PCF2131 losing seconds.

[PepeTheSkrilla]

Hi,

I have a custom PCB board with a PCF2131 RTC.

it is supplied by a main power of 3.3V and a supercap of 12F when main power supplies cut off.

I have disabled the WD and tried multiple Aging Offset but it keeps losing up to -10 seconds in a week or so.

Right now the supercap is running at around 2.8V, which should be enough.

Out of curiosity: Does RTC drift time, or reset time, when voltage drops below 1.8 for few milliseconds?

Don't really understand what could cause this.

[TomasVaverka]

Hi,

The PCF2131 should operate reliably down to a specified minimum voltage, typically 1.2V. If the voltage drops below this, the oscillator supply also fails and a reset occurs. See section 7.6 of the datasheet for more information.

At 2.8V, the supercap should indeed be sufficient for the PCF2131.

Voltage transients on the supercap when switching from main power could potentially impact the oscillator. Consider placing a small decoupling capacitor close to the RTC’s VDD pin to help smooth any voltage fluctuations from the supercap during switchover.

Is the PCF2131 exposed to large temperature variations? Is the TC_DIS bit = 0?

BRs, Tomas

[PepeTheSkrilla]

Hi Tomas, Thanks for the answer.

i will check for TC_DIS, but I think it will be 1.

if TS1, TS2, TS3 and TS4 are connected to 3V3 in series with a 4KOhm Resistor, can it cause any sort of problem of drifting?

[TomasVaverka]

Hi,

Connecting the  timestamp inputs to 3.3V with pull-up resistors should not affect the RTC timekeeping accuracy.

BRs, Tomas

[PepeTheSkrilla]

Hi,

We connected the oscilloscope to the CLKOUT pin and we retrieved this guassian distribution of the frequency of the oscillator (around 90hz) at ambient temperature. (See image)

Could be this the cause of the drift?

[TomasVaverka]

Hi,

This is likely a primary contributor to your time drift issue. I see you created a support case, so please follow up on this internally with my colleague.

BRs, Tomas