Posted on July 03, 2018 at 03:35LSM6DS3 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
---
Why am I not seeing expected FIFO_FULL flag and what should I change.
Please feel free to suggest another method altogether!
