RTC Circuitry

[Beko]

Hi,

Our design does not have (or need) a battery backup to keep OSD335x runnning. (VINBAT/BAT_VOLT is pulled to ground) However we would like to implement a battery powered RTC for time/date keeping during black-outs or power-downs. The OSD335x does have an internal RTC and a crystal connection (OSC1). Is it possible to power this RTC with an external button type battery in case of power down, so the RTC inside the SIP keeps working? (We will only be powering the RTC module, not the entire SIP.)

Alternatively, is it possible to disable the internal RTC and use an external RTC IC with IC2 (like this one https://www.nxp.com/docs/en/data-sheet/PCF8523.pdf). If we use this IC, how should we do it? Also, this IC has its own crystal connection, we should not be connecting the 32.746kHz crystal at OSC1, correct?

As a general note, what is the best strategy to still keep date/time in power-down situations with no internet connectivity?

Thanks!

Beko

[Neeraj Dantu]

Hey Beko,

OSD335x_BAS cannot support RTC only mode. So, you will not able to use a coin cell battery to power the RTC subsystem to keep time.

However, as you suggested, you can use an external battery powered RTC to keep time and generate alarms if you need.

We suggest the following:

1. Use the external RTC powered by a back-up battery and connect to OSD335x as needed.

2. Use 3.3V of the OSD335x output to pull-up the I2C lines to avoid any pull-up when OSD335x is not powered.

3. Keep the RTC subsystem of OSD335x ON by configuring the device as shown in the reference design: https://octavosystems.com/octavo_products/osd3358-sm-red/. This will allow U-boot and SPL to not encounter errors during boot up initialization. See https://octavosystems.com/forums/topic/osd335x-c-sip-bootloader-issues/ for more information on the issues that are encountered when RTC subsystem is powered off.

Please let us know if you need any more help.

Best,

Neeraj

[Beko]

Hi Neeraj,

Thank you for your response. I just wanted to clarify your suggestion #3. When you say keep the RTC subsystem of OSD335x ON, do you mean to enable RTC by doing the connections shown in Figure 7.2 and Figure 7.4 of the OSD335x C-SiP Family Datasheet (Rev3)? Eventhough we will be using the external RTC, you suggest we enable the internal RTC module, correct? If yes, will we also need to include a crystal at OSC1 or leave them floating? Any other things we need to do to not face RTC related bootup issues? Thanks.

Regards,

Beko

[Neeraj Dantu]

Beko,

Yes. The recommendation is to have the RTC subsystem turned ON in function as well as power. You will need to add an RTC crystal on OSC1 in order to enable the RTC subsystem.

Please let us know if you have more questions.

Best,

Neeraj

[Beko]

Hi Neeraj,

I do find it weird that we disabling the RTC subsystem causes issues. This should probably be noted in your tutorials, because at the moment they clearly show a method to disable it. We will go ahead and do as you as and enable it to avoid any potential problems. Thank you for your response.

[Neeraj Dantu]

Beko,

Thanks for the inputs. We will create some documentation around the issue.

Best,

Neeraj

[Mario]

Neeraj,
Do you have (updated) documentation meanwhile?
Cheers,
Mario

[Neeraj Dantu]

Mario,

If you are interested, the information is just scattered.

1. The datasheet (https://octavosystems.com/docs/osd335x-datasheet/) contains the RTC related circuit configurations in section 7.5.1

2. This forum post has all the information needed on the software side: https://octavosystems.com/forums/topic/osd335x-c-sip-bootloader-issues/

There is also a very helpful guide put together by Aleix: https://docs.google.com/document/d/1mLe8aKckKvg9QwjWJkdTg8gPltMKCRK__lkcNBFa6k0/edit for a software work-around to disable RTC.

We do plan to update the tutorial series with this info, but if you need it, the above links should have you covered.

Best,

Neeraj

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