Motorola MC68VZ328 6.3 Programming Model

6-4 MC68VZ328 User’s Manual 
Programming Model
6.2.3 Overlapping Chip-Select Registers
Do not program group address and chip-select registers to overlap, or the chip-select signals will overlap.
Unused chip-selects must be disabled. Map them to an unused space, if possible.
When the CPU tries to write to a read-only location that has already been programmed, the chi p-sel ect and
DTACK signals will not be generated internally. BERR will be asserted internally if the bus error time-out
function is enabled.
NOTE:
The chip-select logic does not allow an address match during interrupt
acknowledge cycles.
6.3 Programming Model
The chip-select module contains registers that are programmed to control external devices, such as
memory. Chip-selects do not operate until the register in a particul ar group of devices is initialized and the
EN bit is set in the corresponding chip-select register. The only exception is the CSA0 signal, which is the
boot device chip-select.
6.3.1 Chip-Select Group Base Address Registers
The upper 15 bits of each base address register selects the starting address for the chip-select address
range. The GBAx field is compared to the address on the address bus to de termine if the group is decoded.
The chip-select base address must be set according to the size of the corresponding chip-select signals of
the group. For example, if CSA1 and CSA0 are each assigned a 2Mbyte memory space, the CSGBA
register must be set in a 4Mbyte space boundary, such as system address 0 ×0, 0×4 Mbyte, 0×8Mbyte,
and so on. It cannot be set at 0×1 Mbyte, 0 ×2Mbyte, 0×3 Mbyte, 0×5 Mbyte, and so on.
CSGBA Chip-Select Group A Base Address Register 0x(FF)FFF100
BIT
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 BIT 0
GB
A28 GB A27 GB A26 GB A25 GB A24 GB A23 GB A22 GB A21 GB A20 GB A19 GB A18 GB A17 GB A16 GB A15 GB A14
TYPE rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw
RESET 000000000000000 0
0x0000
Table 6-2. Chip-Select Group A Base Address Register Description
Name Description Setting
GBAx
Bits 15–1 Group A Base Address—These bits select the high-order bits (28–14) of the starting
address for the chip-select range.
The chip-select base address must be set
according to the size of the corresponding
chip-select signals of the group.
Reserved
Bit 0 Reserved This bit is reserved and should be set to 0.