About RTC in the Apalis iMX8QM

Technical Support
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1

Hello,

I want to develop our own original carrier board. However, there is too little HW information in the Apalis iMX8 QuadMax module. It greatly confuse me from developing an original carrier board.
Sorry if I have missed some datasheets. This is one of the questions.
There is the RTC circuit in the Apalis iMX8 Quad Max module.
(“5.9.1 Real-Time Clock (RTC) recommendation”
https://docs.toradex.com/105526-apalis-imx8-datasheet.pdf)

1,
How much deviation is there in the crystal oscillator?
Do you have documentation to show this? Schematics and BOM are required.

2,
The carrier board has the RTC. Why does the Carrier Board have the RTC, even though the Aparis iMX8QM module has it? Is it a problem of current consumption?

3,
How do I switch between the RTC in the module and the RTC on the carrier board? Is that just the SW setting?
Which is the default and how do I switch to the other?
Does the one not in use sleep? Do I need to deliberately perform a sleep to make it do that?

Best Regards,
Yukio Oyama

2

Dear @YukioOyama , thanks for reaching out.

Have you checked our “Apalis carrier board design guide”? Secion 2.14.2 covers the implementation of an RTC.

Find here other design guides as well: Carrier Board Design Guides

As for your questions:

  1. We will check the SOM schematics and get back to you.
  2. Yes, the internal RTC of the SOM power consumption is much higher than a discrete IC in the carrier board, so this is option is preferred.
  3. Yes, it is just a SW setting. Please, check here for more info about the RTC in Linux.

Kind regards,
Alvaro.

3

Hello Alvaro-san,

Thanks your answer and please continue to follow up on question 1.

I was concerned that the VCC_BACKUP line consumes current even with a ultralow current consumption RTC. With the design-guide you have given, I found that VCC_BACKUP can be left unconnected. Thank you.

Best Regards,
Yukio Oyama

4

Hi Oyama-san,

Regarding question 1, we have several XTAL connected to the iMX8 SoC but the following would be of interest for you:

RTC_XTAL (BN47, BL47 pads): CM7V-T1A 32.768kHz, 12.5pF±20ppm

I was concerned that the VCC_BACKUP line consumes current even with a ultralow current consumption RTC. With the design-guide you have given, I found that VCC_BACKUP can be left unconnected. Thank you.

Yes, however take into account that while powered on, the RTC consumes power from the main power supply, not the battery. You would only consume power from the battery when you have no main power. You can make a quick calculation with the battery capacity and the RTC consumption, and it should give you several years of supply.

Kind regards,
Alvaro.

5

Hello Alvaro-san,

Thanks for the information about the X’tal. I could find the deviation to be 20 ppm.

Is there a published schematic and BOM for the computer-on-module showing that component? Is it confidential?
Sorry if I have missed something.

Best Regards,
Yukio Oyama

6

Hi Oyama-san,

While carrier boards schematics are fully public, SOM schematics and BOMs are confidential and not available to the public. We can indicate certain parts to help in the development but we cannot share the whole documentation for these.

Sorry for the inconvinience.

Kind regards,
Alvaro.

7

Hello Alvaro-san,

I understood. I will enquire if necessary. Thank you.

Best Regards,
Yukio Oyama