How does the timestamp is formatted and generated on the MAXSENSORBLE_EVKIT board for the some of the PPG sensor's Evaluation Kits?

For all the evaluation kits (MAXM86161EVKIT/SYS, MAXM86146EVKIT/SYS, MAX86176EVSYS, MAX86171EVSYS, MAX86140/1EVSYS)
that contain the MAXSENSORBLE_EVKIT board, the timestamp format and how timestamps are generated is explained in the following summary:

Format
The timestamps are in milliseconds (ms) from the epoch (converter).

Generation
FW logic:

  1. SW sends wall clock time
  2. SW sends start command (enables RTC and sensors)
  3. Delay before ADC starts (i.e., MAX86141 DAC cal = ~180ms, other sensors = ~20ms)
  4. FIFO data is read periodically
  5. Every ~1s the RTC value is saved (RTC is read on FIFO interrupt)
  6. SW sends stop command
  7. The final RTC value is sent with the last FIFO sample(s)
  8. SW sends wall clock stop time (unless the device loses power before SW can send it)


Parser logic:

  1. If a start and stop wall clock timestamp are available:
    1. The RTC tick period is: (start - stop - delay)/ (final RTC value)
      1. The default delay is 180ms for MAX8614x, 20ms for other sensors, but is adjustable via a parser parameter
  2. Otherwise:
    1. The RTC tick period is (RTCscale +1)/32.768kHz
      1. Default RTCscale = 31
  3. The number of samples between two successive RTC values (which is saved every ~1s) gives the sample period for that group of samples
  4. The group of samples before the first RTC value uses (start – delay)/(number of samples before first RTC value). Hence if the delay is way off the mark, the sample period for this group will also be.
    1. It’s best to ignore the sample period for this group of samples
  5. If there isn’t a wall clock stop time (due to power loss), then the last group of samples will not have a sample period
  6. Each sample’s timestamp is the previous sample’s timestamp + its group sample period
    1. The first sample’s time stamp is the wall clock timestamp + its group sample period
Tags: MAX86171EVSYS MAXM86146EVSYS timestamp Optical AFEs format Show More