Maxim Integrated MAXQ610 - Manual (Page 167)

13-3
MAXQ610 User’s Guide
13.2 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 MAXQ610 microcontroller equipped with a utility ROM that
permits in-system programming, in-application programming, or in-circuit debugging grants full access to those utili-
ties 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 pass-
word 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 .
13.2.1 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 .