
System Integration Module
5-30
MC68307 USER’S MANUAL
MOTOROLA
CD2–CD0—Low-Power Clock Divider Ratio
Program these bits to specify the frequency of clock signal for the EC000 core processor
when CPU clock division is enabled by the CDEN bit.
After a cold reset , these bits are cleared, so the EC000 core would run at half the system
clock frequency, if the CDEN bit was set. These bits are unaffected by wake-up. For main
low-power sleep (LPEN) operation, the CD2–CD0 and CDEN bit values are ignored.
NOTE
The clock divider ratio should only be changed from one divider
value to another when the CDEN bit is zero, i.e. when the CPU
is operating at full speed. Changing from one reduced speed to
another is not recommended.
5.2.2 Chip Select Registers
Each of the four chip select units has two registers that define its specific operation. These
registers are a 16-bit base register (BR) and a 16-bit option register (OR) (e.g., BR0 and
OR0). These registers may be modified by the EC000 core. The BR should normally be
programmed after the OR since the BR contains the chip select enable bit. Programming
both registers at once using a long word write is also recommended.
5.2.2.1 BASE REGISTERS (BR3–BR0). These 16-bit registers consist of a base address
field, a read-write bit, a function code field and an enable bit.
FC2–FC0—Function Code Field
These bits are used to set the address space function code. The address compare logic
uses these bits to determine whether an address match exists within its address space
and, therefore, whether to assert the chip select line. Although the FC2–FC0 signals are
Value
Division Ratio
Frequency from
16MHz EXTAL
000
÷2
~8MHz
001
÷4
~4MHz
010
÷8
~2MHz
011
÷16
~1MHz
100
÷32
~512kHz
101
÷64
~256kHz
110
÷128
~128kHz
111
÷256
~64kHz
BR0, BR1, BR2, BR3
MBASE+$040, $044, $048, $04C
15
13
12
2
1
0
FC2
FC1
FC0
A23
A22
A21
A20
A19
A18
A17
A16
A15
A14
A13
RW
EN
RESET:
1
0
1
Read/Write
Supervisor or User