Question about MSP-WDS430BT2000D Hardware

Thank your for your answer, 

Could you post what you consider a proper protection for external exposed pins, both to be used for chargin purposes and communication.

PS: In general i really dont like RF connection as most of my users like something reliable that you can just plug, connect and extract data. RF sometimes just do not works. Annd the antenna design is not straight forward (not for our team).

PS2: I forgot to mention it is soposed to be a high-reliability circuit. So it must not fail easily. Not life support, but users really hates when it fails as it is used as a long term datalogger.

Regards,

LR

What's your battery voltage, chemistry, capacity and nominal charge current and is it possible to put battery charger (charge controller) in the device?

It is a LIR2032 battery (http://www.eemb.com/pdf/Li-ion/LIR2032.pdf). The battery charger (MCP73832) is already embedded at the device. When the battey is charging i am  using a standard FET+diode to bypass the battery and power the circuit. 

My big concern about not isolating the external pins is due to the ground. If a big surge happens i am afraid my MSP430 can get reseted.

One more question if is not a problem. One possible solution is to use USB as my external interface and some ESD protection IC as TPD4S012 or MAX4987. Does these ICs would be able to protect my USB lines if they are totally exposed to the enviroment. 

Thank you for your help!

Luis Filipe Rossi said:

One possible solution is to use USB as my external interface

I think, USB communication for application that uses LIR2032 is epic overkill. I would make it simple as 1-wire charging and communication interface that operates same way as 1-wire devices do. Your Battery charge max current is 35mA which is low enough for both charging and communication for a cheap using 1-wire protocol (with external transistor). Host side charger shall be made as 35mA current source. How to do 1-wire communications- there's lot of application notes around. 5V ESD/surge USB VCC protection diode (Infineon for instance) is perfect for you. You shall put series ferrite bead between diode and connector to dampen fast transients. Zillions of mobile phones have USB connectors exposed to lot of nasty ESD events and they don't die. You will be fine too. If you are in doubt, send your prototypes to lab which do ESD testing.

Thank you for your answer, 

Actually it is not an overkill as there is a 16Mb memory that is used to log data. With iRDA is taking very long to pull the whole memory. USB would be of great help. Our dock system is already USB based so we would just change our MSP inside of the device from MSP430F5308 to MSP430F5508 for instance. Do you think something like tpd4s014 would be enough to protect my USB and power lines of my product? How about shortcircuit between lines (lets say USB and GND or VCC)? And how to protect system gorund and prevent resets? A ferrite bead would be enough?

I will take a look at infineon website. Sorry for all questions, i got no real experience with system level protection (thats the reason that i used galvanic isolation at the first version...).

And finally. Usually all cellphones are not tested under watter flow. What sort of extra poblems i can experience lets say insinde a swimming pool or shower? In such situation all the pins are going to be constantly short circuited

Regards,

Luis

Luis Filipe Rossi said:

insinde a swimming pool or shower?

There you shall be afraid of electrolysis, not ESD.

16MB is hellova data indeed. Maybe USB is right choice then. I am curious - mow much energy is needed to flash 16 Megabytes? Did you calculate? Any working prototype? Your battery is really tiny.. (actually maybe it's enough).

Infineon have ESD protection application notes, you shall check them.

Luis Filipe Rossi said:

And how to protect system gorund and prevent resets?

OMG... Please think harder about what you just asked.

Idea of I/o protection is to clamp high currents to ground as close as possible to entry point (close to connector), so those currents are not flowing through your circuit. That's it!

And yes - always make good ground planes, ideally covering nearly whole PCB.

No i am not afraid of electrolysis, just short-circuit or any enviroment with lots of accumulated charges. I know it sounds stupid but i really never worked with any electronic device that might get covered by water, so i can expect anything. 

About the ground, i was just afraid it might be some entrance for conducted noise...  but if that happens to be true i belive just a bad design would be a problem.

Yes i got about 10 working prototype in field test. Their average consumption is 16uA, so i got more than 3 months battery live. What i am doing is powering down the flash with a mosfet and powering up when i need to save something. Actually most of the board is powered off when not needed. I think the only device that keeps on all the time is an accelerometer that needs to be sampled all the time...In general the rest of the design is working well. Well not all, as the RTC clock is drifting more than expected, but still within acceptable ranges. The firmware is very solid right now with Watchdog and so on.. even if the device gets reseted it can get back to where it stoped.. but thats something that i am not expecting to count with.

Most of my designs in the past are related to digital hardware and firmware design. Most of analogue and power designs were very straightforward. That's why i got very low experience in this field.

Thank you for all your help. Any further suggestion is more than well appreciated.

Taking a look at chronos watch schematic i just realized that the VCC is connected to the stainless steal plate of the back of the watch. Also, when a side switch is pressed the VCC is tied to MCU pin that is also tied to stainless steal plate, with no extra protection, not even series resistor... probably i was really over protecting my circuit...

Luis Filipe Rossi said:

No i am not afraid of electrolysis

If your pins have even slightest potential difference between each other, then you will get electrolysis with all the consequences like leakage leading to unwanted battery drain, salt sediments and even disappearing contacts. As I said - discharges will be a problem in dry environments. VCC is easy - just low leakage diode (behind ESD protection) will do the job. I don't know about USB data lines, especially one having pull-up - never did USB protection against shower. Think about watertight cup over USB.

Luis Filipe Rossi said:

RTC clock is drifting more than expected, but still within acceptable ranges.

Do you have any RTC temperature correction? Even rough approximation of manufacturer's data for correction will greatly help. Or you can do your temperature drift measurements yourself. Other option is to use RTC module containing crystal and RTC logic which have built-in temperature correction.

Thank you for your help here and on the other thread.

Yes you are right about electrolysis. As you commented on the other post i can tie both pins to GND when USB is not used. About Pull-up, it will be floating as it will be pulled on external VSB, which wont exist. I believe i wont have any problem with that.

What you mean about watertight cup? Mechanically covering it? If yes, well that would be a real easy and good solution if we had enough volume, but for now to use a mold to fab our pieces is far too expensive so we got problems creating custom pieces. 

Thank you about the tip on the RTC clock. I was not implementing temperature correction for now but probably i will get better results using datasheet curve. Also i am using internal MSP capacitors. Not sure if i can get better results with external ones.

Regards,

LR

Luis Filipe Rossi said:

Not sure if i can get better results with external ones.

Most probably not. You shall check datasheet of your oscillator and try various internal capacitor settings first.

I can change the internal capacitor, but i am already using the specified at the crystal datasheet (12.5pF for instance), so i believe i wont have any better result.

Regards,

LR

Luis Filipe Rossi said:

i am already using the specified at the crystal datasheet (12.5pF for instance)

Well.. for example x2xx series data says that XCAP=3 setting leads to 11pF effective load. In datasheet we can read note "Since the PCB adds additional capacitance, it is recommended to verify the correct load by measuring the ACLK frequency. For a correct setup, the effective load capacitance should always match the specification of the used crystal.". Yet microcrystal application note says that to have effective capacitance 12.5 pF you shall use XCAP=2 for x2xx series. So, better measure than simply assume, especially if every of your RTC's is lagging or running too fast.