Chapter 21 64 KB Flash Module (S12ZFTMRZ64K2KV2)
S12ZVHY/S12ZVHL Family Reference Manual Rev. 1.05
Freescale Semiconductor
739
21.4.10 Stop Mode
If a Flash command is active (CCIF = 0) when the MCU requests stop mode, the current Flash operation
will be completed before the MCU is allowed to enter stop mode.
21.5
Security
The Flash module provides security information to the MCU. The Flash security state is defined by the
SEC bits of the FSEC register (see
). During reset, the Flash module initializes the FSEC
register using data read from the security byte of the Flash configuration field at global address
0xFF_FE0F. The security state out of reset can be permanently changed by programming the security byte
assuming that the MCU is starting from a mode where the necessary P-Flash erase and program commands
are available and that the upper region of the P-Flash is unprotected. If the Flash security byte is
successfully programmed, its new value will take affect after the next MCU reset.
The following subsections describe these security-related subjects:
•
Unsecuring the MCU using Backdoor Key Access
•
Unsecuring the MCU in Special Single Chip Mode using BDM
•
.Mode and Security Effects on Flash Command Availability
21.5.1
Unsecuring the MCU using Backdoor Key Access
The MCU may be unsecured by using the backdoor key access feature which requires knowledge of the
contents of the backdoor keys (four 16-bit words programmed at addresses 0xFF_FE00-0xFF_FE07). If
the KEYEN[1:0] bits are in the enabled state (see
), the Verify Backdoor Access Key
command (see
) allows the user to present four prospective keys for comparison to the
keys stored in the Flash memory via the Memory Controller. If the keys presented in the Verify Backdoor
Access Key command match the backdoor keys stored in the Flash memory, the SEC bits in the FSEC
register (see
) will be changed to unsecure the MCU. Key values of 0x0000 and 0xFFFF are
not permitted as backdoor keys. While the Verify Backdoor Access Key command is active, P-Flash
memory and EEPROM memory will not be available for read access and will return invalid data.
The user code stored in the P-Flash memory must have a method of receiving the backdoor keys from an
external stimulus. This external stimulus would typically be through one of the on-chip serial ports.
If the KEYEN[1:0] bits are in the enabled state (see
), the MCU can be unsecured by the
backdoor key access sequence described below:
1. Follow the command sequence for the Verify Backdoor Access Key command as explained in
2. If the Verify Backdoor Access Key command is successful, the MCU is unsecured and the
SEC[1:0] bits in the FSEC register are forced to the unsecure state of 10
The Verify Backdoor Access Key command is monitored by the Memory Controller and an illegal key will
prohibit future use of the Verify Backdoor Access Key command. A reset of the MCU is the only method
