MC9S12HZ256 Datasheet, PDF (51/692 Pages) Freescale Semiconductor, Inc – HCS12 Microcontrollers

Chapter 1 MC9S12HZ256 Device Overview
to the mode select bits in the MODE register (which is allowed in special modes) can change this after
reset. All of the port E pins (except PE4/ECLK) are initially configured as general-purpose
high-impedance inputs with pull-downs enabled. PE4/ECLK is configured as the E clock output in this
mode.
The pins associated with port E bits 6, 5, 3, and 2 cannot be configured for their alternate functions IPIPE1,
IPIPE0, LSTRB, and R/W while the MCU is in single chip modes. In single chip modes, the associated
control bits PIPOE, LSTRE and RDWE are reset to zero. Writing the opposite value into these bits in
single chip mode does not change the operation of the associated port E pins.
Port E, bit 4 can be configured for a free-running E clock output by clearing NECLK = 0. Typically the
only use for an E clock output while the MCU is in single chip modes would be to get a constant speed
clock for use in the external application system.
1.8 Security
The device will make available a security feature preventing the unauthorized read and write of the
memory contents. This feature allows:
• Protection of the contents of FLASH
• Protection of the contents of EEPROM
• Operation in single-chip mode
• Operation from external memory with internal FLASH and EEPROM disabled
The user must be reminded that part of the security must lie with the user’s code. An extreme example
would be user’s code that dumps the contents of the internal program. This code would defeat the purpose
of security. At the same time the user may also wish to put a back door in the user’s program. An example
of this is the user downloads a key through the SCI which allows access to a programming routine that
updates parameters stored in EEPROM.
1.8.1 Securing the Microcontroller
After the user has programmed the FLASH and EEPROM (if desired), the part can be secured by
programming the security bits located in the FLASH module. These non-volatile bits will keep the part
secured through resetting the part and through powering down the part.
The security byte resides in a portion of the Flash array.
Check the Flash block description chapter for more details on the security configuration.
1.8.2 Operation of the Secured Microcontroller
1.8.2.1 Normal Single Chip Mode
This will be the most common usage of the secured part. Everything will appear the same as if the part was
not secured with the exception of BDM operation. The BDM operation will be blocked.
MC9S12HZ256 Data Sheet, Rev. 2.04
Freescale Semiconductor
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