Datasheet

Analog Integrated Circuit Device Data
Freescale Semiconductor 40
MC34708
Functional Block Description
7.5.3.2 Silent Restart from PCUT Event
If a short duration power cut event occurs (such as from a battery bounce, for example), it may be desirable to perform a silent
restart, so the system is reinitialized without alerting the user. This can be facilitated by setting the PCUTEXPB bit to “1” at booting
or after a Cold Start. This bit resets on RTCPORB, therefore any subsequent Cold Start can first check the status of PCUTEXPB
and the PCI bit. The PCUTEXPB is cleared to “0” when transitioning from PCUT to Off. If there was a PCUT interrupt and
PCUTEXPB is still “1”, then the state machine has not transitioned through Off, which confirms the PCT timer has not expired
during the PCUT event (i.e., a successful power cut). In this case, a silent restart may be appropriate.
If PCUTEXPB is found to be “0” after the Cold Start where PCI is found to be “1”, then it is inferred the PCT timer has expired
before power was re-established. This indicates an unsuccessful power cut or first power up, so the startup user greeting may
be desirable for playback.
7.5.3.3 Silent System Restart with WDI Event
A mechanism is provided for recovery if the system software somehow gets into an abnormal state which requires a system reset,
but it is desired to make the reset a silent event so as to happen without user awareness. The default response to WDI going low
is for the state machine to transition to the Off state (when WDIRESET
= 0). However, if WDIRESET = 1, the state machine will
go to Cold Start without passing through Off mode (i.e., does not generate an OFFB signal).
A WDIRESET event will generate a maskable WDIRESETI interrupt and also increment the PCCOUNT counter. This function is
unrelated to PCUTs, but it shares the PCUT counter so the number of silent system restarts can be limited by the programmable
PCMAXCNT counter.
When PCUT support is used, the software should set the PCUTEXPB bit to “1”. Since this bit resets with RTCPORB, it will not
be reset to “0” if a WDI falls and the state machine goes straight to the Cold Start state. Therefore, upon a restart, software can
discern a silent system restart if there is a WDIRESETI interrupt and PCUTEXPB
= 1. The application may then determine an
inconspicuous restart without fanfare may be more appropriate than launching into the welcoming routine.
A PCUT event does not trip the WDIRESETI bit.
Note that the system response to WDI is gated by the Watchdog timer—once the timer has expired, the system will respond as
programmed by WDIRESET as described above.
Applications should make sure there is time for switching regulator outputs to discharge before re-asserting WDI.
7.5.3.4 Turn On Events
When in Off mode, the circuit can be powered on via a Turn On event. The Turn On events are listed by the following. To indicate
to the processor what event caused the system to power on, an interrupt bit is associated with each of the Turn On events.
Masking the interrupts related to the turn on events will not prevent the part to turn on except for the time of day alarm. If the part
was already on at the time of the turn on event, the interrupt is still generated.
• Power Button Press: PWRON1 or PWRON2 pulled low with corresponding interrupts and sense bits PWRON1I, or
PWRON2I and PWRON1S, or PWRON2S. A power on/off button is connected from PWRONx to ground. The PWRONx can
be hardware debounced through a programmable debouncer PWRONxDBNC
[1:0] to avoid a response upon a very short (i.e.,
unintentional) key press. BP should be above UVDET to allow a power up. The PWRONxI interrupt is generated for both the
falling and the rising edge of the PWRONx pin. By default, a 30 ms interrupt debounce is applied to both falling and rising
edges. The falling edge debounce timing can be extended with PWRONxDBNC[1:0] as defined in the following table. The
PWRONxI interrupt is cleared by software or when cycling through the Off mode.