SAADC has strange periodic noise after power off/on

RE: SAADC has strange periodic noise after power off/on

kxJiang — Mon, 22 Jan 2018 06:22:23 GMT

I have enabled it in the IAR project's Preprocessor section. I may add some code to test whether the chip is still in debug mode or not when reset. Thank you for your patience and kindness.

RE: SAADC has strange periodic noise after power off/on

Jørgen Holmefjord — Fri, 19 Jan 2018 15:59:56 GMT

Have you enabled pinreset using the symbol CONFIG_GPIO_AS_PINRESET? You might still be in debug mode until power cycling the board. Unfortunately, we do not have any more detailed information on when the DC/DC will be active, it will choose the most suited mode depending on the current draw.

RE: SAADC has strange periodic noise after power off/on

kxJiang — Fri, 19 Jan 2018 02:59:26 GMT

But when we reset the board using nRESET pin without power cycling, we will not have this issue. Does that mean in such situation, even though we try to enable DC/DC, the chip may still use LDO as it is more efficient? Why reset differs from power cycling in such circumstance. In my situation, the chip uses LDO when reset without power cycling since the sadc has little noise, but uses DC/DC when power cycling since there's much noise. In both cases, I enabled the DC/DC in my code. Any detailed information on how the chip decide which regulator to use in these two cases ,depending on current consumption? I hope u can stand my verbose question. Thanks for your help.

RE: SAADC has strange periodic noise after power off/on

Jørgen Holmefjord — Thu, 18 Jan 2018 12:08:23 GMT

The reason you are not seeing this issue before power cycling the chip is that after programming the chip will be in debug mode. The DC/DC operates in two modes, normal and refresh mode. In debug mode the chip will draw much current, and then the most efficient mode will be automatically selected. The running mode will not be DC/DC refresh, and maybe not even DC/DC normal mode, as LDO normal mode might be best suited since it provide the most current. The DC/DC regulator is a switching regulator, which might introduce much noise. If you want to use DC/DC mode to save current during normal run, I would recommend that you disable DC/DC mode before sampling the SAADC, and enable it again when sampling is completed. This should reduce the noise.

RE: SAADC has strange periodic noise after power off/on

kxJiang — Thu, 18 Jan 2018 11:37:30 GMT

Hi, we use the DC-DC's 1.3V output as OP's reference voltage. But if that's the problem, why It is normal without power off/on.

RE: SAADC has strange periodic noise after power off/on

kxJiang — Thu, 18 Jan 2018 10:47:52 GMT

Hi, sorry for the late reply. I have figured out the problem by comparing the two projects. The difference is that I did not enable DC-DC in SDK12.0, while enabling it in SDK13.0. That is if I add the code sd_power_dcdc_mode_set(NRF_POWER_DCDC_ENABLE);afterble_stack_init();,That strange thing will happen, the saadc will have large jitters when power off/on. When I remove the sd_power_dcdc_mode_set(NRF_POWER_DCDC_ENABLE); ,everything works fine. But I still don't know why. Any comments on that? Thanks for your kindness!

RE: SAADC has strange periodic noise after power off/on

Jørgen Holmefjord — Mon, 15 Jan 2018 16:15:48 GMT

Can you test with the DK, to see if this is a board specific issue?

RE: SAADC has strange periodic noise after power off/on

kxJiang — Fri, 12 Jan 2018 03:42:10 GMT

No, JLink is disconnect. But the fact is that, reset NRF52 using JLink and then remove JLink connections. there will be no jitters. The jitters only happen when the board's power is removed and then power on again. Oh, man. that strange thing really makes me having no idea. Thanks for your reply, buddy!

RE: SAADC has strange periodic noise after power off/on

kxJiang — Fri, 12 Jan 2018 03:18:28 GMT

Hi, sorry for the lack of information. On reset condition, we get the second result. In SDK13, we first did the sampling in gpio interrupt handler but then changed that in FreeRTOS's periodic timer's handler in order to find out the problem. Either circumstances had the same problem. Here is some code snippets:

first method

// this gpio interrupt is triggered every 2ms
void gpio_int_handler(){
    ... 
    nrf_drv_saadc_sample_convert(NRF_SAADC_INPUT_AIN4, &raw_data);
    ...
}

second method

static void timeout_handler(TimerHandle_t xTimer) {
    ... 
    nrf_drv_saadc_sample_convert(NRF_SAADC_INPUT_AIN4, &raw_data);
    ...
}

period_timer = xTimerCreate("perio", 2, pdTRUE, NULL, timeout_handler);

we do not use PPI in SDK13. All of the sampled values stored in a frame buffer are transmitted to phone through BLE. This is done on our custom board. I have the DK, but haven't tested it on it. Thanks for your response！

RE: SAADC has strange periodic noise after power off/on

koniho — Thu, 11 Jan 2018 22:39:49 GMT

Do you have the JLink connected when the signal is jittering? Having the JLink connected could cause periodic halts that may explain the jitter.

RE: SAADC has strange periodic noise after power off/on

Jørgen Holmefjord — Thu, 11 Jan 2018 16:19:36 GMT

Do you only see the first case after programming, or could you get this case also after reset? Are you triggering sampling using PPI in SDK 13 as well, or some other method? Is this on a custom board or on the nRF52-DK? Could you upload some code that replicates this issue?