iMXRT1064 Coin Cell or Supercapacitor usage

[nksawane]

Hello, 

I am working on a project with iMXRT1064. 

I am using a SuperCapacitor as a replacement for a coin cell battery. 

As per indications on the development kit schematic, I connected supercap to VDD_SVNS_IN.

However, the MCU does not start working until the capacitor has been discharged. 

So after use, I have to wait 15 to 20 minutes to get the board running again. 

Looking at the datasheet, i found that VDD_SVNS_IN pin is multi-featured. RTC is one of the feature.

Could it be that this pin is by default set to GPIO which could be a high drain source?

This could explain why the board does not power up till capacitor has been discharged? 

If yes, in order to use a coin cell or supercap, do we need to configure this pin to RTC?

Thank you,

Nikhil 

[nksawane]

To understand the effect of SuperCap, I removed Supercap from the board and replaced it with a variable +5V source. 

If the voltage is below +3V, the board powers up every time. No issues. This explains why the issue goes away if Supercap removed.

If the voltage reaches +3.3V, the board works but if you cycle power, the board does not turn on. This simulates the charging of the capacitor to around +3.3V.

Now lower the voltage below +3V, as you reach 2.8V or 2.9V, the board automatically turns on and starts functioning again.

This is the issue we are dealing with. The issue exists with the coin cell as well.

Any ideas about what might be happening?

Thanks,

Nikhil

[fangfang]

Hello,

1. SNVS working voltage is 2.4V~3.6V.
2. The work of super capacitors When the board is powered off, the super capacitor will supply power to the SNVS IP, and its voltage will continue to drop, and may even fall below 2.4V or 2V.
3. When powering up the board VSNVS_IN(LDO_3V3) supplies power to SNVS IP, the super capacitor will absorb current to charge itself, the 3.3V input voltage will be pulled down to the same value as the super capacitor voltage, and then its voltage will slowly rise until its voltage When it rises above 2.4V, it will meet the requirements of the SNVS module and the board will start. So, the 15 minutes you mentioned is the time it takes for LDO_3V3 to charge the super capacitor and make its voltage reach above 2.4V.

Have a nice day.

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[nksawane]

Thank you for your reply. 

If that is the case, then if I have it running for a day where the SuperCap has received enough charge. 

If I turn the power OFF and then back ON again. LDO_3V3 should have voltage close to 3.3V and well beyond 2.4V. 

So I should not be having this problem. 

If I have the board turned OFF for couple of days where the Supercap had enough time to discharge. In that scenario, it should take some time for Supercap to charge back to at least 2.4V. 

Do I need to specify something in the software?

Thanks,

Nikhil

[fangfang]

Hello ,

You are welcome.

>>Do I need to specify something in the software?

 No, you don't. Software can't solve the problem.

>>suggestions:

You can do the test like below.

Test 1:

Remove the capacitor, then boot board, checking if it can be normally boot.

Test 2:

Replace the capacitor with a Li coin cell battery, then power up board, checking if it can boot normally.

 Then tell me results of above 2 tests, please!

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- We are following threads for 7 weeks after the last post, later replies are ignored
Please open a new thread and refer to the closed one, if you have a related question at a later point in time.
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[nksawane]

I already completed Test 1 - Remove the capacitor, the issue goes away. 

Any comments from data on Test 1?

Test 2 - I will check this option and let you know.

Thanks,

Nikhil

[fangfang]

Hello,

>>Any comments from data on Test 1?

It indicates that when the capacitor is fully charged, the internal RTC power consumption is relatively large, resulting in the capacitor discharging too fast, and then it can not meet the requirements of VSNVS_IN(LDO_3V3) voltage (2.4V to 3.6V).

You may refer to the suggestions:
1. Power the SRTC with Li coin cell battery (the best rechargeable one);

Or 2. Use independent RTC clock chip + Li coin cell battery (preferably rechargeable)

Have a nice day.