MC9S08AW60 Datasheet, PDF (54/320 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 4 Memory
4.4.5 Access Errors
An access error occurs whenever the command execution protocol is violated.
Any of the following specific actions will cause the access error flag (FACCERR) in FSTAT to be set.
FACCERR must be cleared by writing a 1 to FACCERR in FSTAT before any command can be processed.
• Writing to a FLASH address before the internal FLASH clock frequency has been set by writing
to the FCDIV register
• Writing to a FLASH address while FCBEF is not set (A new command cannot be started until the
command buffer is empty.)
• Writing a second time to a FLASH address before launching the previous command (There is only
one write to FLASH for every command.)
• Writing a second time to FCMD before launching the previous command (There is only one write
to FCMD for every command.)
• Writing to any FLASH control register other than FCMD after writing to a FLASH address
• Writing any command code other than the five allowed codes ($05, $20, $25, $40, or $41) to
FCMD
• Accessing (read or write) any FLASH control register other than the write to FSTAT (to clear
FCBEF and launch the command) after writing the command to FCMD.
• The MCU enters stop mode while a program or erase command is in progress (The command is
aborted.)
• Writing the byte program, burst program, or page erase command code ($20, $25, or $40) with a
background debug command while the MCU is secured (The background debug controller can
only do blank check and mass erase commands when the MCU is secure.)
• Writing 0 to FCBEF to cancel a partial command
4.4.6 FLASH Block Protection
The block protection feature prevents the protected region of FLASH from program or erase changes.
Block protection is controlled through the FLASH Protection Register (FPROT). When enabled, block
protection begins at any 512 byte boundary below the last address of FLASH, $FFFF. (see Section 4.6.4,
“FLASH Protection Register (FPROT and NVPROT)”).
After exit from reset, FPROT is loaded with the contents of the NVPROT location which is in the
nonvolatile register block of the FLASH memory. FPROT cannot be changed directly from application
software so a runaway program cannot alter the block protection settings. Since NVPROT is within the last
512 bytes of FLASH, if any amount of memory is protected, NVPROT is itself protected and cannot be
altered (intentionally or unintentionally) by the application software. FPROT can be written through
background debug commands which allows a way to erase and reprogram a protected FLASH memory.
The block protection mechanism is illustrated below. The FPS bits are used as the upper bits of the last
address of unprotected memory. This address is formed by concatenating FPS7:FPS1 with logic 1 bits as
shown. For example, in order to protect the last 8192 bytes of memory (addresses $E000 through $FFFF),
the FPS bits must be set to 1101 111 which results in the value $DFFF as the last address of unprotected
memory. In addition to programming the FPS bits to the appropriate value, FPDIS (bit 0 of NVPROT) must
MC9S08AW60 Data Sheet, Rev.1.0
54
Freescale Semiconductor