RTC drift

[Vineet Nair]

Posted on June 08, 2018 at 21:35

I am Using the #RTC #M41T94 for keeping time on my project.Have noticed on multiple occasions that there is a significant #drift in the time keeping. Have reviewed the datasheet multiple times to check if my schematic or firmware is doing something wrong but no success.

Was wondering if anyone here has had similar issues ?

Any help is appreciated#.

Thanks & Regards,

Vineet Nair. Note: this post was migrated and contained many threaded conversations, some content may be missing.

[T J]

Posted on June 16, 2018 at 02:39

You cant connect to the XO, it should not work at all. does anything get hot ?

did you check the current has jumped up now ?

I checked the pin numbers are correct, they are good.

From the clock source to the XI pin.

as

Turvey.Clive.002

suggested, capacitively couple with a low value like 10pF - 1nF, I guess

or you could try a 100k resistor which may be more stable.

I use the DS3232,

https://www.digikey.com/product-detail/en/maxim-integrated/DS3232SN/DS3232SN-ND/1197580

it has the integral TCXO Xtal, but the price....

I guess that is the reason that you are not using the HAT.

aside :

for your information, the primary reason that clocks drift, is the humidity affects the capacitors value.

normally you would dry the board for a few hours and cover the caps with conformal coating.

and of course the temperature of the crystal.

[Vineet Nair]

Posted on June 18, 2018 at 17:52

Hi @TJ,

Thanks for your response.

- Checked the current - current consumption still the same

- Feel of it, does not seem like IC gets hot.

- Will check with a thermal camera later today.

Could you help me understand why you say that it should not work when connected to XO ?

Also regarding the moisture - the boards on which we see the drift(almost all board) were already potted so that give improved moisture protection. We have also baked them to remove moisture on connectors is any.

[henry.dick]

Posted on June 19, 2018 at 03:06

'

i should connect the clock source to XO instead of XI.'

it is generally not advisable to drive the xo pin - instead, driving the xi pin is preferred. however, for low-frequency / low drive power oscillators, the xo pin has fairly high impedance (to limit drive power) so driving it is OK.

your mistake &sharp1 here is to pick an external oscillator - the device is asking for an external crystal. if you have to drive it actively, use a small resistor to isolate the oscillator from the rtc.

and go from there.

[Vineet Nair]

Posted on June 19, 2018 at 15:09

Thank you

henry.****

The oscillator used here has been tested on some of our other products last 5-6 years and has performed extremely well. Especially at high temperatures at 150C, we were not sure we could get the same using a crystal & the inbuilt RTC circuitry. Hence the reasoning behind using a dedicated oscillator.

I will replace R63 with a 50ohm resistor and then do some long duration test and temperature test on it.

WIll update back with the results.