
MOTOROLA
MC68341 USER’S MANUAL
4- 35
4.3.4.4 MAP SELECT REGISTER. The map select register can map byte wide
peripherals on the M68000 bus to either the upper or lower half of the data bus (even or
odd address space).
Map Select Register
$03E
15
14
13
12
11
10
9876543210
RESERVED
MSR7
MSR6
MSR5
MSR4
MSR3
MSR2
MSR1
MSR0
RESET:
0000000000000000
Supervisor Only
Setting a bit in the MSR defines the corresponding CS≈ bit to be a byte-wide peripheral.
Even/odd select capability is provided by pairing of chip selects, so the base address for
an even/odd pair is specified in the even chip select register (CS2 provides base address
and range for CS2 and CS3 if MSR3 is set). The base address and range information is
combined with the size and A0 information drive the corresponding chip select pins for an
even/odd pair of chip selects. If an access addresses both even and odd bytes, then the
corresponding even and odd chip select pins are asserted.
The chip select implementation requires that odd byte chip selects used for M68000 byte
peripherals be paired with even byte chip selects of similar address, but does not restrict
even byte peripheral chip selects. For example, if CS2 is used for an M68000 byte
peripheral, but there is no byte peripheral in that address range on the lower half of the
data bus, then CS3 can be used as a general purpose chip select for any address range,
bus type, or port size.
4.3.4.5 CHIP SELECT REGISTERS PROGRAMMING EXAMPLE. The following listing is
an example of programming a chip select at starting address $00040000, for a block size
of 256 Kbytes, accessing supervisor and user data spaces with a 16-bit port requiring two
wait states. There will be no write protection, no fast termination, and no CPU space
accesses.
base address 1 = $0004
base address 2 = $0013
address mask 1 = $0003
address mask 2 = $FF49
NOTE
If an access matches multiple chip selects, the lowest
numbered chip select will have priority. For example, if CS0
and CS2 "overlap" for a certain range, CS0 will assert when
accessing the "overlapped" address range, and CS2 will not.
4.3.5 External Bus Interface Control
The following paragraphs describe the registers that control the I/O pins used with the
EBI. Refer to the Section 3 Bus Operation for more information about the EBI. For a list
of pin numbers used with port A and port B, see the pinout diagram in Section 13
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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