MAXQ612/MAXQ622 User’s Guide
Maxim Integrated 15-3
15.2 Password-Protected Access
Some applications require preventive measures to protect against simple access and viewing of program code
memory. To address this need for code protection, any MAXQ612/MAXQ622 microcontroller equipped with a utility
ROM that permits in-system programming, in-application programming, or in-circuit debugging grants full access to
those utilities only after a password has been supplied. The password is defined as the 16 words of physical program
memory at addresses 0010h to 001Fh of each memory area (system, user loader, user application, see Figure 2-7).
Note that using these memory locations as a password does not exclude their usage for general code space if a unique
password is not needed.
Multiple password lock bits (PWL/PWLS/PWLL) are implemented in the SC register. When a PWL bit is set to 1, a pass-
word is required to access the in-circuit debug and in-system programming utility ROM routines that allow reading
or writing of internal memory. When a PWL is cleared to 0, these utilities are fully accessible through the utility ROM
without password.
The PWL bits default to 1 after a power-on reset. To access the ROM utilities, a correct password is needed; otherwise,
access to the utility ROM utilities is denied. Once the correct password has been supplied by the user, the utility ROM
clears the password lock. The PWLs remain clear until one of the following occurs:
• Power-on reset
or
• Set to 1 by user software
For flash-less devices with ROM program memory, the end user supplies the ROM code, thus the user always knows
the password if needed. It is expected that the password is rarely needed since the utility of memory programming and/
or in-circuit debug to the end user is minimal once the decision has been made to freeze the code in program ROM.
For devices with reprogrammable nonvolatile memory, the password is always known for a fully erased device since
the unprogrammed state of these memories is fixed. Once the memory has been programmed, a password is estab-
lished and can be used for access protection. The utility ROM code denies access to the protected routines when
PWL indicates a locked state.
15.2.1 Entering Passwords
A password can be entered in one of two ways:
• Through the in-system programming interface established by the PSS[1:0] bits when SPE bit is set to 1; the util-
ity ROM bootstrap loader dictates the protocol for entering the password over the specified serial communication
interface.
• Through the TAP interface directly by issuing the unlock password debug mode command. The unlock password
command requires 32 follow-on transfer cycles each containing a byte value to be compared with the program
memory password.
(93 Seiten)
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