Current measurement guide: Measuring current with nRF52832 Engineering B and S132 v2.0.0-7.alpha

###Other relevant posts

###Outline

  • Errata affecting the current measurements
  • DCDC does not automatically switch off in System ON IDLE
  • NFCT can draw current when not enabled
  • Measurements
  • Results
  • Test setup: Power analyzer
  • Power Analyzer Measurements
  • Test setup: Oscilloscope and ampere-meter
  • Oscilloscope Measurements

Errata affecting the current measurements

###DCDC does not automatically switch off in System ON IDLE Please read this erratum for more information: [63]

The DCDC draws a base current while active. When the device load current is very low, this base current will be higher than the current saved by efficient buck regulation. The DCDC converter is therefore switched off by the hardware power management system when the load current is low. The Errata means the DCDC is not switched off when it should be, and its base current will impact the measurement of current when the device goes to low power modes.

In application code, you have the option to either turn on or off the DCDC. If the DCDC is on all the time for the Engineering B revision of the nrF52832, you will get the correct current consumption during radio and CPU activity, but too high consumption during low power modes. If you turn off the DCDC, you will see higher than expected average current for BLE events because the Radio and CPU peak currents are higher without the DCDC converter.

To overcome this issue some modifications to the application code need to be implemented to manually control the DCDC to be on when the Radio is on, and off when in low power modes.

Note: The DCDC is not on all the time the CPU is running. This will mean the CPU run current is higher than expected for the SoftDevice and the application. If the application executes code when measuring current, the CPU current will be higher than expected.

Instead of the regular sd_app_evt_wait() function called in the main loop in main.c, we can write a custom code that waits for an event, and then turns the DCDC on or off based on the radio state (on or off).

Replace the main for loop with this code:

for (;;)
{
    do{
        __WFE();
    }while (!NRF_RADIO->POWER);     
    NRF_POWER->DCDCEN = 1;          
    NRF_TIMER0->TASKS_START = 1;    
    NRF_TIMER0->TASKS_STOP = 1;
    do{
       __WFE();
    }while (NRF_RADIO->POWER);      
    NRF_TIMER0->TASKS_SHUTDOWN = 1; 
    NRF_POWER->DCDCEN = 0;          
    (void)sd_power_mode_set((nrf_power_mode_t) 1); 
}

This code is only useful for doing current measurements and should not be used when developing an application.

###NFCT can draw current when not enabled Another relevant Errata is regarding the NFCT peripheral which can cause increased current consumption in System OFF and System ON IDLE. Please see this erratum for more information: [62]

The problem does not occur until the supply voltage (VDD) is higher than 2.5V, but you should be aware of this issue in case you see increased consumption during low power modes.

##Measurements This section will show current consumption measured by Nordic using nRF52832 Engineering B on the PCA10040 kit with S132 v2.0.0-7.alpha and using the example application code for measurements in different use cases. These use cases include advertising and connection with different intervals, 100 ms and 1000 ms. It will also show the current draw of a complete adv/con event, the current in System ON IDLE + RTC between BLE events and in System OFF mode. Measurements have been taken using a power analyzer as a reference and to show current profile plots, an ampere meter and an oscilloscope. The results and the setup for each instrument are included.

###Results The results show good correlation between the measurement methods. The Ampere meter does not have results for BLE events further than 100ms apart and the Oscilloscope does not have results for very low current measurements. This is explained in the following parts.

These results are measured using a 2V supply voltage due to Errata 62 affecting current consumption when the supply voltage is high. Normal operating conditions for measurement is 3V supply which would result in between lower average current measurements because the DCDC is more efficient at higher supply voltages. The table contains power analyzer measurements taken with 3V supply on a prototype circuit that resolves Errata 62. The prototype circuit is not available on development kits so you may experience higher averages on the development kit with 3V supply.

Interval
(ms)
Data
(bytes)
Power
analyzer (uA)
Vdd=2V
Power
analyzer (uA)
Vdd=3V
Ampere-
meter (uA)
Vdd=2V
Oscilloscope (uA)
Vdd=2V
Advertiser 1000 6 14.3 11.4 - 14.3
Advertiser 100 6 120 94.6 120 118
Connection 1000 20 9.47 9.31 - 9.3
Connection 100 20 80.0 65.9 80 78.1
Sys ON IDLE - - 1.75 1.72 2 -
Sys OFF - - 0.225 0.224 0.2 -

###Test setup: Power analyzer

  • Chip: nRF52832 QFAABA
  • Softdevice: s132_nrf52_2.0.0-7.alpha
  • Application: modified app_pwr_profiling example from nRF52_SDK_0.9.2
  • Equipment: Keysight N6705B Power Analyzer with N6781A module
  • External supply Voltage: 2 V
  • Data Log Range, Current: Auto
  • Sampling frequency: 195312 samples / sec
  • TX power: 0 dBm

Advertising mode is using directed advertisement packets to get rid of scan requests, in order to get consistent measurements. This means that the payload of the packets is fixed at 6 bytes.

