Figure 6-19 shows the timing when the bus right is requested by an external bus master during a
read cycle in a two-state-access area. There is a minimum interval of two states from when the
BREQ
signal goes low until the bus is released.
Figure 6-19 External-Bus-Released State (Two-State-Access Area, During Read Cycle)
ø
Data bus
AS
HWR
BREQ
BACK
RD
,
LWR
,
T
1
T
2
Address
2
1
3
4
5
6
High
CPU cycles
External bus released
CPU cycles
Minimum 2 cycles
High-impedance
High-impedance
High-impedance
High-impedance
1
2
3
4, 5
6
Low signal is sampled at rise of T state.
signal goes low at end of CPU read cycle, releasing bus right to external bus master.
pin continues to be sampled while bus is released to external bus master.
High signal is sampled twice consecutively.
signal goes high, ending bus-release cycle.
BREQ
BREQ
BREQ
BREQ
BACK
1
Address
bus
CS
n
High level
n = 7 to 0
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