
Integrated Memories
MOTOROLA
ColdFire2/2M User’s Manual
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5-17
5.3.2.2 WP: ROMBAR[8]—WRITE PROTECT.
This bit is reserved for future use. This bit can be used for debug purposes; i.e. it could set
a trip point when a write to ROM occurred.
0 = No effect
1 = An attempted write access will generate an access error exception in the processor.
5.3.2.3 AS ROMBAR[5:1]—ADDRESS SPACE MASKS.
This five-bit field, specified by ROMBAR[5:1], allows certain types of accesses to be
“masked” or inhibited, from accessing the RAM module. The mask bits are defined as:
AS5 - Mask CPU Space and Interrupt Acknowledge Accesses
AS4 - Mask Supervisor Code Accesses
AS3 - Mask Supervisor Data Accesses
AS2 - Mask User Code Accesses
AS1 - Mask User Data Accesses
If a given mask bit is set, then references of that type are NOT allowed to access the ROM
module.
If ASn = 0, then accesses of the given type are allowed by the ROM.
If ASn = 1, then accesses of the given type are not allowed by the ROM. If an access is made
to a space that is masked, it simply becomes mapped to the next valid space.
5.3.2.4 V:
ROMBAR [0]—VALID.
The valid bit is specified by ROMBAR[0]. This bit is cleared by a hardware reset. When set,
this bit enables the ROM module, otherwise the module is disabled.
If ROM_SZ is set to zero, this bit is set to zero and the ROM module is disabled. Any attempt
to set the valid bit is disabled when ROM_SZ is set to zero.
5.3.3 ROM INITIALIZATION / ROM BOOT
After a hardware reset, the contents of the ROMBAR depend upon the ROM_VLD signal
polarity and the ROM_SZ[2:0] configuration. If ROM_SZ[2:0]
=
0, the valid bit is held
cleared, even if a write is attempted during a load to ROMBAR.
ROM_VLD determines if the ROM module should be active immediately after a hard reset.
Thus, if ROM_VLD is asserted, the first fetches ($0, $4) go to ROM instead of external
memory. Note, if ROM_VLD is asserted, ROM_SZ cannot be set to zero. ROM_VLD
controls the reset value of the ROM base address register (i.e. the ROM must be based at
$0000 if ROM_VLD is asserted).
To map the ROMBAR, a load to the ROMBAR mapping the ROM module to the desired
location within the address space, must be performed.
F
Freescale Semiconductor, Inc.
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