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Chapter 2 Basic CPU Functions
36
Reset
2-5 Reset
The CPU contents are reset and registers are initialized when the RST pin is
pulled to low.
■ Initiating a Reset
There are two methods to initiate a reset.
(1) Drive the RST pin low for at least four clock cycles.
Figure 2-5-1 Minimum Reset Pulse Width
(2) Set bit 7 (P2OUT7 flags) of the P2OUT register to "0." After reset is released,
the P2OUT flag will be "1."
■ Releasing the Reset
When the RST pin changes from low to high, an internal 15-bit counter begins
counting at the oscillation clock frequency. The interval from when this counter
begins counting until it overflows is known as the stabilization wait time. After
waiting for this amount of time, the internal reset is released and the CPU begins
operation.
Figure 2-5-2 Reset Release Sequence
RST pin
4 clock cycles
(200ns for a 20MHz oscillation)
For the reset to be stable, the
low pulse must be maintained
for at least four clock cycles.
However, it is important to
minimize noise, since a reset
may occur in a smaller number
of clock cycles.
Oscillation
stabilization wait time
2 /fosc
RST pin
Peripheral
register
CPU
internal reset
15
When returning from the STOP mode is terminating, the
software can use the DLYCTR register to select an
oscillation stabilization wait time of 0, 2
7
/fosc, 2
11
/fosc, or
2
15
/fosc.