mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 53

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

3.6 CPU16 Memory Mapping
MC68HC16Y3/916Y3
USER’S MANUAL
Each member of the M68HC16 family is comprised of a set of modules connected by
the intermodule bus (IMB). The full IMB has a 16-bit data bus, a 24-bit address bus,
and three function code lines, and ideally provides eight distinct memory maps, each
with 16 megabytes of address space. In practice, only four of these memory maps are
available for user code and data. Three are inaccessible because the function codes
lines are never driven to states that allow them to be decoded, and one is devoted ex-
clusively to control information not associated with normal read and write bus cycles.
The total amount of addressable memory is further limited on the CPU16. While the
CPU32 can operate in both the user and supervisor modes denoted by the function
code lines, the CPU16 operates only in supervisor mode. Excluding the CPU space
memory map used for special bus cycles, the CPU16 can access only the supervisor
program space and supervisor data space memory maps. The CPU16 also has only
20 address lines. This limits the total address space in each of the two memory maps
to one megabyte.
Although the CPU16 has only 20 addresses lines, it still drives all 24 IMB address
lines. IMB address lines [19:0] follow CPU address lines [19:0], and IMB address lines
[23:20] follow the state of CPU address line 19 as shown in Figure 3-5. This causes
an address space discontinuity to appear on the IMB when the CPU16 address bus
rolls over from $7FFFF to $80000.
Figure 3-5 Address Bus Connections Between the CPU16 and IMB
CPU ADDR10
CPU ADDR11
CPU ADDR12
CPU ADDR13
CPU ADDR14
CPU ADDR15
CPU ADDR16
CPU ADDR17
CPU ADDR18
CPU ADDR19
CPU ADDR0
CPU ADDR1
CPU ADDR2
CPU ADDR3
CPU ADDR4
CPU ADDR5
CPU ADDR6
CPU ADDR7
CPU ADDR8
CPU ADDR9
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IMB ADDR0
IMB ADDR1
IMB ADDR2
IMB ADDR3
IMB ADDR4
IMB ADDR5
IMB ADDR6
IMB ADDR7
IMB ADDR8
IMB ADDR9
IMB ADDR10
IMB ADDR11
IMB ADDR12
IMB ADDR13
IMB ADDR14
IMB ADDR15
IMB ADDR16
IMB ADDR17
IMB ADDR18
IMB ADDR19
IMB ADDR20
IMB ADDR21
IMB ADDR22
IMB ADDR23
CPU16 ADDRESS CONNECTIONS
MOTOROLA
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