Surviving 1 second without battery power

[peu]

Im trying to understand how this LED flashlight driver works, so I can implement the same technique on my own design.

The driver is powered by a single 3.6v Li-ion battery and by powering it off-on in less than a second it changes modes: Hi 100% > Mid 35% > Lo 20% > Strobe > SOS

Here is the link to it:
http://www.dealextreme.com/details.dx/sku.6190
AMC7135 Datasheet

And here is the Schematic I drawn from inspecting the PCB:


It obviously detects when power goes off, but I don't have the slightest clue on how its done, if I have to think of some PIC function that does that it would be BOD, but to be honest, Im lost.

The capacitor in the schematic may work as a battery backup, but its rather small, so upon battery power lost the PIC must be configured to keep consumption to a minimum.

Any help will be appreciated!

[skimask]

It might not have to survive without power.
If you power up, read a byte from eeprom, you can know what mode you were in last time around, increment that value, then power down.
Next time you power up, you read the same byte, the mode has incremented, save the value, power down...wash lather rinse repeat.
And there's probably some slick single-button power on/off trickery going on.
Do a search and you'll find a slick single-button power on/off trickery circuit here too...

[peu]

It might not have to survive without power.
If you power up, read a byte from eeprom, you can know what mode you were in last time around, increment that value, then power down.
Next time you power up, you read the same byte, the mode has incremented, save the value, power down...wash lather rinse repeat.
And there's probably some slick single-button power on/off trickery going on.
Do a search and you'll find a slick single-button power on/off trickery circuit here too...

Thanks for your prompt reply Skimask (as always I must add )


If you power up say after 10 seconds it remembers the last mode, but if you do an ON-OFF-ON cycle in less than a second it changes modes as described. Thats the only way to change modes, fast ON-OFFs

No button trickeries, just fast ON-OFFs I have the driver at hand, and this technique is rather common in new multimode LED flashlights.

I know how to do it by using an extra diode and a PIC pin to detect power lost, but this circuit uses a more efficient way for doing that.

[skimask]

If you power up say after 10 seconds it remembers the last mode, but if you do an ON-OFF-ON cycle in less than a second it changes modes as described. Thats the only way to change modes, fast ON-OFFs

So you've got a counter...that counts...and it sits there...counting...
when the power button is pushed...
If the count < x then it saves the next mode and powers down
If the count > x then it doesn't switch modes in eeprom and just powers down

[Acetronics2]

Is it a Power on Reset

or

Is it a Brown Out Reset ...

Just test the PCON bits ... to know it. and do what's to do.

Alain

[skimask]

But if it's one of those single button power on/off type circuits, it'll always be a power on reset.

[peu]

So you've got a counter...that counts...and it sits there...counting...
when the power button is pushed...
If the count < x then it saves the next mode and powers down
If the count > x then it doesn't switch modes in eeprom and just powers down

But you must count while the power is out... there is no backup power. The mode is changed if you do an ON-OFF-ON cycle in less than a second.

I Just updated the schematic to clearly show the entire circuit is battery powered with no backup power, reload the image so the new one is shown.


Maybe Im missing something from your answer <scratching head>

[Acetronics2]

Maybe Im missing something from your answer <scratching head>

YES !!!

The capacitor holds enough power to the PIC not to reset all registers While you do the "quick on-off-on"

remember V b.o.r. is more than the min running voltage of the Pic ... and driving your current sinks do not drain much current from the PIC ...

so that gives enough time for the Pic to reset AND test the BOR Bit, change the mode, ... ... after that you re-apply the power ...

Héhé ...

[skimask]

Maybe Im missing something from your answer <scratching head>

Ok, I smell what your cooking...
Put a meter on the cap and see what it reads and how fast it decays.
Maybe it's a version of a super-cap in a really small package.
PIC's (nano-watt PIC in particular) can draw some seriously LOW current in a sleep mode.