IB Peter Feucht <feuchts <at> kabelbw.de> writes:
> I developped a small battery operated device, using a 16F684, everything
> works fine. Now the customer shows up, and wants to have any sort of
> batt-low indication. For already 1000 PCS are made and assembled, there i=
st

You can use statistics to determine if the BORs are becoming too frequent (=
a
sign for low battery). It works like so:
- A new battery may cause BORs at turn on and turn off, several even. Let's=
 call
this number of BORs N1, say, 3. Also, they will occur during the 'turn on' =
or
'turn off' phase, which you can account for with a state machine in the sof=
tware.
- A weak battery will cause BORs every now and then, when external loads ar=
e
connected especially. They will occur in normal operation (not when startin=
g and
stopping, see above state machine). Let's call this new number of BORs N2, =
which
will normally be way bigger than 3, but let's say 5 for now.
- If any of your comparator or ADC inputs depend on battery voltage directl=
y you
can change your code to 'measure' battery voltage using those.

Now you can implement a state machine like:

if( ResetOccurredByBor() && (State !=3D ST_STOP) ) {
  // do not count BORs when stopping power - assumes mcu can turn its
  // own power off in some way
  IncrementNumBors();
}
if( UserPressedNumBorReset() ) { // start fresh cycle, hope batteries are n=
ew
  SetNumBors(0);
  SetLockout(0);
  while( UserPressedBorReset() ); // wait ...
}
tmp =3D GetState();
if( (t =3D=3D ST_START) || (t =3D=3D ST_STOP) ) {
  if( GetNumBors() > MAX_N1 ) SetLockout(2); // most severe, battery very w=
eak
or bad contacts
} else {
  if( GetNumBors() > MAX_N2 ) SetLockout(1); // likely weak battery, resets=
 in
mid-operation,=20
}
switch( GetLockout() ) {
  case 0: ; // all is well, operate normally
    break;
  case 1: ; // battery may be low, turn on low battery ind., try to work
    break;
  case 2: ; // we don't want to work at all, battery end ind., loop forever
    lockout: goto lockout;
}
     =20
Where Lockout is a variable you set which shuts down the device (or makes i=
t=20
in-operable and indicates low battery). You can even set the Lockout bit in
EEPROM so it will require a device reset on battery change from the user
(pushing a specified button, not the reset mcu signal). The State and NumBo=
rs
variables above should me located in a place that is not wiped by RESET (us=
ually
RAM assuming the mcu always has power and is sleeping between active uses),=
 and
if you use a C compiler be sure to edit the startup code so those locations=
 are
really left alone during initialization, if your compiler's startup code wi=
pes
all ram by default.

You can refine this by having additional steps of NumBors > LOW_N1 etc to
indicate imminent battery failure.

And, yes, this is a hack, unless you have comparator or ADC information abo=
ut
something directly related to battery state, but you have 1000 products and=
 your
good name (?) to save.

<aside>I "love" people who come back after 5 years for added features they
definitely decided they do not want, in despite of having been explained wh=
y
that may be needed 5 years before. Such as 10 cents for a flash upgrade
connector or 2 cents for a pin to be able to attach a serial terminal and d=
ebug
or reprogram via boot loader or similar.</aside>

-- Peter


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