I think the circuit will be easier to understand if the 
names of the pins and the internal structure of the PIC
port pins are in the picture.

I am not so sure if I understand the circuit correctly
but I will give it a try.

The one connected to the decompling capacitor (pin14) is the
Vdd pin. Normally it is connected to the supply, but not in
this case. Port pins have diode protection to Vdd and Vss.
Therefore the external power (5.5V and limit at 50mA) 
will charge the Vdd decoupling capacitor through the
port protection diode to about 5.5V-0.6V=4.9V.
This will make the PIC happy and running. Pin 8 then 
will be configured as an output pin and output high 
(to turn the high sider FET on). 

Now we can pulse-charge the two backup batteries or supercaps 
by configuring pin6/pin7 as output pins and outputing high 
to charge and configuring as input to stop charging.

To detect DC voltage on Pin 8, we need to configure it 
as an input pin from time to time (thus turn the high 
side FET off). Once it reads low, we know that the DC 
voltage drops quite a lot and we'd better turn off the 
high side FET totally. In this case, the backup
batterie(s)/supercap will take over.

Since pin 8 should not sink more than 25mA, the total
current consumption should be less than this. The voltage
should not be higher than maximum allowed input voltage
(Vdd+0.3V). That is why the external DC voltage to pin 8 
is typically 5.5V and can drive 25mA at most.

I think this is quite non-standard. ;-) 


Regards,
Xiaofan

-----Original Message-----
From: Peter
Sent: Wednesday, October 12, 2005 4:25 AM
To: Microcontroller discussion list - Public.
Subject: RE: [PIC] Battery backup

On Mon, 10 Oct 2005, James Newtons Massmind wrote:

> Can you comment a bit more on what is happening in that circuit? In the
pdf,
> you say "The bulk diodes are used during startup, then code turns on the
> relevant high side output FET which conducts both ways, so it can be used
> for charging and/or extra power (as a low power scheme). Pulsed charging
can
> achieve very low current charging of the backup battery(es)."
>
> When you refer to bulk diodes and FETs, you are talking about components
> that are inside the PIC?

Yes. The circuit as shown powers the pic from either battery or the dc 
input, without the code doing anything. When the code wakes up it should 
turn on the high side fet where the power comes in and then pulse the 
high side fets of the batteries to charge them. It cannot really measure 
the voltage but it can make sure that the integrated current does not 
exceed the trickle charge current of the NiCd or supercap used.

When external dc drops the pic can detect this (f.ex. by not keeping the 
high side fet on on the dc input pin, but pulsing it off every few 
msec) and turn the high side fet off.

> I assume the PIC can read the pin connected to the external power supply
to
> see if it is producing, then it would set the pin as an output and drive
it
> high to connect that power source to the internal Vdd and set the pin
> connected to one of the batteries to high to charge that battery?
>
> Your statement "using the outputs to drive low at full (20mA) current from
> another power supply within +/-0.3v+0.6v of the voltage on the decoupling
> cap" totally confuses me.
>
> Finally, is there any particular reason why you have two batteries?

The idea is to use two coin cells so they can be changed one by one 
wihout losing data. Of course you would not charge the coin cells. If 
using a proper supercap or battery then just one is enough.

I have used such a scheme in a hours meter. It could also work in other 
systems, like alarms, dongles and clocks.

Peter
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