MOTOROLA
MC68306 USER'S MANUAL
3- 9
CLK
A31–A1
AS
S0
S1 S2
S3 S4
S5 S6
S7
S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19
INDIVISIBLE CYCLE
UDS OR LDS
R/W
DTACK
D15–D8 OR
D7–D0
FC2–FC0
Figure 3-9. Read-Modify-Write Cycle Timing Diagram
The descriptions of the read-modify-write cycle states are as follows:
STATE 0
The read cycle starts in S0. The processor places valid function codes on
FC2–FC0, a valid address on the address bus, and drives R/
W
high to
identify a read cycle.
STATE 1
During S1, no bus signals are altered.
STATE 2
On the rising edge of S2, the processor asserts
AS
and
UDS
/
LDS
.
STATE 3
During S3, no bus signals are altered.
STATE 4
During S4, the processor waits for a cycle termination signal (
DTACK
or
BERR
). If neither termination signal is asserted before the falling
edge at the end of S4, the processor inserts wait states (full clock cycles)
until either
DTACK
or
BERR
is asserted.
Case R1:
DTACK
only.
STATE 5
During S5, no bus signals are altered.
STATE 6
During S6, data from the device are driven onto the data bus.
STATE 7
On the falling edge of the clock entering S7, the processor accepts data from
the device and negates
UDS
/
LDS
. The device negates
DTACK
at this
time.
STATES
8–11
The bus signals are unaltered during S8–S11, during which the arithmetic
logic unit makes appropriate modifications to the data.