NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

RE: NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

pragathi — Wed, 09 Mar 2022 19:44:36 GMT

Update: after some testing, we identified a couple causes, thank you for your help!

The battery voltage was too high when fully charged
The copper interconnects would sometimes de-laminate and crack, making the connection unstable when the device was bent
We're working on implementing the external capacitors, this might be an issue as well.

I also probed a few new devices and they are drawing current within the spec sheet limits, so I'm not sure why the older devices drew that much current. (Nothing was changed between the new and old devices)

RE: NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

ketiljo — Thu, 17 Feb 2022 12:14:10 GMT

The external caps are mandatory. Especially the cap on DEC4 is needed for the internal regulators operate properly. So you need to implement those.

Max supply voltage is 3.6 V. Absolute max is 3.9 V for a _very short_ period of time only. Any higher will damage the device. With a LiPo battery, you need a regulator as the voltage can easily be higher than this, up to 4.3 V.

RE: NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

hmolesworth — Wed, 16 Feb 2022 21:54:16 GMT

The battery on the leg is shown as a (nominal, so could be higher) 3.7 volt Lithium-Ion battery, which will (eventually) damage the nRF52832 in the sense that side effects similar to what you describe start to occur. The rated supply of the nRF52832 (module) is only 3.6 volts, and although the absolute maximum of the nRF52832 is 3.9V note that "Maximum ratings are the extreme limits to which the chip can be exposed for a limited amount of time without permanently damaging it. Exposure to absolute maximum ratings for prolonged periods of time may affect the reliability of the device". A quick fix here is a series schottky diode.

With low expected BLE throughput it's worth compressing the EMG data with a simple compression routine that (for example) sends offsets from the most positive peak value in a group of EMG readings and also that peak. 50 16-bit readings can be transmitted as a single 16-bit reading plus 49 4-bit offsets but of course this depends on the anticipated dynamic range of groups of EMG readings. If not noisy sigma-delta encoding also works.

BLE at 2.4GHz does not perform well close to a ground plane (such as the floor) or through (moist) muscular tissue; try standing on a table and see if that improves performance (for confirmation).

RE: NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

pragathi — Wed, 16 Feb 2022 21:00:52 GMT

We do not have those capacitors implemented -

When the sEMG was being developed, we contacted the manufacturer to see if they were necessary, and were told they were only recommended. I think they weren't implemented because they were a bit too large (our flexible substrate relies on chips being only a couple mm wide for them to withstand routine bending), but now that we are redesigning the device's layout, adding the capacitors shouldn't be a problem.

We also looked for deep-trench capacitors, as they were the most compatible with our interconnects (thin, 0.1-1mm pitch copper interconnects)

We connect the chip to a 3.3V Li-ion battery, and the schematic you linked above is to prevent the chip from rapid voltage fluctuations - are you thinking that the power source may be unreliable during flexing?

RE: NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

ketiljo — Tue, 15 Feb 2022 14:19:57 GMT

Ok, that should be ok. The module requires external capacitors on VCC and DEC4:

Do you have these?

cheers,

Ketil

RE: NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

pragathi — Mon, 14 Feb 2022 18:34:27 GMT

Also wanted to add that the module is from Taiyo-Yuden, the datasheet can be found here, and no it is not on a rigid PCB.

https://www.yuden.co.jp/wireless_module/document/datareport2/en/TY_BLE_EYSHSNZWZ_DataReport_V1_3_20180530E.pdf

RE: NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

pragathi — Mon, 14 Feb 2022 18:16:07 GMT

Before testing, we did have the NRF chip wirebonded to a chip-carrier like so:

But on the sEMG device, the chip is embedded in PDMS with no back plane.

How would we test if the device is shutting off due to mechanical stress? Is there an error log that's stored in the chip that we can access after testing the sEMG on a person?

Thank you,

Pragathi

RE: NRF52832 embedded in flexible device unexpectedly disconnects and shuts down when transmitting, now also draws too much current – what is causing it?

ketiljo — Mon, 14 Feb 2022 09:59:40 GMT

Hi Pragathi

There's no reason for the device to degrade over time. It should withstand thousands of flash programming cycles as well. So I'm thinking that it's relates to mechanical stress caused by the flexing of the PCB. Do you have the nRF52832 on a separate module? If so, how does the module look like? Is it on a rigid PCB?

best regards,
Ketil Aas-Johansen