The adxl367 uses fifo, non-interruptible mode, and is unable to obtain a specified number of data

Question

I use the FIFO of ADXL367 and set it to stream mode, hoping to save 64 sets of X, Y, and Z-axis data with a sampling frequency of 400Hz. I poll the FIFO data every 1 second (without using interrupts), but I always find 513 data entries when reading the FIFO, which is inconsistent with the manual description. How should I configure it to achieve my desired functionality?

this is my code

uint16_t adx1367_get_fifo_xyz(uint16_t *buff)
{
uint8_t data_buff[512 * 2] = {0};
uint16_t fifo_entries = 0;
uint8_t reg_val[2] = {0};

adxl367_get_register_value(&reg_val[0], ADXL367_REG_STATUS, 1);
if(reg_val[0] & 1 << 2){
adxl367_get_register_value(reg_val, ADXL367_REG_FIFO_ENTRIES_L, 2);
fifo_entries = ((reg_val[1] & 0x03) << 8) | reg_val[0];
if((0 == fifo_entries)){
return 0;
}

adxl367_get_register_value(data_buff, ADXL367_REG_I2C_FIFO_DATA, fifo_entries);
fifo_entries = (fifo_entries > SAMPLE_DEFAULT_AXI_HZ*6 ? SAMPLE_DEFAULT_AXI_HZ*6:fifo_entries);
for(int index = 0; index < fifo_entries; index++){
buff[index] = (data_buff[index * 2] << 6) + (data_buff[index*2 + 1] >> 2);
}
return fifo_entries;
}

return 0;
}

void adxl367_fifo_setup(void)
{
sample_mode_info_t *p_sample = NULL;
uint8_t reg_value = 0;

p_sample = sample_config_get();

// Set fifo stream mode
adxl367_get_register_value(&reg_value, ADXL367_REG_FIFO_CONTROL, 1);
reg_value &= ~(3);
reg_value |= ADXL367_STREAM_MODE;

// Set fifo sample x,y,z
reg_value &= ~(0x0F << 3);

// set fifo sample 64/1
reg_value &= ~(1 << 2);
adxl367_set_register_value(reg_value, ADXL367_REG_FIFO_CONTROL);

// set fifo sample 64/1
reg_value = 0;
adxl367_get_register_value(&reg_value, ADXL367_REG_FIFO_SAMPLES, 1);
reg_value = p_sample->sample_axi_hz;
adxl367_set_register_value(reg_value, ADXL367_REG_FIFO_SAMPLES);
}

MarioSM · Accepted Answer

Hello jike I am glad to help :) 
 jike said: I believed that when reading from the FIFO via I2C, there should be 64*2 sets of three-axis data, with older data being discarded; however, in practice, I observe that the FIFO is always full (513 data points) when I read it, which is confusing to me. Could you help explain this? 
 This is how the stream mode is expected to work, use this application note as reference: AN-1025: Utilization of the First In, First Out (FIFO) Buffer in Analog Devices, Inc. Digital Accelerometers | Analog Devices 
 To read the latest 128 samples (almost 42 samples per axis), then you should read from the hold register down to FIFO[385]. (FIFO[0] contains the oldest sample, and FIFO [511] the latest) I would also note that writing a 64 in FIFO_SAMPLES while recording 3 axis while lead you to fire the watermark after 192 samples. (64 per axis) hope this helps, Mario SM)

MarioSM · Answer

Hello jike excuse me seems like I am not getting notified for the updates on the asian MEMS forum. Hope you had a great weekend, I am afraid that there is no way to make the FIFO work in the way you need, it will always get completely filled. Interrupt + host logic is the only way to simulate fixed-sample capture, using the FIFO watermark/fifo_samples and disabling the FIFO after receiving the interruption. But this process would be non-autonomous since it must be completely driven by the host cpu. In any case I must insist that in stream mode you will always have available the latest 192 samples, the rest of older data that you can just ignore. hope this helps somehow, Mario SM

MarioSM · Answer

Hello jike thanks for reaching us! What kind of configuration do you want to achieve? Taking also into account the holding reg, you can read 513 values (sets of data of 2 bytes, FIFO watermark and FIFO ENTRIES only track the 512 inside the FIFO memory) In stream mode after 1 second, the FIFO will be full with data (mix of new and old data) if you do not follow the watermark bit you will lose the oldest data available, you can also check the overrun bit. On a first sight, the behavior of the part and your code are both consistent. Let me know how I can help you achieving a different functionality. have a nice week, Mario SM

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The adxl367 uses fifo, non-interruptible mode, and is unable to obtain a specified number of data

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