Rev. 1.00, 09/03, page 125 of 704
6.5.5 Wait
Control
When accessing the external address space, this LSI can extend the bus cycle by inserting the wait
states (T
W
). Ways of inserting wait states: Program wait insertion, pin wait insertion using the
WAIT
pin, and the combination of program wait and the
WAIT
pin.
In Normal Extended Mode:
1. Program Wait Mode
When the external address is accessed in the program wait mode, the T
W
of the number of
states which are set by the WCn1 and WCn0 pins of the BCRAn, is always inserted between
the T
2
and the T
3
states.
2. Pin Wait Mode
A specified number of wait states T
W
are always inserted between the T
2
state and T
3
state
when accessing the external address space. The number of wait states T
W
is specified by the
settings of the WCn1 and WCn0 bits. If the
WAIT
pin is low at the falling edge of
φ
in the last
T
2
or T
W
state, another T
W
state is inserted. If the
WAIT
pin is held low, T
W
states are inserted
until it goes high.
Pin wait mode is useful when inserting four or more T
W
states, or when changing the number
of T
W
states to be inserted for each external device.
3. Pin Auto-Wait Mode
A specified number of wait states T
W
are inserted between the T
2
state and T
3
state when
accessing the external address space if the
WAIT
pin is low at the falling edge of
φ
in the last
T
2
state. The number of wait states T
W
is specified by the settings of the WCn1 and WCn0 bits.
Even if the
WAIT
pin is held low, T
W
states are inserted only up to the specified number of
states.
Pin auto-wait mode enables the low-speed memory interface only by inputting the chip select
signal to the
WAIT
pin.
Figure 6.25 shows an example of wait state insertion timing in pin wait mode.
The settings after a reset are: 3-state access, 3-state program wait insertion, and
WAIT
pin
input disabled.
Содержание H8S/2437
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