Clocking LPC5411x RTC from external oscillator? (TCXO)

[OldRadioHam]

Hi All,

My design needs an accurate 32.768kHz oscillator, temperature compensated. Can I feed the clock into the LPC5411's RTC input directly?

Other micros I used (PIC, MSP430) allowed this. The LPC datasheets talk about crystal selection but nothing about external clocking.

I have the budget for an accurate TCXO from Epson or MicroCrystal, that generates a good logic level signal.

Thanks for any replies.

[OldRadioHam · Accepted solution]

Hi Daniel,

An LPCXpresso54102 board is here now. I removed the 32k crystal and will feed in an external clock to check to RTC is running. There is just time to do this before the dealine to order production PCBs.

We are doing a conversion from Atmel XMEGA to NXP LPC family.

You didn't really answer but thankyou for your efforts. The devil is in the detail with micros, so it may not be long before another question comes up...

[OldRadioHam]

Hi Daniel,

An LPCXpresso54102 arrived, so we will feed in a 32kHz signal to RTCIN and prove the RTC clocks. The existing crystal is easy to remove. There is just enough time to test this.

Good that I got a response from NXP! There may be some other questions as we have to port a product from Atmel XMEGA to NXP LPC family. The devil is in the detail...

[Daniel-Aguirre]

Normally, where you can connect a crystal you can connect an oscillator (but not the other way). The output of the oscillator needs to be connected to the low impedance pin of the microcontroller, if using a single output oscillator (normally should be XOUT due to being the low impedance pin, but naming is not an standard).

For the LPC5411, the recommended way of handling the unused pins is as follows:

RTCXIN -> Connect to GND

RTCXOUT -> Leave unconnected

This could state that, aside from the convention I mentioned before, RTCXOUT is a Hi-Z pin, which leaving unconnected is a common practice. In this particular case then, the output of the oscillator should be connected to the RTCXIN pin (maybe to give a more intuitive name to the pin).

Still, no examples using TCXO are available at this moment for LPC. Nonetheless, we do recommend for you to contact an NXP representative for a better support.

Please, let us know if this information helps you or not.

[OldRadioHam]

Hello,

Sorry but the datasheet does not say an external oscillator rather than crystal can be used for the RTC. Probably it will work but my company is spending thousands of $$ on a PCB and I have to be certain.

As you say there is a development board but they are very scarce. I have one on order to test the 32kHz external input, but it will arrive after the deadline for ordering in the PCBs. If the datasheet said the external oscillator would work this pressure would be avoided.

Thankyou for the reply anyway.

[Daniel-Aguirre]

Hi,

LPC5411 has 2 pins in which you could connect an external oscillator (RTCXIN and RTCXOUT). This is mentioned in LPC541x Product Datasheet. In these pins, you can connect the oscillator which complies with your specific requirements.

LPC541XX | NXP Semiconductors

There is also an EVK which implements LCP5410 and could give a guidance on this topic. For this schematic you will need to sign-in into NXP site, so take that in mind. The schematic can be found on the Design Files section.

LPCXpresso™ Board for the LPC54100 family of MCUs | NXP Semiconductors

Let us know if there something else we can help you with.