Author Topic: Three bad breakouts? Or is it me...?  (Read 555 times)

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Offline Aggressive_Doughnut

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Three bad breakouts? Or is it me...?
« on: June 30, 2018, 07:10:07 am »
I recently became the proud owner of three LSM6DS3 (6-axis accel/gyro sensor by STM, same one in my phone!) breakout board (the cheap little purple ones). I excitedly soldered headers on one of them, hooked it up to a breadboard with a STM32L4 Nucleo, fired up some SPI code the same as I have done several times (successfully) before, and I get a weird signal. Unplug the breakout and I see what I expected to see. So I think it's the board. Same thing with the next two boards, being increasingly more careful with the soldering thinking that could be the culprit. But none of them work. One of them just sits there, and the other two give me a very strange result indeed.

I suppose I am looking for confirmation of my analysis. Looking at the captures, I see that my signal out look correct when the boards aren't plugged in: I am setting the SS low, the clock looks good, and the data is 143 (=0b10001111), as expected. With the boards I get a complete mess, but the signals look partially additive, like some of each signal crosses into the others. Am I seeing a short in the board itself here? My contacts are clean and clearly separated, and I cleaned off all the extra flux to be sure that it wasn't conducting, but no difference. My only conclusion is that each board is somehow leaking across contacts, is that right?

I attached two LA captures, the first is the "good" signal, with no sensor attached. The other shows the crazy behavior when I hook up the sensor.
Note: I was sending 0b10001111, which is supposed to be the WHO_AM_I command for the sensor, as you can see in one of the captures. In the other you see that I am only sending 0xF, which was just part some experimentation on my end, but I get the analogous result for any value.
 

Offline RobertHolcombe

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Re: Three bad breakouts? Or is it me...?
« Reply #1 on: June 30, 2018, 09:34:01 am »
Do the breakout boards include decoupling capacitors?
 

Offline Aggressive_Doughnut

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Re: Three bad breakouts? Or is it me...?
« Reply #2 on: June 30, 2018, 10:26:20 am »
Yes, they appear to have all of the correct caps, resistors and even a voltage regulator on board. Are you thinking that if the caps weren't there, that would cause the voltage bounces that I see?
 

Offline tsman

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Re: Three bad breakouts? Or is it me...?
« Reply #3 on: June 30, 2018, 11:39:06 am »
How have you wired the breakout? That sensor has several different modes it can be in. It has multiple different interface options as well with I2C + SPI or SPI by itself and a secondary interface for other devices.

It could be possible all 3 of your breakouts are duds though. I've had similar boards where the manufacturer swapped out parts without really understanding what they're doing or some that just came dead.

Can you try the I2C interface to see if that responds?
« Last Edit: June 30, 2018, 11:41:02 am by tsman »
 

Offline a59d1

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Re: Three bad breakouts? Or is it me...?
« Reply #4 on: June 30, 2018, 11:47:12 am »
Are you sure your Nucleo is outputting on the right SPI bus?
 

Offline rstofer

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Re: Three bad breakouts? Or is it me...?
« Reply #5 on: July 01, 2018, 01:27:56 am »
It sure looks to me like there are some shorted outputs.  That is, two outputs connected together.

I like to put 300 Ohm resistors in series with all IO pins.  The resistance is insignificant for signal integrity because of the high input impedance of other devices yet it is high enough to prevent damage if the outputs are shorted.

Bottom line:  I think the gadget is wired wrong.
 

Offline Aggressive_Doughnut

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Re: Three bad breakouts? Or is it me...?
« Reply #6 on: July 02, 2018, 07:56:43 am »
Thanks all. I have checked the wiring and such, everything is definitely hooked up correctly. I can swap in, e.g., an EEPROM pin for pin and it works as expected on the same interface. I2C gives analogous behavior, with no sensor wired, I see the expected comms on the LA; once I wire up the sensor, boom, just one ugly pulse on clock that pushes SDA part of the way up. Happens the same on each one. I think the only possibility is that the circuit board is shorted across the connectors.

It's a shame, there are plenty of reviews for these little boards (maybe they are fake, or I just got unlucky) but not much internet activity surrounding them. On searching, I largely find info related to the (no doubt better-assembled) Sparkfun version. I ordered a few more, this time a variety of vendors and three variations of the board, since they are cheap enough and I really wanted to try them out.

Thanks for your help everyone!
 

Offline a59d1

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Re: Three bad breakouts? Or is it me...?
« Reply #7 on: July 03, 2018, 12:38:09 am »
Care to name the vendor? Or was it some noname dropshipper on Alibaba?
 


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