FM8P51
Rev1.2 Mar 15, 2005
P.38/FM8P51
FEELING
TECHNOLOGY
2.12 Reset
FM8P51 devices may be RESET in one of the following ways:
1. Power-on Reset (POR)
2. Brown-out Reset (BOR)
3. RSTB Pin Reset
4. WDT time-out Reset
Some registers are not affected in any RESET condition. Their status is unknown on Power-on Reset and
unchanged in any other RESET. Most other registers are reset to a “reset state” on Power-on Reset, RSTB or WDT
Reset.
A Power-on RESET pulse is generated on-chip when Vdd rise is detected. To use this feature, the user merely ties
the RSTB pin to Vdd.
On-chip Low Voltage Detector (LVD) places the device into reset when Vdd is below a fixed voltage. This ensures
that the device does not continue program execution outside the valid operation Vdd range. Brown-out RESET is
typically used in AC line or heavy loads switched applications.
A RSTB or WDT Wake-up from SLEEP also results in a device RESET, and not a continuation of operation before
SLEEP.
The TO and
PD
bits (STATUS<4:3>) are set or cleared depending on the different reset conditions.
2.12.1 Power-up Reset Timer(PWRT)
The Power-up Reset Timer provides a nominal 18/4.5/288/72ms delay after Power-on Reset (POR), Brown-out
Reset (BOR), RSTB Reset or WDT time-out Reset. The device is kept in reset state as long as the PWRT is active.
The PWDT delay will vary from device to device due to Vdd, temperature, and process variation.
2.12.2 Oscillator Start-up Timer(OST)
The OST timer provides a 16 oscillator cycle delay (from OSCI input) after the PWRT delay (18/4.5/288/72ms) is
over. This delay ensures that the X’tal oscillator or resonator has started and stabilized. The device is kept in reset
state as long as the OST is active.
This counter only starts incrementing after the amplitude of the OSCI signal reaches the oscillator input thresholds.
2.12.3 Reset Sequence
When Power-on Reset (POR), Brown-out Reset (BOR), RSTB Reset or WDT time-out Reset is detected, the reset
sequence is as follows:
1. The reset latch is set and the PWRT & OST are cleared.
2. When the internal POR, BOR, RSTB Reset or WDT time-out Reset pulse is finished, then the PWRT begins
counting.
3. After the PWRT time-out, the OST is activated.
4. And after the OST delay is over, the reset latch will be cleared and thus end the on-chip reset signal.
The totally system reset delay time is 18/4.5/288/72ms plus 16 oscillator cycle time.