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#1 July 11, 2017 16:52:55

jjones7432
Registered: 2017-07-11
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Stability of external 32K crystal circuit in high humidity conditions

I'm using the 32.768K external crystal oscillator circuit shown in the TIWI-UB1 EM user manual in my TIWI-UB1 application in an IP54 and IP67 enclosures. This external crystal oscillator stops operating in high humidity application, although there is no visible moisture on the board. Apparently this oscillator drives chip operation so when the oscillator stops, the chip stops operating.

Is anyone using the TIWI-UB1 in a Bluetooth LE application in a high humidity application (outdoors, refrigerator, etc.)?

If so, what is your 32.768K external oscillator (crystal or chip) circuit design?

Thanks.

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#2 July 11, 2017 17:02:01

SupportEngineer
Registered: 2016-03-11
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Stability of external 32K crystal circuit in high humidity conditions

We are not clear on why the humidity is having an affect, do you have a value to provide? Unfortunately we have not had this issue reported before and so have not looked into a recommended replacement.

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#3 July 11, 2017 17:51:49

jjones7432
Registered: 2017-07-11
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Stability of external 32K crystal circuit in high humidity conditions

I explain more below but do you have an TIWI-UB1 BLE application that operates outside in raining conditions. If so, we'd like to use those lessons learned.

I say humidity related because there is no moisture detected visually on the PCB but the failure is easily reproducible by operating outside in the rain, even with a water resistance rated enclosure.

I don't have a specific rH humidity value yet, but try the following test case which we use:
1. develop a TIWI-UB1 UIT that continually advertises so that you know when the device stops AND that produces a test pulse train on an output pin, used to determine if the chip is running.
2. place this UIT in an IP54 enclosure. (Higher IP are addressed below.)
3. Place the enclosure in the continuous stream of a fountain, shower, etc., simulating operation outside during the rain.
4. periodically use a host device to check that the UIT is still advertising.

In my testing, the UIT will fail (stop advertising) in about 30 minutes, which is good for trying solutions. In higher IP enclosures, the time to failure increases to 3-7 days. The target time to failure is 300 days. Typically, there is no moisture seen on the UIT PCB, although there may be condensation showing in the enclosure. Conformal coating, liquid tape, and electrical tape solutions used in many ways have not been successful. Placing the enclosure in a zip lock bag works but is impractical.

Perhaps your testing will be successful and we can learn from your application's layout.

The longer the UIT is in the test environment after the UIT stop, the longer it take the UIT to restart, unless the UIT's board is dried, typically with a hot air gun.

Verification was done by probing for the test pulse with an Oscope (and the leads of the crystal oscillation which show oscillations even using a 10x probe). When the UIT stop advertising, there are no oscillations and there is no test pulse train.

Thanks.

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#4 July 11, 2017 18:20:37

jjones7432
Registered: 2017-07-11
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Stability of external 32K crystal circuit in high humidity conditions

I forgot to mention that when we program the UIT to use the internal 32K oscillator, the UIT is very robust in outdoor testing in rain, in that the UIT advertises even in enclosures that allow water into the enclosure and on the UIT.

UITs using the internal 32K oscillators last for months, but of course, will not connect and send data in BLE applications.

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#5 July 12, 2017 19:45:55

jjones7432
Registered: 2017-07-11
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Stability of external 32K crystal circuit in high humidity conditions

Support, I have some specifics that are of interest:

I tested your TIWI-UB1 EM board (750-00490-r1.x) against BlueRadios BR-DEV-LE4.0-S2A, Development Board. Both of these boards use the TI CC254X, albeit the TIWI-UB1 uses the CC2541 and the BR-DEV-LE4.0-S2A uses the CC2540.

TIWI-UB1 EM BOARD: Oscope shows a clear oscillation across C5. Oscillation shows briefly on unlabeled component, which is assumed to be C7. Touched finger to water and then touched C5, C7 and X1 which are all tightly clustered together. Device immediately stopped advertising. Oscope shows no oscillation across C5. EM Board was hot air-dried for 10 minutes with no oscillation. Board took 1 hour to begin oscillation across C5 and device began advertising.

BR-DEV-LE4.0-S2A, Development Board: 32K circuit is included with the BR-LE4.0-S2A module, under the module shielding. Blueradios provides no information about the layout of the module under the shielding. I removed the shielding and identified a crystal and two component adjacent to it. Oscope showed a 32K signal across each component which are assumed to capacitors. Again I touched my finger to water and then to the crystal/capacitor cluster. Advertising stops. It took about 10 minutes for the board to start.

Since the BlueRadios product protects the 32K oscillator circuit under the module's shield, any suggestions on properly protecting the external 32K oscillaltor for the TIWI-UB1?

Liquid tape, conformal spray, electrical tape and similar techniques tend to affect the 32K oscillator before the moisture/humidity does.

Edited jjones7432 (July 12, 2017 22:13:36)

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