134
4.4.5
Wait Control
When accessing external space, the H8S/2678 Series chip can extend the bus cycle by inserting
one or more wait states (T
w
). There are two ways of inserting wait states: (1) program wait
insertion and (2) pin wait insertion using the
WAIT
pin.
Program Wait Insertion: From 0 to 3 wait states can be inserted automatically between the T
2
state and T
3
state on an individual area basis in 3-state access space, according to the settings in
WTCRA and WTCRB.
Pin Wait Insertion: Setting the WAITE bit to 1 in BCR enables wait input by means of the
WAIT
pin. When external space is accessed in this state, a program wait is first inserted in
accordance with the settings in WTCRA and WTCRB. If the
WAIT
pin is low at the falling edge
of ø 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.
This is useful when inserting seven or more T
w
states, or when changing the number of T
w
states to
be inserted for different external devices.
The WAITE bit setting applies to all areas.
Figure 4.17 shows an example of wait state insertion timing.
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