Motorola MC68020 User Manual

5- 26 M68020 USER’S MANUAL MOTOROLA
5.3.1 Read Cycle
During a read cycle, the processor receives data from a memory, coprocessor, or
peripheral device. If the instruction specifies a long-word operation, the MC68020/EC020
attempts to read four bytes at once. For a word operation, it attempts to read two bytes at
once and for a byte operation, one byte. For some operations, the processor requests a
three-byte transfer. The processor properly positions each byte internally. The section of
the data bus from which each byte is read depends on the operand size, A1–A0, and the
port size. Refer to 5.2.1 Dynamic Bus Sizing and 5.2.2 Misaligned Operands for more
information on dynamic bus sizing and misaligned operands.
Figure 5-19 is a flowchart of a long-word read cycle. Figure 5-20 is a flowchart of a byte
read cycle. Figures 5-21–5-23 are read cycle timing diagrams in terms of clock periods.
Figure 5-21 corresponds to byte and word read cycles from a 32-bit port. Figure 5-22
corresponds to a long-word read cycle from an 8-bit port. Figure 5-23 also applies to a
long-word read cycle, but from 16- and 32-bit ports.
PROCESSOR
ADDRESS DEVICE
1) ASSERT ECS/OCS FOR ONE-HALF CLOCK
2
) SET R/W TO READ
3
) DRIVE ADDRESS ON A31–A0
4
) DRIVE FUNCTION CODE ON FC2–FC0
5
) DRIVE SIZ1, SIZ0 (FOUR BYTES)
6
) ASSERT AS
7
) ASSERT DS
8
) ASSERT DBEN
ACQUIRE DATA
1) LATCH DATA
2
) NEGATE AS AND D
S
3
) NEGATE DBEN
START NEXT CYCLE
PRESENT DATA
1) DECODE ADDRESS
2
) PLACE DATA ON D31–D0
3
) ASSERT DSACK1/DSACK0
TERMINATE CYCLE
1) REMOVE DATA FROM D31–D0
2
) NEGATE DSACK1/DSACK0
EXTERNAL DEVICE
*
*
*
This step does not apply to the MC68EC020.
For the MC68EC020, A23–A0.
*
**
Figure 5-19. Long-Word Read Cycle Flowchart

Motorola MC68020 Specifications

General

Brand	Motorola
Model	MC68020
Category	Computer Hardware
Language	English