Function Generator instead of LFCLK

[prosper]

I am working on synchronizing RTC timers on several nrf52832s and i would like to have a single clock clocking all the RTCs on all the boards. I understand that the LFCLK can be bypassed and a signal provided on XL1 and XL2 pins. I have been providing a 250mV pp sine wave on XL1 and grounding XL2, and i have configured the solder bridges appropriately and the low frequency clock wont start. Is there something i am missing or is it that using a function gen as a clock source is not possible?

[Jørgen Holmefjord]

Hi,

It is possible to source the LFCLK in nRF52832 with external signal, but you need to set the correct configuration.

The supported configurations are shown in 32.768 kHz crystal oscillator (LFXO). Your input corresponds to:

• Low swing clock signal applied to the XL1 pin. The XL2 pin shall then be grounded.

Have you configured LFCLKSRC registers with the following values:

• SRC=1 (Xtal)
• EXTERNAL=1 (Enabled)
• BYPASS=0 (Disabled)

The input amplitude looks appropriate according to the electrical specifications:

Best regards,
Jørgen

[Jørgen Holmefjord]

Just to make sure, you have also started the LFCLK through TASKS_LFCLKSTART or similar method?

What is the value of LFCLKRUN and LFCLKSTAT registers?

[prosper]

The image shows the values of the registers. The LFCLKSTAT register is defaulting to RC oscillator for some reason

[Jørgen Holmefjord]

Hi,

prosper said:

The LFCLKSTAT register is defaulting to RC oscillator for some reason

I think this is caused by the debug session. Entering debug interface mode will likely force the LFRC on, as this clock source is needed in this mode. I see the same on my end.

I tested the low swing external clock input on one of my DKs. It is working as expected with 200mVpp sine input from a signal generator. I tested with different offsets (0mV, +50mV, +100mV) and all work on my end. Can you try out this test application to see if it works for you?

rtc_ext_lfxo_pca10040.hex

It will turn on LED4 on the DK (clear P0.20) when clock have started, and toggle LED1 at a regular interval.

Best regards,
Jørgen