Freescale Semiconductor, Inc.
Operating Modes
Internal Resource Mapping
EXSTR1, EXSTR0 — External Access Stretch
This two bit field determines the amount of clock stretch on accesses
to the External Address Space. In Single Chip and Peripheral Modes
this bit has no meaning or effect.
Table 5-7. EXSTR Stretch Bit Definition
Number of E Clocks
EXSTR1
EXSTR0
Stretched
0
0
1
1
0
1
0
1
0
1
2
3
ROMHM — FLASH EEPROM only in second Half of Map
This bit has no meaning if ROMON bit is clear.
0 = The 16K byte of fixed Flash EEPROM in location $4000–$7FFF
can be accessed.
1 = Disables direct access to 16K byte Flash EEPROM from
$4000–$7FFF in the memory map. The physical location of
this16K byte Flash can still be accessed through the Program
Page window.
In special mode, with ROMTST bit set, this bit will allow overlap of the
four 32K Flash EEPROM arrays and overlap the four 4-byte Flash
register space in the same map space to be able to program all arrays
at the same time.
0 = The four 32K Flash arrays are accessed with four pages for
each.
1 = The four 32K Flash arrays coincide in the same space and are
selected at the same time for programming.
CAUTION: Bit must be cleared before reading any of the arrays or registers.
ROMON — Enable FLASH EEPROM
These bits are used to enable the Flash EEPROM
0 = Disables Flash EEPROM in the memory map.
1 = Enables Flash EEPROM in the memory map.
MC68HC912DG128 — Rev 3.0
Technical Data
Operating Modes
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