Memory Controller
MOTOROLA
MPC801 USER’S MANUAL
15-71
15
15.4.4 Option Register
The option register (OR) contains the address and address type mask bit for address bus
comparison. It also includes the CS general bits and all the GPCM parameters.
AM—Address Mask
These read/write bits provides masking on any corresponding bits in the associated base
register. By masking the address bits independently, external devices of different size
address ranges can be used. Any clear bit masks the corresponding address bit and any set
bit causes the corresponding address bit to be used in address pin comparison. Address
mask bits can be set or cleared in any order in the field, thus allowing a resource to reside
in more than one area of the address map. Be aware that following a system reset, these
bits are reset in OR0.
ATM—Address Type Mask
These bits can be used to mask certain address type bits, thus allowing more than one
address space type to be assigned to a chip-select. Any set bit causes the corresponding
address type code bits to be used as part of the address comparison and any cleared bit
masks the corresponding address type code bit. The ATM bits should be cleared so that
address type codes are ignored as part of the address comparison. Be aware that following
a system reset, these bits are reset in OR0.
CSNT—Chip-Select Negation Time
This bit is used to determine when the CS/WE signals are negated during an external
memory write access handled by the general-purpose chip-select machine. This helps in
meeting address/data hold time requirements for slow memories and peripherals. Be aware
that following a system reset, this bit is set in OR0.
0 = CS
1 = CS
/
/
WE are negated normally.
WE are negated a quarter of a clock earlier.
SAM—Start Address Multiplex
This bit determines how the address is output on the first cycle of an external memory
access read or write when the memory access is handled by
the
UPMA or UPMB.
0 = Address pins are the address requested by the internal master.
1 = Address pins are the address requested by the internal master multiplexed
according to the AMA field (if UPMA is selected to control the memory access) or
the AMB field (if UPMB is selected).
OR
BIT
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
FIELD
AM
BIT
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
FIELD
AM
ATM
CSNT
/SAM
ACS/
G5LA,G5LS
BI
SCY
SETA
TRLX
EHTR
RES