Motorboating == USB overcurrent -> emergency shutdown -> NoPower -> autostart

Andy Green andy at
Mon Jun 30 19:19:42 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:
| Somebody in the thread at some point said:
| | Andy Green wrote:
| |> No need, everything that happens on there is synchronous to pcf50633
| |> 32kHz clock... these two in the pic are locked together.
| |
| | It does make sense if the start of all this is synchronized with some
| | clock. However, doesn't the duration of the Vsys drop event change
| | when you add an external capacitor ? It does in my experiments.
| Yeah it does change the slope of the V part, so it's not all tick-tock.
| ~ But it remains locked to pcf50633 32kHz.... the whole thing on Vsys is
| managed by the pcf logically.
| What fits it is it throws out Vsys "permanently" for whatever reason
| (overcurrent on USB is the guess), and that is only countermanded when
| Vsys goes below 2V (lowsys action I guess).  How long it takes to hit 2V
| on the downward part of the V once Vsys is no longer driven by anything
| is just a function of the capacitive load on Vsys.
| The Vsys comes back, and again the time taken to charge the capacitors
| on Vsys defines the slope back up again, that's why it's symmetrical.

Hey notes on p5 of pcf50633 datasheet looks interesting for this.  Here
they are annotated with values for our variant.

''If the system voltage drops below Vth(sysok) [3.0V], a lowsys
interrupt is generated and the time-out timer is started. The SYSOK
status bit is reset (see Table 14 and Table 20).

If the system voltage drops below Vth(sysmin) [2.5V], an emergency
shutdown is initiated and the system transitions to the Save state.
If the system voltage drops below Vth(syspres) [2V], and no backup
battery is present (bubpres = 0; see Table 14), the system will be reset
and enter the NoPower state. If a backup battery is connected (VBUBAT >
Vth(BUBPRES), the PCF50633 will continue in Save mode, powered
by the backup battery.

If the system voltage is below Vth(syspres) [2V], the system will
continue to operate in the Save state as long as bubpres = 1 (VBUBAT >
Vth(bubpres); see Table 14). If bubpres changes to 0, the system will be
reset and enter the NoPower state.''

I just saw my backup battery is at 0.4V.

It can work out if during startup Vsys is going under Vth(sysmin) which
is hardcoded at 2.5V, it forces an "emergency shutdown" and pushes us
into SAVE -- that is what gets us the sticky power loss.  Then it goes
under Vth(syspres) 2V limit and we "reset and enter the NoPower state".
~   I guess having being forced to NoPower, it then sees USB power is
present and after debouncing it transitions right out of it into an
autostart action repeating the cycle.

- -Andy
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