RTC_PWRONRSTN pin

Takahiro Ogo

Genius 5940 points

Hi,

I have a simple question about am335x pin configuration.

If RTC is not used, how should we configure the RTC_PWRONRSTN pin?

I checked am335x datasheet, trm and schematic checklist (wiki). I coluldn't find it.

Best Regards.

over 12 years ago

0 Biser Gatchev-XID over 12 years ago

TI__Guru**** 393215 points

Hi Takao,

RTC_PWRONRSTn should be pulled up to VRTC.

Best Regards

Biser

0 Takahiro Ogo over 12 years ago in reply to Biser Gatchev-XID

Genius 5940 points

Hi Biser,

Thank you for your quick reply.

I understood that RTC_PWRONRSTn should be pulled up to VDDS_RTC when RTC is not used.

By the way, Where is that written?

Best Regards

Takao

0 Biser Gatchev-XID over 12 years ago in reply to Takahiro Ogo

TI__Guru**** 393215 points

Hi Takao,

It's not written anywhere, just good common sense. It's just not a good idea to leave a reset input floating. You can also tie it together with PWRONRSTn, but only if VDDSHV6 voltage is 1.8V.

Best Regards

Biser

0 peaves over 12 years ago in reply to Biser Gatchev-XID

TI__Guru 63055 points

Please hold for a final answer to your question.

I'm concerned the suggestion from Biser does not properly reset the RTC logic. The RTC may need to be reset even when it is not being used. I will try to provide an answer in the next couple of days.

Regards,
Paul

0 Takahiro Ogo over 12 years ago in reply to peaves

Genius 5940 points

Hi Paul

Thank you for your posting.

I will wait for your answer. Your cooperation is greatly appreciated.

Best Regards

Takao.

0 Takahiro Ogo over 12 years ago in reply to Takahiro Ogo

Genius 5940 points

Hi Paul
I apologize for rushing you, but any update on this?

-Takao

0 Biser Gatchev-XID over 12 years ago in reply to Takahiro Ogo

TI__Guru**** 393215 points

Hi Takao,

Sorry for delaying you, we had a similar question these days and missed your thread. Anyway, here is the link to the other post. It should answer your question too:

http://e2e.ti.com/support/dsp/sitara_arm174_microprocessors/f/791/t/219777.aspx

Best Regards

Biser

0 eugenio over 12 years ago in reply to Biser Gatchev-XID

Expert 1505 points

Thank you, I had the same doubt about RTC_PWRONRSTn pin.

0 Takahiro Ogo over 12 years ago in reply to Biser Gatchev-XID

Genius 5940 points

Hi Biser,

AM335x Schematic Checklist on Wiki page was updated and I was confused again about  RTC_PWRONRSTN.

http://processors.wiki.ti.com/index.php/AM335x_Schematic_Checklist#RTC

Wiki says RTC_PWRONRSTn connect to "Digital GND" if RTC is not used.

The last time I undersood RTC_PWRONRSTn should be connectted to "PORz"  if RTC is not used.

Which is correct ?

-Takao

0 Biser Gatchev-XID over 12 years ago in reply to Takahiro Ogo

TI__Guru**** 393215 points

Hi Takao,

This was dicussed again internally a couple of days ago. Here are two quotes from this discussion:

Biser Gatchev: "The RTC_PWRONRSTn signal can be tied together with PWRONRSTn as described in AM335X TRM, Section 8.1.4.3.6. Please note that the two signals are in different power domains - VDDS_RTC and VDDSHV6. Therefore if VDDSHV6 is at 3.3V (VDDS_RTC is 1.8V) you will need a level translator with a high-voltage tolerant input between PWRONRSTn and RTC_PWRONRSTn. This level translator should be powered from VDDS_RTC."

Peaves: "I would like to provide some new information that will be published in a new revision of the AM335x Data Sheet which should be on the web later this week. The PWRONRSTn input will have its own section in the Electrical Characteristics table. This section will describe how the PWRONRSTn input voltage threshold is not a function of VDDSHV6. The new Vih min will be 1.35 volts, Vil max will be 0.5 volts. So it may be possible to source a 1.8 volt signal to this input while VDDSHV6 is operating at 3.3 volts. However, you should never source a 3.3 volt signal to this input while VDDSHV6 is operating at 1.8 volts. This will allow you to generate a 1.8 volt reset signal that can be connected to both PWRONRSTn and RTC_PWRONRSTn inputs even if VDDSHV6 is operating at 3.3 volts. The RTC oscillator is disabled during and after reset, so you will not be able to hold RTC_PWRONRSTn low if you need this clock.

The new AM335x data sheet that will be published on the web later this week has a new note. The AM335x CLKOUT1 and CLKOUT2 clock outputs should not be used as a synchronous clock for any of the peripheral interfaces because they were not timing closed to any other signals. These clock outputs also were not designed to source any time critical external circuits that require a low jitter reference clock. The jitter performance of these outputs is unpredictable due to complex combinations of many system variables. For example, CLKOUT2 may be sourced from several PLLs with each PLL supporting many configurations that yield different jitter performance. There are also other unpredictable contributors to jitter performance such as application specific noise or crosstalk into the clock circuits. Therefore, there are no plans to specify jitter performance for these outputs."

This is confirmed and the new datasheet is available for download on the AM335X product pages now. Hope this helps.

0 Takahiro Ogo over 12 years ago in reply to Biser Gatchev-XID

Genius 5940 points

Thank you for the information. it was very helpful.

By the way, I'd liket to confirm this subject again. We will never use the RTC features.
If RTC is not used, RTC_PWRONRSTn should be connected to "PWRONRSTn ", or "Dgital GND"?

Best Regards.
-Takao

0 Biser Gatchev-XID over 12 years ago in reply to Takahiro Ogo

TI__Guru**** 393215 points

Hi Takao,

Connect RTC_PWRONRSTn to PWRONRSTn directly only if PWRONRSTn signal is at 1.8V level. If PWRONRSTn signal is at 3.3V level you will need a level transltor from 3.3V level PWRONRSTn to 1.8V level for RTC_PWRONRSTn.

0 Takahiro Ogo over 12 years ago in reply to Biser Gatchev-XID

Genius 5940 points

Thank you for the quick reply.

Wiki says RTC_PWRONRSTn connect to "Digital GND" if RTC is not used.
http://processors.wiki.ti.com/index.php/AM335x_Schematic_Checklist#RTC

So Wiki is wrong?

Best Regards
-Takao

0 Biser Gatchev-XID over 12 years ago in reply to Takahiro Ogo

TI__Guru**** 393215 points

I don't have information when the wiki page has last been updated. If you are not sure what to do I would suggest that you provide both connections on your PCB and see which one will work better for you.

0 sviss over 12 years ago in reply to Biser Gatchev-XID

Expert 1530 points

Yes, people do both: pull it up using AND gate, and provide 100kOhm pull-down,

(probably) to have a covenient way resetting the RTC with a pincette.

You may notice this know-how in most reference design schematics. :o

0 Biser Gatchev-XID over 12 years ago in reply to sviss

TI__Guru**** 393215 points

The purpose of these pulldown resistors is to hold reset lines low during voltage ramp-up.

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RTC_PWRONRSTN pin