Connection mode is using notifications to send 20 bytes of data in one packet.

###Power Analyzer Measurements In all plots, the larger peaks are BLE events (advertising or connection) and the smaller peaks in between are the low power regulator drawing current periodically in refresh mode.

**Advertising 1000ms **

Avg: 14.3 uA.

Advertising 100ms

Avg: 120 uA

Connection 1000ms

Avg: 9.47 uA

Connection 100ms

Avg: 80.0 uA

System ON IDLE

Measured in between advertising events

Avg: 1.75 uA

System OFF

Avg: 0.255 uA

###Test setup: Oscilloscope and ampere-meter

  • Chip: nRF52832 QFAABA
  • Softdevice: s132_nrf52_2.0.0-7.alpha
  • Application: modified app_pwr_profiling example from nRF52_SDK_0.9.2
  • Equipment: Agilent Infinium 54815A
  • External supply voltage: 2V
  • Voltage offset: 2V
  • Sensitivity: 20mA/div
  • Memory depth: 32768 pts
  • Sampling rate: 250 kSa/s
  • Probes: 1X attenuation
  • Other: 9-bit bandwidth limit
  • TX power: 0 dBm

It is difficult to get accurate results when measuring low average currents (< 100 uA) using an oscilloscope because the sensitivity is not high enough. The low current periods between BLE events are not accurately measured. The best choice is using an ampere meter together with an oscilloscope.

You can use an ampere meter to measure the average current during System ON IDLE + RTC and System OFF as well as advertising/connection events when the interval is not too long. We have successfully been able to measure the current with an ampere meter during advertising or connection with intervals as long as 100 ms. Intervals above 100 ms are not easy to read because the display will jump between the low and high currents.

You can use an oscilloscope to capture BLE events as well as measuring average current higher then around 100 uA.

###Oscilloscope Measurements

Advertising event

Advertising event

Connection event

Connection event

Calculating average current from Oscilloscope and Ampere meter measurements

These plots show that the average current consumption during an advertising event is,

24.8 mV / 10 Ohm = 2.48 mA

The captured event lasts for,

500 us/div * 10 divs = 5ms

We then calculate the average current consumption during 1000ms adverting intervals:

5ms/1000ms * 2.45 mA = 12,25 uA

If we add the System ON IDLE + RTC current measured by the ampere-meter, we get:

12,25 uA + 2,0 uA = 14,25 uA

Similarly, for 1000 ms connection interval:

5ms/1000ms * 1.45 = 7.25 uA
7.25 uA + 2,0 uA = 9.25 uA

For higher average current we can use the average values measured on the oscilloscope.

Advertising 100 ms

Avg: 1.18 mV / 10 Ohm = 118 uA

Connection 100 ms

Avg: 781 uV / 10 Ohm = 78.1 uA

image description

Anonymous
  • @metch, I updated the links. Thanks for letting me know. The spikes you see in the plots are the refresh mode spikes. This is just how the regulator is designed, to charge up caps regularly to provide power to the chip.

  • @stian, FYI, these two links are broken:

    1. Please read this erratum for more information: [63]
    2. Please see this erratum for more information: [62]
  • Hi @stian,

    Could you tell me what are those spikes which appear each 250ms ? They seems to appear in System Idle and System OFF.

    For instance, take a look at the latest picture of the Power analyzer. Could you give me some information about these spikes ?

    We used a PCA10040 dev board (nrf52). We have a very simple code running without SoftDevice (while(true) __WFI();). We also see such spikes every ~40ms. Is this normal ?

  • @endnode:

    We are working on this. Sorry for the delay.

    @wenxue:

    1. The reason is that this reverts the workarounds provided by the errata, and may cause the chip to be unstable. But for developing and during current measurements it is fine.

    2. If you enable DCDC the regulator will automatically choose the mode that is most power efficient. So it will turn on or of DCDC automatically. However, this is only the case on the production chip (AAB0) and Engineering C (AABB). On previous Engineering versions (Eng A and B), the DCDC did not turn on or off automatically, hence the workaround provided in the code above (Because this tutorial was written for Eng B). The code above should not be used on the production chip.

  • thanks´╝ü but I have the following questions:

    1. you said "This code is only useful for doing current measurements and should not be used when developing an application." .So what's the reason?

    2. datasheet said:Automatic switching between LDO and DC/DC regulator based on load to maximize efficiency I don't understand automatic switching? If I set DCDCEN=1, it only use DCDC?