PPK2 Interfering with Device Operation in NCP Wi-Fi Setup

[Palmer Huang]

Dear Nordic Support Team,

We have a system where an MCU is connected to a Wi-Fi module operating in NCP (Network Co-Processor) mode. The MCU controls the Wi-Fi chip to connect to an MQTT broker, subscribe to topics, and publish messages periodically.

We are using the Nordic Power Profiler Kit II (PPK2) in Ampere meter mode, connected in series with our device to measure current consumption.

Issue:
When running our system with PPK2, the device initially works correctly—MQTT connects, messages are received, and the Wi-Fi chip wakes up once every second as expected. However, after a period of time, the system stops working at random intervals — sometimes after 4.5 minutes, other times after just 10 to 30 seconds.

• The periodic current spikes indicating Wi-Fi wake-ups disappear

• No MQTT messages are received

• The system seems to stop functioning entirely

This issue only happens when using PPK2. If we remove the PPK2, or use a Keysight N6705C to measure current instead, the system continues to operate normally, with regular 1-second wakeups.

The screenshot below is from the PPK2 measurement software, showing the current profile. As you can see, in this particular run, the current suddenly drops after about 4 minutes and 30 seconds, and the system stops its periodic wakeups.

Question:
Why does using the PPK2 cause the system to stop working?

Best regards,

Gray

[hmolesworth]

Not just an issue with the PPK2, but also with (for example) a standard current meter with auto-ranging. The problem is due to range-changes; when measuring  a high current of some mA a low-value series resistor is used which has a voltage drop across the series resistor which indicates the current value. A low current measurement requires a much higher value of series resistor when the same current (actually voltage) measurement resolution is required and this change of resistance is automatic but takes a finite time. When the high-current transmission burst occurs - after a period of time with the low-current high series resistance measurement - the meter/PPK2 switches in the lower-value series resistor, but during the short time while this happens the high-value series resistor is not able to supply the required current and so the delivered voltage starts to fall - sometimes, not always, far enough to cause a device reset.

There are two options to fix this. The first is to increase the bulk capacitance downwind of the meter/PPK2 such that the delivered voltage droop - while the sense resistor change takes place - is reduced far enough to avoid the reset. The second is to simply sacrifice high-resolution low-current measurement resolutions and fix the meter/PPK2 scale to always measure with the same series resistance - ie turn off auto-ranging.

Regarding the PPK2, Series Resistor Rc for current sense is 1k0 in parallel with none or more of 110R, 11R0, 1R0 and 0R05

// Resistor Rc for current sense is 1k0 in parallel with none or more of 110R, 11R0, 1R0 and 0R05
//
//              <- +ve Supply to DUT From VOUT                               External Battery +ve To VIN <-
//             +------------------------------+                             +-----------------------+
//             |                              |                             |                       |
//             |              +---------+     |                             |     +---------+       |
//             |              |     GND |     | VOUT                   VIN  |     | GND -ve |       |
//   +=========O=========+    |    +====O=====O=============================O=====O====+    |       | Battery
//   |        Vdd (+)    |    |    |    |     #----+                        |     |    |    |       | + 3 volts
//   |                   |    |    |    |     |    |Rc        AMPERE \      |     |    |    |     +-O-+
//   |                   |    |    |    |     |   +++                 \     |     |    |    |     |   |
//   |                   |    |    |    |     |   | |   +-----------O  O----#     |    |    |     |   |
//   | Device Under Test |    |    |    |     |   | |   |                   |     |    |    |     |   |
//   |       DUT         |    |    |    | 120k|   | |   |                   |120k |    |    |     |   |
//   |                   |    |    |    |    +++  +++   |   \  SOURCE      +++    |    |    |     |   |
//   |                   |    |    |    |    | |   |    |    \             | |    |    |    |     +-O-+
//   |                   |    |    |    |    | |   +----#--O  O-- PPK LDO  | |    |    |    |       | Gnd (-)
//   |                   |    |    |    |    | |                           | |    |    |    +-------+
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |     |                             |     |    |
//   |                   |    |    |    |     #-- VDUT                VIN --#     |    |
//   |                   |    |    |    |     |  Sense                Sense |     |    |
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    | |                           | |    |    |
//   |                   |    |    |    |    +++                           +++    |    |
//   |                   |    |    |    |     | 30k                     30k |     |    |
//   |                   |    |    |    +--#--+                             +--#--+    |
//   |                   |    |    |       |                                   |       |
//   |                   |    |    |     =====      <<== Common Gnd ==>>     =====     |
//   |                   |    |    |      ===           (Common -ve)          ===      |
//   |        Gnd (-)    |    |    |       =                                   =       |
//   +=========O=========+    |    +===================================================+
//             |              |     PPK-II
//             +--------------+