short on VBAT?

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ag123
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short on VBAT?

Post by ag123 » Tue Sep 19, 2017 4:16 pm

recently i tried to meddle with RTC on my Baite MM clone, RTC 'simply works' thanks much for the really good effort here.
then i hooked up a CR3032 coin cell on VBAT pin on the MM, it seemed to run quite well initially keeping the clock going after removing USB power.
after less than a month the CR3032 drains from some 3+ volts to around 1.5v, i'm rather surprised as VBAT is intended for low power and basically keep the LSE crystal osc and RTC running?
i then tried to measure resistance and voltages, if the coin cell is connected, it reads zero volts across to ground. but if i measure resistance across with a multimeter (VBAT to ground removing the coin cell) i get some 200 ohms.
This is pretty much a short between VBAT and ground?
it seemed the specs gives typical currents of 1 uA, this would seem much greater than 1 uA
would there be any need to put MM into 'low power modes' rather than simply disconnecting USB power?
:?

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Pito
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Re: short on VBAT?

Post by Pito » Tue Sep 19, 2017 6:38 pm

You do something wrong with your measurement. 200ohm would drain the cr2032 in 10hours completely.
3V/200ohm = 15mA. Cr2032's capacity is 150mAh.
According to the schematics I saw the Vbat is connected directly to pin Vbat.
If you measure with a multimeter (plus on Vbat, minus on GND), which uses several volts for measuring the resistance, it may happen you open the upper clamp diode (ESD protection at every pin) at the VBat pin and you may measure the forward resistance of the upper clamping diode against VCC/GND (could be 200ohm).
https://training.ti.com/clamp-diodes

With 20uA (doublecheck the datasheet plz) when RTC is running you drain the cr2032 in 150000/20 hours.
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ag123
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Re: short on VBAT?

Post by ag123 » Thu Sep 21, 2017 12:29 am

thanks pito ! it seemed the coin cell which has already drained to 1.5v is possibly completely drained, as measuring it across when connected to VBAT - GND gives 0 v, i think you are right about the clamping diodes. i'd use a new coin cell and possibly connect a small series resistor and measure voltages across again, that would tell if after all VBAT is literally shorted, i think there are also some other posts about BP, MM pins shorting across not just VBAT.
it is not easy to identify the tiny features on the boards simply examining them with the naked eye, there might be some shorts at the solder joints etc

actually i'm starting to wonder if disconnecting the usb cable supplying power could have forward biased the clamping diodes, that would increase the currents from microamps to milliamps literally a thousand times higher. if that's true for the general case it would be bad news as it may mean coin cells won't last on VBAT pin on MM board designs when power is removed

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Pito
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Re: short on VBAT?

Post by Pito » Thu Sep 21, 2017 1:48 am

The Vbat current when RTC is on should be 1.2uA according to the datasheet.
Simply insert an ammeter between Cr2032+ and Vbat (or Cr2032- and GND) and you may see the current.
I do not have MMini Baite schematics handy (do we have it somewhere?).
The Vbat pin could be setup differently (on silicon) so the clamp diodes (if any) could be arranged such they will not be fw biased when Vcc off and Vbat on. You are right the 3V at Vbat with Vcc at 0V could mess with the upper clamp diode.

PS: My 2 MMini clones when not powered
First shows 7Mohm with multimeter's + at Vbat pin (and 2Mohms with - at Vbat pin) against GND
Second shows 5Mohm with multimeter's + at Vbat pin (and 2Mohms with - at Vbat pin) against GND
That would be 3V/6Mohm = ~0.5uA, plus add current of the running RTC, it may get 1.2uA in total.

The Baite's Vbat goes directly to stm's pin 1 here.

Btw, MMinis do not have 32kHz RTC crystal mounted, how did you wire it?
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RogerClark
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Re: short on VBAT?

Post by RogerClark » Thu Sep 21, 2017 3:02 am

I looked on the original MM schematic and VBat is only supposed to be connected to pin 1 on the MCU
maplemini(1).pdf
(24.3 KiB) Downloaded 9 times
Looking at a MM clone, it also looks like VBat is just connected to pin 1

Of course your MM clone may be different

ag123
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Re: short on VBAT?

Post by ag123 » Thu Sep 21, 2017 12:31 pm

Pito wrote:
Thu Sep 21, 2017 1:48 am
Btw, MMinis do not have 32kHz RTC crystal mounted, how did you wire it?
thanks pito, this is tad ugly but it here is the hack, guess it's about time i solder a new MM :lol:
Image
looks ilke that huge blob of solder at the crystal joints looks pretty bad, would probably need to 'drain' all that solder off
VBAT is 3rd pin from the bottom

i think baite is basically MM clone hence presumably the specs would be same as/similar to the original MM

and the usual places to get those 32k crystals, it is a gamble that they even works, but for this piece it works (certainly not that epson 5pf high precision feather weight crystals) :lol:
https://www.ebay.com/sch/i.html?_nkw=32768&_sacat=0

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