December 4, 2009
about 5 years ago
To follow up on my two earlier comments regarding sensitivity to noise and ringing: apparently there was some RFI getting into my circuit as well as the power and ringing problems I experienced. A solid ground is essential. Again, this is running 4-wire SPI on 2.7V digital power.
This really is the most finicky part I have ever used. It doesn’t take much to make it misbehave by not receiving and/or sending SPI data as expected. But with the extra power filtering, the damping resistor on the SPI output line, and a solid ground with limited RFI pickup, it finally seems to be happy.
It would appear that the ADXL345 SDO line – at least when configured for 4-wire SPI, the only configuration I’m testing – is VERY SENSITIVE TO RINGING.
I STRONGLY RECOMMEND applications insert a damping resistor in series with the ADXL345 SDO line. I used 27 ohms here to solve the problem.
Before this fix, I could not get the correct device ID response. Pressing my thumb all over the breakout board changed the device ID response and sometimes made it OK. After much gnashing of teeth I finally discovered this issue.
This module has just two 0.1 uF decoupling capacitors, BUT the AD documentation suggests much heavier noise isolation – 10 uF and 0.1 uF in parallel AFTER a resistance up to 100 ohms.
Turns out AD is not kidding. I had terrible problems with inconsistent behavior until adding 10 uF decoupling and inserting a 27-ohm resistor in series with the 2.8V power supply I was using.
I am still having trouble talking to the blasted thing via SPI, can’t get the expected device-id value out, but at least the behavior is now consistent.
Those having trouble with this breakout board might want to try adding this heavy-duty decoupling/isolation in accordance with AD recommendations! Wow what a finicky part.
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