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LSM6DS3 Continuous FIFO interrupts

Question asked by Knut Odman on Jul 3, 2018

LSM6DS3 Continuous FIFO interrupts

For this discussion 1 sample = 6 bytes of 3d accelerometer data.
I want to use FIFO to save power on CPU.
As long as all sample data fits in the FIFO, all is straight-forward.
I'm unable to figure out how to use Continuous mode to read more samples
than fit in the FIFO.
Example:
I want's to read 2048 samples x 6 bytes.
Because FIFO is 8k I intend to read 2 x 1024 samples. I have tried many combinations
of fifo status bits, but no approach works. Here's one attempt:

setup
-----

FIFO_CTRL3.DEC_FIFO_XL = 1
CTRL4.DRDY_MASK = 1
FIFO_CTRL1/2.FTH = 3093 // (1024 samples + 7 discards (my setting)) *3
INT1_CTRL.INT1_FTH = 1
FIFO_CTRL5.FIFO_MODE = 6 // Continuous mode

isr
---

if (FIFO_STATUS2.FIFO_FULL == 0)
   // first read
   // discard initial samples
   // read first 1024 samples
   // next watermark = 1024 - (MAX_FIFO - already read). I usually get 1031 samples in first round,
   // so 1024 - ( 4095/3 - (1031 ) = 1024 - (1365 - 1031) = 1024 - 334 = 690. 690 * 3 = 2070
   FIFO_CTRL1/2.FTH = 2070
   INT1_CTRL.INT1_FTH = 0
   INT1_CTRL.INT1_FULL_FLAG = 1 // This status bit never gets set when isr is called again
   CTRL4.STOP_ON_FTH = 1 // should set full bit but doesn't. Only FTH bit set.
   TAP_CFG.LIR = 1 // makes no difference
else
   // last read.
   // Read ending 1024 samples and shut down XL
   // note! "else" code never executes because FIFO_FULL status never gets set

result
----
1st interrupt
FIFO_STATUS1/2.DIFF_FIFO == 3093 // correct
FIFO_STATUS2.FTH == 1 // correct
FIFO_STATUS2.FIFO_FULL == 0 // correct

2nd interrupt
FIFO_STATUS1/2.DIFF_FIFO == 2068 // WRONG
FIFO_STATUS2.FTH == 1 // correct
FIFO_STATUS2.FIFO_FULL == 0 // WRONG

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Why am I not seeing expected FIFO_FULL flag and what should I change.
Please feel free to suggest another method altogether!

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