LPC5460x without oscillator input (XTALIN) possible?

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LPC5460x without oscillator input (XTALIN) possible?

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topkinschen
Contributor II

Hi

    From the datasheet, I can generate 180M system clock from FRO_12M clock or CLK_IN(see attached).

but in datasheet page 53 table 5(attached below), says:

XTALIN-Connect to ground. When grounded, the RTC oscillator is disabled.

   Does this mean, I can't run 5460x with out oscillator input? I just want to simplify the design.

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carstengroen
Senior Contributor II

Yes, you can run the CPU without external xtal (I use the 12 MHz internal to generate 180 MHz clock)

You need to check requirements if using USB (high/full speed etc)!!

The text in the datasheet is wrong, it says ("Connect to ground. When grounded, the RTC OSCILLATOR is disabled")

This is clearly a typical "cut&paste" error from the RTCXIN pin description :smileyhappy:

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soledad
NXP Employee
NXP Employee

Hi, 

You can run the CPU without external xtal like Carsten mentioned, if you don't need the RTC the RTC xtal it is not necessary.  If you need the RTC you need to put the RTC xtal.

Have a great day,
Sol

 

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topkinschen
Contributor II

Hi Sol

    From the datasheet, if XTALIN is not connected and grounded then RTC is disabled, that means if I don't mount XTALIN, RTC will not work whether or not RTCXIN is mounted.

    I just want to double check:

                                        if the datasheet make a "copy&paste" mistake as Carsten say.

     OR,

                                        datasheet is correct, it's designed behavior?

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soledad
NXP Employee
NXP Employee

Hi, 

Yes, this is a mistake as Carten say. I will inform to our documentation team. 

Thank you for your feedback

Sol

 

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carstengroen
Senior Contributor II

Yes, you can run the CPU without external xtal (I use the 12 MHz internal to generate 180 MHz clock)

You need to check requirements if using USB (high/full speed etc)!!

The text in the datasheet is wrong, it says ("Connect to ground. When grounded, the RTC OSCILLATOR is disabled")

This is clearly a typical "cut&paste" error from the RTCXIN pin description :smileyhappy:

View solution in original post

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topkinschen
Contributor II

Hi Carsten

    Thanks for help.

    I don't use USB, and I just want to double check with you: you don't use external xtal, and your RTC works fine,  is that right?

    Thanks.

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carstengroen
Senior Contributor II

Sorry Liang,

I was not aware that you meant the RTC and not the normal CPU clock!

In case of the RTC, I don't know, but I would assume that an external xtal is needed for it to work (several of NXPs other processors does need external RTC xtal for the RTC to work)

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topkinschen
Contributor II

Hi Carsten

     I don't make it clear, Sorry for misunderstanding.

what I just want to say is :you don't use external xtal for MCU main clock(of course you use 32k  RTC oscillator as normal ),  and your RTC works fine,  is that right?

     I just want to double check the datasheet about this should say"

XTALIN-Connect to ground. When grounded, the XTAL oscillator is disabled.

"

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carstengroen
Senior Contributor II

Liang,

I do NOT have a RTC xtal mounted and I do also NOT use the RTC, so can't help you.

But, I would think that the RTC will NOT work without the RTC xtal, most NXP processors I use require the RTC xtal if you are going to use RTC. I would expect the same for the 546x series, but need to check with others.

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