FUJITSU Semiconductor Controller MB89950/950A - Manual (Page 58)

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CHAPTER 3 CPU
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Power-on reset
Products can be set to with or without power-on reset (optional). On products with power-on reset, turning
on the power generates a reset. The reset operation is performed after the oscillation stabilization delay time
has passed. Moreover, external reset signal is outputted by the reset output option.
On products without power-on reset, an external reset circuit is required to generate a reset when the power
is turned on.
■ Main clock oscillation stabilization delay time and the reset source
Whether there will be an oscillation stabilization delay time depends on the operating mode when reset
occurs, and the power-on reset option selected.
Following reset, operation always starts out in the normal main clock operating mode, regardless of the
kind of reset it was, or the operating mode (the clock mode and standby mode) prior to reset. Therefore, if
reset occurs while the main clock oscillator is stopped or in a stabilization delay time, the system will be in
a "main clock oscillation stabilization reset" state, and a clock stabilization period will be provided. If the
device is set for no power-on reset, however, no main clock oscillation stabilization delay time is provided
for power-on or external reset.
In software or watchdog reset, if the reset occurs while the device is in main clock mode, no stabilization
time is provided.
Table 3.5-2 "Reset source and oscillation stabilization delay time" shows the relationships between the
reset sources and the main clock oscillation stabilization delay time, and reset mode (mode fetch)
operations.
Table 3.5-2 Reset source and oscillation stabilization delay time
Reset source Operating state
Reset operation and main clock oscillation stabilization delay time
With power-on reset Without power-on reset
External reset
(*1)
At power-on,
during stop mode
After the main clock oscillation
stabilization delay time, if the
external reset is waked up, reset is
operated.
(*2)
Reset state is held until external reset
is waked up; then the reset is
operated.
Software and
watchdog reset
Main clock mode After 4-instruction-cycle reset occurs, reset is operated.
(*3)
Power-on reset
Device enters main clock oscillation
stabilization delay time at power-on.
Reset is operated after delay time
ends.
(*2)
An external circuit must be provided
to hold external reset asserted at
power-on until main clock has had
time to stabilize.
*1: No oscillation stabilization delay time is required for external reset while main clock mode is operating. Reset is
operated after external reset is waked up.
*2: If the reset output option is selected, "L" is output at RST pin during the main clock oscillation stabilization delay time.
*3: If the reset output option is selected, "L" level is output at RST pin during 4-instruction-cycle.