HFXO does not work on nRF52832

[silvio3105]

Hi,

I cannot start HFXO on nRF52832. I'm using 32MHz crystal TSX-3225 32.0000MF20X-W0(from Mouser) wtih 12pF caps UMK105CG120JV-F(also from Mouser).

Checked with beeper on multimeter, no shorts between pins and between pin and GND.

Looks ok on MCU pin side(QFN package).

This is code

int main(void)

{

    NRF_LOG_INIT(NULL);

    NRF_LOG_DEFAULT_BACKENDS_INIT();

    NRF_CLOCK->TASKS_HFCLKSTART = 1;

    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);    

And PCB

I'm out of ideas.

[Kazi Afroza Sultana]

Hello,

We have an [68] CLOCK: EVENTS_HFCLKSTARTED can be generated before HFCLK is stable errata that could be relevant to your issue. If EVENTS_HFCLKSTARTED starts before HFCLK is started.

Can you check the Crystal start-up time of your 32MHz?

[rmptxf]

We also run into a similar issue, and the start time was more than a 1ms on best cases (not even fixed).

What should we do in that case of high start time?

[rmptxf]

This is how we get the startup time on NCS.

NRF_TIMER1->BITMODE = TIMER_BITMODE_BITMODE_32Bit << TIMER_BITMODE_BITMODE_Pos;
NRF_TIMER1->TASKS_CLEAR = 1;
NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
NRF_TIMER1->TASKS_START = 1;
NRF_CLOCK->TASKS_HFCLKSTART = 1;
while(NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);
NRF_TIMER1->TASKS_CAPTURE[0] = 1;

LOG_INF("HF Clock has started. Startup time: %d uS", NRF_TIMER1->CC[0]);

[hmolesworth]

Most useful, we will adopt this. For other users it may be helpful to do the measurement in uSecs:

   NRF_TIMER1->PRESCALER = 4; // Time in uSecs when using 1MHz timer clock. fTIMER = 16 MHz / (2^PRESCALER)

[silvio3105]

Mine app stucks at while loop.

[hmolesworth]

The data sheet for that crystal implies 7pF CL although the Mouser listing states 12pF, so 12pF load caps are not correct in either case.

CL = (CaCb)/(Ca+Cb) + Cstray where Ca=Cb=12pF+Cpin=15pF and Cstray (not pins) say 1pF
     = 8.5pF

CL = (CaCb)/(Ca+Cb) + Cstray where Ca=Cb=9pF+Cpin=12pF and Cstray (not pins) say 1pF
     = 7pF

If the Mouser description of 12pF is correct then try 19pF

CL = (CaCb)/(Ca+Cb) + Cstray where Ca=Cb=19pF+Cpin=22pF and Cstray (not pins) say 1pF
     = 12pF

[silvio3105]

Mouser says 7pF https://hr.mouser.com/ProductDetail/Epson-Timing/TSX-3225-32.0000MF20X-W0?qs=f9yNj16SXrLFb%2FNL%252B2Lczg%3D%3D

Datasheet says 7pF to infinity

DigiKey says 12pF https://www.digikey.com/en/products/detail/epson/TSX-3225-32-0000MF20X-W0/7731972

I made mistake with not including 4pF for Cpin.
For 7/5pF caps have to be 4pF while for 12/5pF caps have to be 14pF, according to this calculator - https://ecsxtal.com/crystal-load-capacitance-calculator/

Seems like CL is 7pF since I guess 12pF will be enough for 12pF CL.

[silvio3105]

Tried 4pF, 8pF(stack od 2x4pF in parallel) and 20pF(2x4pF + 12pF in parallel) - still cannot start it.

I belive CL is 7pF as stated in datasheet. Calculated caps with "((8+4+1)*(8+4+1))/(2*(8+4+1))" - from nRF52832 clock section.

C = C1 = C2

(C + Cpin + Cs)^2 / (2 * (C + Cpin + Cs))

Cs = 1pF
Cpin = 4pF

Result is 6.5pF which is close to 7pF from datasheet.
Connection is tested, solders on MCU pins look good.

 Kazi Afroza Sultana