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CC2541: SCAN max number

frank tsai
Intellectual 435 points
Part Number: CC2541


Dear expert,

I have question when I set the max scan number to 8 when I have about 20 ble devices around it. 

The scan result found almost around 8 or 7 devices one time (set 400ms scan interval).

My question is,

What's the priority that the 8 of 20 devices been chosen by the BLE stack ?

Is it possible that  can't find a specific device ex. device 5 if it is far away or near form the scan device ?

Thanks in advance

  • 0
    Guru 735685 points
    The priority is the time BLE scan the advertising from different devices.
  • 0 in reply to YiKai Chen
    Intellectual 435 points

    Dear expert,

    Do you means each device has the equal opportunity been found(scaned) by scan device no matter it's far or near scan device ?

    Is it  also imply that for speific devie ex. device 5 maybe can't been find for a long time (worest case) even it is closest to the scan device ?

    Best regards,

  • 0 in reply to frank tsai
    Guru 735685 points
    When devices do advertising, BLE scan gets them in sequential. Yes, there is a chance that a very close device cannot be scanned. But I never see my device always cannot be scanned when I do BLE scan.
  • 0 in reply to YiKai Chen
    Intellectual 435 points

    Dear expert,

    1. What's the different when scan whitelist devies in this case, if the device #5 in the whitelist and use scan with whitelist methid ? 

    The device #5 still has the chance can't be found for a long time (worest case) even it is in the whitelist ?

    2. Moreover what's the difference if device #5 has connected to scan device ? Is it still has the chance can't communicate with center for a long time and time out (worest case) because of too many devices around center.

    I am curious if the BLE spec. can guarantee the communication within a resonable response time (10ms) in such case (many devices arround a scan/center device) ?

    Thanks

  • 0 in reply to frank tsai
    Guru 735685 points

    1. In white list case, only when device#5 is scanned and in the white list, the device would show on the scan result.

    2. If device#5 is connected to scan device, I don't think it won't have the chance can't communicate with center for a long time and time out (worest case) because of too many devices around center. However, if there are too many interferences on BLE 2.4G band, the connection might be broken.

  • 0 in reply to YiKai Chen
    Intellectual 435 points

    Dear expert,

    I observed that if device is more far away from scan device compared to others it will be less chance been scanned, that means the devices around scann device are not equal opportunity, stronger signal get more

    chance.

    About whitelist, I know scan device will filter device only allow device in the whitelist but if others which not in the whitelist has more stronger signal casues the device in list less change been scanned , it will take long time to find it no matter it is in the whitlist or not.   

    Also for the connected situation no matter if devices is conneced or not still will timeout if other devices have much more chance  been scanned by center.

    It seesms no guarantee that BLE devices communication can be stable and with resonsable response time ! 

    Is it correct of my understanding, please give me more detail and clear explanation if know.

    Thanks