measuring RTC Battery voltage

[lpcware]

Content originally posted in LPCWare by Keith Ross on Wed Dec 17 16:44:11 MST 2014
Hi,

We require to provide information about the battery that drives the RTC. Is there any way for the condition or way to tell if the battery is near failure from within the LCP1830? I suspect not. If no I will have to add a fet sampling circuit to the battery in hardware.

Regards,
Keith

[lpcware]

Content originally posted in LPCWare by Keith Ross on Tue Jan 06 17:11:33 MST 2015
Thanks for this info. On the LPC1830 datasheet under 17.18.9 Power Control, the same diodes are shown so I will connect the battery direct to VBAT.

[lpcware]

Content originally posted in LPCWare by rocketdawg on Mon Jan 05 15:58:36 MST 2015

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Quote: starblue
There is a block diagram in the data sheet showing internal diodes, in chapter 7.20.9 Power control, page 83.

Also, MCB1800/4300 has the battery connected directly to the pin (no diode, only a jumper).

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Why yes there are internal diodes, so no external diodes are necessary.

[lpcware]

Content originally posted in LPCWare by starblue on Mon Jan 05 08:59:58 MST 2015
There is a block diagram in the data sheet showing internal diodes, in chapter 7.20.9 Power control, page 83.

Also, MCB1800/4300 has the battery connected directly to the pin (no diode, only a jumper).

[lpcware]

Content originally posted in LPCWare by rocketdawg on Fri Dec 19 08:59:39 MST 2014
a BAV99 has .275v forward voltage drop @ 2 ua.  so 3.0 - (2 * .275)  > 2.2 v minimum RTC voltage, but not by much.

a single diode is all that is required.

[lpcware]

Content originally posted in LPCWare by Keith Ross on Thu Dec 18 14:15:54 MST 2014
Thanks for your reply. It is inline with my thinking.

In a typical RTC powered situation we would use a diodes to connect VDD and Battery and feed the resultant signal to BAT of the micro.

On the LPC devices the RTC is powered from VDD when present. Can anyone confirm that, therefore, only the battery needs to be connected to BAT of the micro?

I have a dual diode (BAV99) to give a voltage drop so that the 3V battery is far enough below the 3.3V VDD as to not draw current from the battery. I assume that is what rocketdawg is meaning for the diode.

[lpcware]

Content originally posted in LPCWare by rocketdawg on Thu Dec 18 12:57:42 MST 2014
First, you posted a LPC1800 question in an LPC800 forum.

Yes you can monitor the voltage by wiring the battery to an ADC pin.
This will probably reduce the life of the battery due to leakage in the ADC path.
my data sheet shows an 1.2M input impedance.
You will have to verify that with the electrical specs in the user manual or data sheet.
if an ADC pin causes too much drain, then you might have to buffer it with some very high impedance switch.

But you should only be running on the RTC battery when the device loses power.
normal one has a diode in series with the Battery so that if VDD is present, then the RTC is powered off VDD.
if you lose power, then RTC runs off Battery.
So life running on battery only @ 2 uA battery current draw, a BR2330 should last ~ 10 years, a BR1632 maybe 7 years.