MC68HC908AS32A Datasheet, PDF (41/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Electrically Erasable Programmable Read-Only Memory (EEPROM)
2.8.1.1 EEPROM Configuration
The 8-bit EEPROM nonvolatile register (EENVR) and the 16-bit EEPROM timebase divider nonvolatile
register (EEDIVNVR) contain the default settings for the following EEPROM configurations:
• EEPROM timebase reference
• EEPROM security option
• EEPROM block protection
EENVR and EEDIVNVR are nonvolatile EEPROM registers that are programmed and erased in the same
way as EEPROM bytes. The contents of these registers are loaded into their respective volatile registers
during a MCU reset. The values in these read/write volatile registers define the EEPROM configurations.
• For EENVR, the corresponding volatile register is the EEPROM array configuration register
(EEACR).
• For the EEDIVNCR (two 8-bit registers: EEDIVHNVR and EEDIVLNVR), the corresponding volatile
register is the EEPROM divider register (EEDIV: EEDIVH and EEDIVL).
2.8.1.2 EEPROM Timebase Requirements
A 35 µs timebase is required by the EEPROM control circuit for program and erase of EEPROM content.
This timebase is derived from dividing the CGMXCLK or bus clock (selected by EEDIVCLK bit in
CONFIG2 register) using a timebase divider circuit controlled by the 16-bit EEPROM timebase divider
EEDIV register (EEDIVH and EEDIVL).
As the CGMXCLK or bus clock is user selected, the EEPROM timebase divider register must be
configured with the appropriate value to obtain the 35 µs. The timebase divider value is calculated by
using the following formula:
EEDIV= INT[Reference Frequency(Hz) x 35 x10^–6 +0.5]
This value is written to the EEPROM timebase divider register (EEDIVH and EEDIVL) or programmed into
the EEPROM timebase divider nonvolatile register prior to any EEPROM program or erase operations
(see 2.8.1.1 EEPROM Configuration and 2.8.1.2 EEPROM Timebase Requirements).
2.8.1.3 EEPROM Program/Erase Protection
The EEPROM has a special feature that designates the 16 bytes of addresses from $08F0–$08FF to be
permanently secured. This program/erase protect option is enabled by programming the EEPRTCT bit in
the EEPROM nonvolatile register to a 0.
Once the EEPRTCT bit is programmed to 0 for the first time:
• Programming and erasing of secured locations $08F0–$08FF is permanently disabled.
• Secured locations $08F0–$08FF can be read as normal.
• Programming and erasing of EENVR is permanently disabled.
• Bulk and block erase operations are disabled for the unprotected locations $0800–$08EF and
$0900–$09FF.
• Single byte program and erase operations are still available for locations $0800–$08EF and
$0900–$09FF for all bytes that are not protected by the EEPROM block protect EEBPx bits (see
2.8.1.4 EEPROM Block Protection and 2.8.2.2 EEPROM Array Configuration Register)
MC68HC908AS32A Data Sheet, Rev. 2.0
Freescale Semiconductor
41