PCF2129 oscillator stops

[tike64]

Dear all,

We have designed in PCF2129AT RTC into one of our products. They were not long in use in the field when some of them lost their date. They had risen their OSF status bit to indicate oscillator stopped failure and then Linux driver refuses to return the date.

It is hard to establish what is the root cause but, nevertheless, I can replicate same kind of an issue at my desk. When I cool down the chip, it sometimes takes abnormally high current from the battery and turns on the OSF bit. This is one example of the events:

CH2 is battery voltage and CH1 voltage at PCF2129 VBAT pin. VDD is off. There is 1k resistor from battery to PCF2129 VBAT. It can be calculated from the picture that the chip took approximately 700uA current. However, the voltage didn't drop even near the 1.2V where the OSF bit should go up but it still did.

I have measured CLKOUT and verified that the oscillator really stops for a moment. The time and date is not corrupted and the chip continues normally after the event, only OSF being set.

There is 1uF at VBAT and 10nF at BBS. We use I2C.

What is going on? Any ideas, how to debug or fix this?

[tike64]

I'm very sorry I originally forgot to mention quite relevant details. The traces were battery (CH2, stable) and the VBAT pin of the PCF2129 (CH1, dropping). I have already edited the question. In this light, I think Jose's answer doesn't apply in my case.

I would suspect the temperature measurement event. But the mystery is, how come the chip sips so much current. According to the datasheet, it should be in the order of a few uA. Here we see almost a mA. And the oscillator stops even if the voltage is not near Vbat min.

Can anyone guide me how to remove the "Assumed Answered" tag?

[reyes]

Hi Timo,

Please share your schematic, it is not clear to me how are the connections of the battery. Typically, battery is directly connected to VBAT pin, so, I do not understand why these pins have different voltage level.

Take Figure 13 on page 20 of the Application note AN11186 as an example of the connections for the PCF2129.

AN11186: https://www.nxp.com/docs/en/application-note/AN11186.pdf

Regards,

Jose

[tike64]

Hi Jose,

Thanks for using your valuable time for this.

Here is the schematic:

In my scope picture CH1 was from pin 19 VBAT and CH2 was directly from the battery. The red fixes were in place.

We have always used a series resistor with the battery for safety. I thought it should be okay also here given that the datasheet gives 3.5uA as a maximum battery current.

[reyes]

Hi Timo,

Can you please include this voltage rail (pin 20) and VBAT (pin 20) on a scope?

I ask this because one reason why the OSF flag is setting could be because of a quick drop of VDD voltage. A slope of 0.7V/ms on the VDD voltage drop needs to be maintained for proper operation of the battery switchover, otherwise OSF will be set.

Other test, please change the value of the VBAT capacitor (C47), change it from 1.0uF to 0.1uF as we recommend in our documents.

Regards,

Jose

[tike64]

Hi again Jose,

I'm sorry to say I'm not able to replicate the problem any more (or should I be glad). I really don't know why. It was a few weeks ago I last investigated this. Now I changed the R19 to 0Ω and tried to prove that I can replicate the problem in this case too - but couldn't. Then I tried 100Ω, 1kΩ and finally 10kΩ. All I was able to achieve was a bump in current consumption from about 2μA to 4μA for a 50ms or so exactly at the time I started cooling.

I know the problem with quickly dropping Vdd but the effects, I saw on my desktop, were definitely not due to that. I never saw to OSF flag setting simply by powering off. Nevertheless, It may be the actual cause in the field.

I'm leaning to a conclusion that the phenomenons happened on my desk were due to extreme stressing of the chips. At the same time I hope, that by correcting the obvious mistakes in the circuit, the problems in the field are resolved.

Thanks again for your time.

Timo

[reyes]

Hi,

The PCF2129 has a backup battery input pin and backup battery switch-over circuit which monitors the main power supply. The backup battery switch-over circuit automatically switches to the backup battery when a power failure condition is detected.

The power management functions are controlled by the control bits PWRMNG[2:0] (see Table 19) in register Control_3 (see Table 11).

I’m assuming that for your system, you have the default setting for these bits, which are ‘00’ and means that battery switch-over function is enabled in standard mode; and battery low detection function is enabled.

When battery switch-over function is set in standard mode, the power failure condition happens when VDD < VBAT AND VDD < Vth(sw)bat

Vth(sw)bat is the battery switch threshold voltage which is 2.5 V.

The mentioned above seem to be happened in the image you sent, but does not totally explain why the OSF is being set, my theory is that the battery switchover is being seen as a power-on event fort the PCF2129, which as you can see in section 8.6 of the datasheet, causes an OSF flag set. Try to clear it (OSF set logic 0) by command after the oscillator starts running and is stable after this power-on. If OSF gets back to ‘0’, then you can continue using it without a problem.

Regards,

Jose