Wait Modes in Areas Where Wait-State Controller is Enabled
External three-state access areas in which the wait-state controller is enabled (ASTn = 1, WCEn =
1) can operate in pin wait mode 1, pin auto-wait mode, or programmable wait mode, as selected
by bits WMS1 and WMS0. Bits WMS1 and WMS0 apply to all areas, so all areas in which the
wait-state controller is enabled operate in the same wait mode.
Pin Wait Mode 1: In all accesses to external three-state-access areas, the number of wait states
(T
W
) selected by bits WC1 and WC0 are inserted. If the
WAIT
pin is low at the fall of the system
clock (ø) in the last of these wait states, an additional wait state is inserted. If the
WAIT
pin
remains low, wait states continue to be inserted until the
WAIT
signal goes high.
Pin wait mode 1 is useful for inserting four or more wait states, or for inserting different numbers
of wait states for different external devices.
If the wait count is 0, this mode operates in the same way as pin wait mode 0.
Figure 6-13 shows the timing when the wait count is 1 (WC1 = 0, WC0 = 1) and one additional
wait state is inserted by
WAIT
input.
Figure 6-13 Pin Wait Mode 1
Address bus
Data bus
AS
RD
HWR
,
LWR
T
1
T
2
T
W
T
W
T
3
Write data
*
Read data
*
Read
access
Write
access
Note: Arrows indicate time of sampling of the pin.
*
WAIT
ø
pin
WAIT
Data bus
External address
Write data
Inserted by
wait count
Inserted by
signal
WAIT
135
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