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Providing external 32kHz signal to 8M Mini

Question asked by adevries on Sep 3, 2019
Latest reply on Sep 6, 2019 by adevries

Hello,

 

In my design, I am powering the 8M Mini using the PF8200. Since this PMIC does not have an RTC source on it, I am trying to find an external 32kHz oscillator that can provide the RTC signal to the 8M Mini instead. Based on the timing diagram of the 8M Mini (see attached), I need an oscillator that will output a valid clock signal no more than 50ms after VDD_SNVS_0P8 starts to turn on, and I need RTC_RESET_B to be de-asserted no earlier than 200us before this clock signal starts and no later than 100us after this clock signal starts.

 

I'm having some difficulty meeting these requirements using a standalone oscillator and was hoping for some insight from NXP on this.

 

Many oscillators I've come across have start-up times between 500ms and 1s, which is obviously much longer than 50ms. Additionally, unless I find an oscillator that has a fast enable time, I'm not certain I'll be able to meet the -200/+100us limit either.

 

Here are my questions:

1. What are the implications of exceeding time limit T2 and not having a stable 32kHz clock signal until 1 second after VDD_SNVS_0P8 starts to turn on?

2. What are the implications of exceeding time limit T3 and not de-asserting RTC_RESET_B until 100ms after a stable 32kHz signal is at the input?

 

Note: I would not begin power up any other voltage rails until after this whole sequence is finished.

 

If anyone has any thoughts on how to accomplish providing this signal in the time specified, I would love to hear them.

 

 

P.S. In the attached images, does the description for T2 contain a number of mistakes? It says "Delay from VDD_SNVS_0P8 high or RTC_SET_B de-assert".

1. Figure 3 shows the delay being measured from when VDD_SNVS_0P8 starts to ramp up, not when it is high.

2. should the "or" be a "to"? As in the "Delay from X *to* Y"?

3. RTC_SET_B isn't a signal. Should that say RTC_RESET_B?

 

 

Thanks

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