nRF52 clock in and out

RE: nRF52 clock in and out

mydadisaplumber — Sat, 06 Jun 2020 17:27:57 GMT

Hi Martin,

I came across this question. I'm confused because in the docs for the Clock of the nRF52 here:

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fclock.html

It seems to suggest you can use a 32 MHz clock with the nRF52 as a source for the HFXO. I've read the clock section of the docs a few time and still don't understand.

If there's no external 32 MHz clock, what's the HFXO source? Internal?

Why is it that all functionality can be done with no HF external clock, but a lot of other forum posts seem to suggest that you can't use Soft Device or BLE if you use an internal RC filter for the LFXO. Is that true? Is the 32.768 kHz external clock required for full nRF52 functionality?

Thanks.

RE: nRF52 clock in and out

Vincent Belaïche — Thu, 08 Feb 2018 14:44:19 GMT

Replying to myself.

I just found why my code does not start the 4MHz code after the start of the BlueTooth. Because I was trying to write clean/futureproof code I used the nrf_drv_clock_hfclk_request function in order to make sure that the fast clock at 16MHz is running and stable before to start the timer.

If instead of this I directly do the NRF_TIMER1->TASKS_START = 1; like in the original code, then the 4MHz clock is generated.

So, my comment is that the way I start the BlueTooth does something wrong with the clock driver. Your help about this is welcome…

RE: nRF52 clock in and out

Vincent Belaïche — Thu, 08 Feb 2018 14:09:16 GMT

Hello,

I am also interested in outputting a 4MHz clock from nRF52 (actually I am using nRF52832). I have a few comments question on the code that you have supplied:

Concerning this piece of code: [quote userid="6606" url="~/f/nordic-q-a/12177/nrf52-clock-in-and-out/46076#46076"]nrf_gpio_range_cfg_output(18); //Configure pin 18 as output[/quote]I am using nRF5_SDK_14.2.0_17b948a, and there seems that the nrf_gpio_range_cfg_output function takes 2 arguments (both ends of the range), so you need nrf_gpio_cfg_output driver function instead.
Concerning this piece of code:[quote userid="6606" url="~/f/nordic-q-a/12177/nrf52-clock-in-and-out/46076#46076"]NRF_CLOCK->TASKS_HFCLKSTART = 1; //Start high frequency clock
while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0) {
//Wait for HFCLK to start
}
NRF_CLOCK->EVENTS_HFCLKSTARTED = 0; //Clear event[/quote]I think it would be cleaner to use the driver functions for code futureproofness/portability, would the following make it:
```
  if(!nrf_drv_clock_init_check())
    {
      err_code = nrf_drv_clock_init();
      APP_ERROR_CHECK(err_code);
    }
```

Same comment for the PPI configuration, wouldn't it be cleaner if we used some driver call for this piece of code [quote userid="6606" url="~/f/nordic-q-a/12177/nrf52-clock-in-and-out/46076#46076"]//Configure PPI
NRF_PPI->CH[0].EEP = (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[0];
NRF_PPI->CH[0].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[0];

NRF_PPI->CHENSET = PPI_CHENSET_CH0_Enabled << PPI_CHENSET_CH0_Pos; [/quote]like this:

    void my_clock_start_handler(nrf_drv_clock_evt_type_t event)
    {
      if(event == NRF_DRV_CLOCK_EVT_HFCLK_STARTED)
        NRF_TIMER1->TASKS_START = 1;
      else
        APP_ERROR_HANDLER(NRF_ERROR_FORBIDDEN);
    }
    
    nrf_drv_clock_handler_item_t my_clock_handler_item_s = {
      .p_next = NULL,
      .event_handler = &my_clock_start_handler
    };
    
    void my_clock_start(void)
    {
    
      nrf_drv_ppi_init();
      err_code = nrf_drv_ppi_channel_alloc	(&my_4MHz_clck_ppi_channel_s);
      APP_ERROR_CHECK(err_code);
      
      err_code = nrf_drv_ppi_channel_assign	(my_4MHz_clck_ppi_channel_s,
     					 (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[0],
     					 (uint32_t) &NRF_GPIOTE->TASKS_OUT[0] );	  
      APP_ERROR_CHECK(err_code);
    
      err_code = nrf_drv_ppi_channel_enable(my_4MHz_clck_ppi_channel_s);
      APP_ERROR_CHECK(err_code);
    
      nrf_drv_clock_hfclk_request(&my_clock_handler_item_s);
    }

Now, last but not least, I have one more question. I noticed that the code above does not work any longer after any call to NVIC_DisableIRQ(POWER_CLOCK_IRQn);
For instance, when the SoftDevice is present, and the nrf_sdh_enable_request function from nrf-sdk/components/softdevice/common/nrf_sdh.c is called, then this function calls function sdh_state_observer_notify from the same file which in turn runs some state observers hooks, amongst which the sd_state_evt_handler from the nrf-sdk/components/drivers_nrf/clock/nrf_drv_clock.c file, which in turns calls NVIC_DisableIRQ(POWER_CLOCK_IRQn);.
I would be quite greatfull to get some feedback why this happens…

