Real Time Clock

Forum: Hardware
bonhomme.corp 
Hello,

I have a problem with the battery that saving the date/time.
Indeed, after 30 days, it is completely empty.

Do you have the same problem?

Best regards,

davef 
Mine works fine.

Measure the voltage with a digital multimeter and report back.

Remco 
not sure if your board is already in a machine, but some ESD (anti static)
bags or foams actually conduct enough do drain a small battery in a few
weeks

remco

bonhomme.corp 
Hello,

It is a mini2440 not yet in a machine.
The voltage across the battery is 3V.

Battery life time = 50mA/hour
R12 = 10 Kohm

Board off:
Across R12, there is a voltage of 0.020V.
So consumption of 0.02/10000 Ohm = 2μA.
50mA/2μA = 25000 hours ---> 1041 days 25000/24hour => 1041/365 = 2.85
years

Board on:
Across R12, there is a voltage of 0.492V.
So consumption of 0.492/10000 Ohm = 49.2 ľA
50mA/49,2μA = 1016 hours ---> 1016/24hour => 42days.

Strange ?

Best reagrds,

bonhomme.corp 
Hello,

It is a mini2440 not yet in a machine.
The voltage across the battery is 3V.

Battery life time = 50mA/hour
R12 = 10 Kohm

Board off:
Across R12, there is a voltage of 0.020V.
So consumption of 0.02/10000 Ohm = 2 microA.
50mA/2 microA = 25000 hours ---> 1041 days 25000/24hour => 1041/365 = 2.85
years

Board on:
Across R12, there is a voltage of 0.492V.
So consumption of 0.492/10000 Ohm = 49.2 ľA
50mA/49,2 microA = 1016 hours ---> 1016/24hour => 42days.

Strange ?

Best reagrds,

davef 
When the board if off the current through R12 comes from the battery.

When the board is on the current through R12 comes from the VDD33V.  There
will be no current coming from the battery.

What is strange is why the current goes up through R12 by a factor of 20
for an increased voltage of about 0.3Volts.  Is this what you mean by
strange?

Another issue is the cell can supply 50ma for one hour, but if you reduce
the current drawn, say by an order of magnitude, you will get more then an
order of magnitude increase in operating time.  This behaviour starts to
fall apart when the current drawn approaches the self-discharge rate of the
cell.

bonhomme.corp 
I'm confused.
I have no problem with the card Mini2440.
I actually have the battery problem with the card Micro2440sdk.
Indeed, the circuit is not the same.
The battery is in series with two diodes D2 and D4, there is no VDD33V...

Thank you for your response davef

davef 
< It is a mini2440 not yet in a machine.

Actually, I initially opened up the schematic for the micro2440 by accident
and couldn't even find R12 or the battery!

If the battery is really in series with BOTH D2 and D4 that means there
would only be 3.0 - 1.2Volts = 1.8 Volts, instead of 2.4Volts, being
supplied to the RTC. 

My micro2440-sdk_schematic.pdf shows a VDD33V.

Measure the actual voltage being supplied to the RTC pin.

You also said the current changed between power ON and power OFF.  If there
is no VDD33V, where is the extra current coming from?