after
programming of the device.
The C508-4R devices, which operate from internal ROM, are always checked for correct
ROM contents during production test. Therefore, unprotected as well as protected ROMs
must provide a procedure to verify the ROM contents. In ROM verification mode 1, which
is used to verify unprotected ROMs, a ROM address is applied externally to the C508-
4R and the ROM data byte is output at Port 0. ROM verification mode 2, which is used
to verify ROM protected devices, operates differently. In this mode, ROM addresses are
generated internally and the expected data bytes must be applied externally to the
device (by the manufacturer or by the customer) and are compared internally with the
data bytes from the ROM. After 16 byte verify operations, the state of the P3.5 pin shows
whether the last 16 bytes have been verified as expected.
This mechanism provides very high security for ROM protection. Only the owner of the
ROM code and the manufacturer who knows the contents of the ROM can read out and
verify it.
The behavior of the move code instruction, when the code is executed from the external
ROM, is such that accessing a code byte from a protected on-chip ROM address is not
possible. In this case, the byte accessed will be invalid.
4.7.1 Unprotected ROM Mode
If the ROM is unprotected, ROM verification mode 1 as shown in Figure 4-4 is used to
read out the contents of the ROM. Please refer to the AC specifications in the C508 Data
Sheet for the AC timing characteristics of the ROM verification modes.
P1.0-P1.7
P2.0-P2.6
Port 0
Address 1
Data 1 OUT
Address 2
Data 2 OUT
Inputs: PSEN, P2.7 = V_SS
ALE, EA = V_IH/V_IH2
RESET = V_IH1
MCS04096
Figure 4-4 ROM Verification Mode 1
User’s Manual
4-10
2001-05