6

M68000 USER’S MANUAL ADDENDUM

MOTOROLA

8-bit mode requires two bus cycles to fetch the immediate data of the STOP instruction. After the

processor clock is disabled, it is often necessary to disable the clock to other sections of your circuit.

This can be done, but be careful that runt clocks and spurious glitches are not presented to the

MC68SEC000. A timing diagram is shown in Figure 4.

Figure 4. MC68SEC000 Clock Stop Timing for 16-Bit Data Bus

pull-up or pull-down resistor.

3.

This step is optional depending on whether your applications require the MC68SEC000 signals with

three-state capability to be placed into a high-impedance state. To place the MC68SEC000 into a

three-state condition, the proper method for arbitrating the bus (as described in 5.2 Bus Arbitration

in the

M68000 User’s Manual, Rev 8) should be completed during the fetch of the status register data

for the STOP instruction. A timing diagram with the bus arbitration sequence is shown in Figure 5.

Figure 5. MC68SEC000 Clock Stop Timing with Bus Arbitration for 16-Bit Data Bus

Write to

Low-Power

Address

Fetch Immediate

Data of STOP

Instruction

Stop

CLK

S0 S1 S2 S3 S4 S5 S6 S7

S0 S1 S2 S3 S4 S5 S6 S7

CPU_CLK

DTACK

RW

AS

BR

BG

CLK

S0 S1 S2 S3 S4 S5 S6 S7

S0 S1 S2 S3 S4 S5 S6 S7

CPU_CLK

DTACK

RW

AS

Write to

Low-Power

Address

Fetch Immediate

Data of STOP

Instruction

Stop