Distance Measurement Using VL53L4CD - Range Measurement Update Delay

Question

Hello all,We are once again using one of the VL53 TOF sensors for a close-proximity ranging application. For this application we are using code from the VL53L4CD_ULD_Driver.After initialising the system using the VL53L4CD_SensorInit function, we are obtaining single measurements by starting the sensor via VL53L4CD_StartRanging and getting results via get_data_by_interrupt()We are noticing that if the target distance is changed, the first 2 or 3 measurements after the target distance change, are incorrect (look like values from the previous target distance). The 3rd or 4th measurement (and onwards) are correct. Please have a look at the below log: --------------------------------------------------------------------------------------------------------------------------------------bVL53L4CD ULD ready ! - VL53L4CD INITIALISEDTARGET PLACED ~85MM FROM SENSOR AND STARTED GETTING SINGLE MEASUREMENTSeStatus =  0, Distance =  86 mm, Signal =  832 kcps/spadeStatus =  0, Distance =  87 mm, Signal =  848 kcps/spadeStatus =  0, Distance =  86 mm, Signal =  857 kcps/spadeStatus =  0, Distance =  87 mm, Signal =  869 kcps/spadeStatus =  0, Distance =  86 mm, Signal =  868 kcps/spadeStatus =  0, Distance =  86 mm, Signal =  877 kcps/spadeStatus =  0, Distance =  86 mm, Signal =  865 kcps/spadeStatus =  0, Distance =  86 mm, Signal =  869 kcps/spadeStatus =  0, Distance =  87 mm, Signal =  869 kcps/spadeStatus =  0, Distance =  87 mm, Signal =  881 kcps/spadeStatus =  0, Distance =  87 mm, Signal =  861 kcps/spadeStatus =  0, Distance =  84 mm, Signal =  870 kcps/spadeStatus =  0, Distance =  86 mm, Signal =  864 kcps/spadTARGET MOVED TO ~45MM FROM SENSOR AND STARTED GETTING SINGLE MEASUREMENTSeStatus =  0, Distance =  88 mm, Signal =  866 kcps/spad - STILL MEASURING 85MM!eStatus =  0, Distance =  86 mm, Signal =  854 kcps/spad - STILL MEASURING 85MM!eStatus =  0, Distance =  43 mm, Signal =  4154 kcps/spad - FROM NOW ONWARDS OKAY AGAINeStatus =  0, Distance =  45 mm, Signal =  6408 kcps/spadeStatus =  0, Distance =  44 mm, Signal =  6408 kcps/spadeStatus =  0, Distance =  46 mm, Signal =  6424 kcps/spadeStatus =  0, Distance =  47 mm, Signal =  6376 kcps/spadeStatus =  0, Distance =  45 mm, Signal =  6344 kcps/spadeStatus =  0, Distance =  46 mm, Signal =  6360 kcps/spadeStatus =  0, Distance =  46 mm, Signal =  6416 kcps/spad-------------------------------------------------------------------------------------------------------------------------------------- Any suggestions for solving this would be greatly appreciated.Thanks in advance for your help! Brian

John E KVAM · Accepted Answer

The sensor does not have much memory. The range is taken, and the data is moved into dual-port memory so the I2C interface can get to it. Then the interrupt is triggered, and the next range starts.

There is no way 2 ranges can be buffered up.

I'm going to guess that if you put a time stamp on your ranges that you find you don't get the timing you expect.

Sometimes people read the prior result twice - but in your case the signal strengths are different, and that means you are in fact reading different ranges.

Somehow your code is buffered.

But if you put a timestamp on the data as soon as you get the interrupt, you will figure out what is going on.

(This has happened to me too.)

Perhaps your stop was issued after the range was taken - and you are reading stale data there. But that would only account for one stale range, not two.

There is an interrupt immediately after doing the start. I'd clear that one without reading any data. The next one would be valid.

- john

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