RE: nRF52 clock in and out

MartinBL — Mon, 29 Feb 2016 14:59:25 GMT

The answer to question # 1 is no, it is not supported. It was possible on the nRF51, but the clock module is redesigned and external 32MHz clocks is not a feature of nRF52. You can try at your own discretion, but the new design is so sensitive to jitter and noise that Nordic will not make any guarantees. A crystal is needed to ensure performance.

RE: nRF52 clock in and out

Greg Nelson — Mon, 29 Feb 2016 14:53:56 GMT

The back-of-the-envelope is very helpful, since I don't have the tools to measure currents in the uA range at my immediate disposal. I was just wondering if the GPIOTE functions would contribute noticeable additional power draw - the data sheet only lists IGPIOTE,IN and doesn't give any info about frequency dependency (uA/MHz). If I assume a 1.8V supply and only consider the pad capacitance (data sheet says 3pF except for NFC, but I'll stick with your 4pF for consistency), your formula and the resulting 5kΩ gives about 360uA per pin. In contrast, the formula from the datasheet (pg. 151: IGPIO=VDD Cload f) gives about 58uA per pin. I'm not sure which of these is a better estimate, but they are rather different.

RE: nRF52 clock in and out

Øyvind Karlsen — Mon, 29 Feb 2016 14:39:35 GMT

Well we have a pin capacitance of 4pF at each GPIO, the pulse moves at 8MHz and we know the relation X_C = 1/ ω C, this gives a equivalent resistance of approximately 5kΩ. Then you will have to add what you end up connecting to the pad. In the end the power consumed will be approximately equal to the power going through the equivalent resistor.

Alternately you can set up this on your development kit and measure the difference in current draw with and without 8MHz output. Please refer to the current measurement guide for practical information.

RE: nRF52 clock in and out

Greg Nelson — Mon, 29 Feb 2016 14:30:08 GMT

Just to be clear, for my second point, it is fine that I can't get 32MHz or 64MHz out ("the high frequency clock"); the design simply requires some clock in the MHz range ("a high frequency clock") and the power drawn by the nRF52 is orders of magnitude lower than typical crystal oscillators drawing several mA. We just want to get the power as low as possible.

RE: nRF52 clock in and out

Greg Nelson — Mon, 29 Feb 2016 14:16:00 GMT

This is excellent information - I believe that 8MHz would work well for our needs. Is there any simple way to estimate of the power impact of the GPIOTE solution listed above? I assume it's more complex that just CfV^2 for the I/O pin.

RE: nRF52 clock in and out

Øyvind Karlsen — Mon, 29 Feb 2016 09:56:47 GMT

Hi,

See Martin's answer.

The peripherals only run at 16MHz, this is why it is not possible to get HFCLK output from the nRF52. Max output is 8MHz, since you need to toggle on and off a pin. This is achieved using GPIOTE and PPI with the following code which creates a 8MHz output on pin 18:

 int main(void)
 {
 nrf_gpio_range_cfg_output(18); //Configure pin 18 as output

 NRF_CLOCK->TASKS_HFCLKSTART = 1; //Start high frequency clock
 while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0)
 {
      //Wait for HFCLK to start
 }
 NRF_CLOCK->EVENTS_HFCLKSTARTED = 0; //Clear event
 
 
 //Configure GPIOTE to toggle pin 18 
 NRF_GPIOTE->CONFIG[0] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                         GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                         18 << GPIOTE_CONFIG_PSEL_Pos | 
                         GPIOTE_CONFIG_OUTINIT_Low << GPIOTE_CONFIG_OUTINIT_Pos;
 
 //Configure timer
 NRF_TIMER1->PRESCALER = 0;
 NRF_TIMER1->CC[0] = 1;  // Adjust the output frequency by adjusting the CC.
 NRF_TIMER1->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos;
 NRF_TIMER1->TASKS_START = 1;
 
 //Configure PPI
 NRF_PPI->CH[0].EEP = (uint32_t) &NRF_TIMER1->EVENTS_COMPARE[0];
 NRF_PPI->CH[0].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[0];
 
 NRF_PPI->CHENSET = PPI_CHENSET_CH0_Enabled << PPI_CHENSET_CH0_Pos;
 
 while (true)
 {
     // do nothing
 }
 }

Best regards,

Øyvind