Maximum Sampling Frequency for BTLE or WiFi
응답
15. 10. 26 오후 2:34
Hi,

The demo code for Wifi and BT say that the maximum frequency for the demos is 10Hz. Is this the maximum that the XDK is capable of via BTLE and WiFi? 

I need at least 100 Hz, prefereably 200Hz, via WiFi or BT. Is this possible? And can this simply be changed in the demo code or do i need to write my own for this?

Thanks
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Maximum Sampling Frequency for BTLE or WiFi
응답
15. 10. 27 오후 1:13 as a reply to Christine Martindale.
Christine,

where did you find this limitation?

While I'm sure there is/will be one limitation, I would imagine it to be higher. The demos are not necessarily optimized for a special purpose but should show you more of a broader idea what you can do with the XDK.

I've opened a ticket with the developers of this demo to see what they have to say about this.

Anyway, I'm sorry I can't help you with an immediate solution right now, but I'll make some test and see if I can assemble something that'll fix your issue.

- Florian
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Maximum Sampling Frequency for BTLE or WiFi
응답
15. 10. 27 오후 1:24 as a reply to Florian Harr.
Florian,

I found the limitation in the following two places:

1.The getting-started demo for SensorMonitor states:

'4.1 Collection interval and sensor selection
Collection interval is limited by a constraint of communication channel on the XDK: data for each
sensor (temperature, humidity and pressure are considered as one sensor) could not be
collected in an interval smaller than 100ms. Per example, the minimum interval for 2 sensors is
200ms.'

Hence, a maximum of 10 Hz.

2. And marcstaller on the demo FAQ said:

' Change the data collection interval 

Solution/hint:
  • The collection interval is limited to 100ms. However if you want to change the data collection interval, you can go to Settings -> collection interval and set it to a number between 300 and 60 000. This will set the collection interval to a different value, initially 1000 is preset here.'

Thanks for the reply, I hope that a relatively quick solution or starting point can be found.

-Christine
 
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Maximum Sampling Frequency for BTLE or WiFi
응답
15. 11. 12 오후 7:54 as a reply to Christine Martindale.
Christine,

as I said, these demos aren't necessarily optimized for performance and they have their flaws...
One of them being the limitations you mentioned. 
We have several posts here in the community that point out how and why the limitations on the SensorMontior Demo are there as well as more info on the limitation that we have on the DataLogger demo. 

To let me answer your question more precise, maybe even with a PoC, do you know which sensors you need/can you limit them to a few or do you need all of them?

Thanks,
Florian
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RE: Maximum Sampling Frequency for BTLE or WiFi
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16. 9. 7 오후 6:26 as a reply to Florian Harr.

Hello,

I have the same question / concern as Christine. There are various statements throughout the documentation and demos that suggest only a limited sampling + data TX frequency is possible. This thread: "http://xdk.bosch-connectivity.com/community/-/message_boards/message/133124" establishes 2 things:

  1. The databus is limited to 1kHz. 
  2. There is a some frequncy X between 1 and 1000 that is the maximum frequency at which a single sensor can be sampled and the data written out. 

In the documentation it states under known issues that "WLAN/BLE transfers at high frequences is not supported". This statement seems to be evidenced by #1, but does not specify what is considered 'high'. 

Touching upon #2, the Sensor Monitor demo uses 100ms (ie, X=10) collection rate as the fastest sample when only 1 sensor is being triggered. For most applications, 10Hz is not adequate enough though. Like Christine, I also need to trigger a sensor and then send the data via BLE or WiFi, possibly at 100-200Hz. Is this able to be accomplished using the XDK? Would it require a more sophisticated approach of writing locally to the SD card and then in separate thread writing to BLE or Wifi? I understand in the demo the actual sensor 'bandwidth' rate can be set anywhere from 7 to 1000Hz, but if it is bottlenecked by a much slower data outrate, then it seems by all intensive purposes the effective bandwidth is greatly reduced. 

 

Thank you.

 

 

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RE: Maximum Sampling Frequency for BTLE or WiFi
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16. 9. 8 오후 1:28 as a reply to Josh Doerr.

Hello Josh,

welcome to the XDK community.
The sampling rate is indeed limited to the data transfer frequency of the SPI data bus that connects the sensors with the MCU. The transfer frequencies of WLAN and BLE are much faster than the 1KHz restriction of the SPI bus. But in all cases the transfer frequency of the SPI bus is the limiting factor to get sensor values for the required time interval. The bus frequency is also divided by the number of enabled sensors. This makes it more complicated if you enable more than one sensor to stream data with a certain transfer frequency.
Based on this it makes no sense to temporarily store sensor data on the SD card unless you get more than one measurement every 5 milli seconds.

The SensorMonitorDemo is not meant to display the XDK’s limits and boundaries. As Florian already mentioned, it rather is a playground to explore implementation possibilities with the XDK.

In general it is possible to send data with 100-200Hz, but this depends on whether you are using Wi-Fi or BLE. In terms of Wi-Fi, you have to be aware which transfer protocol  (UDP,HTTP, CoAP, etc.) you are using. This can slow down your transfer frequency, depending on the used protocol header. BLE is much slower than Wi-Fi. But given its short overhead it should provide transfer frequencies between 100 to 200 Hz. Only the data size is significant if you use 100 or 200Hz.

In summary, both Wi-Fi and BLE should work with 100-200Hz, but it depends on which requirements your application has. This includes the amount of used sensors and, in case of Wi-Fi, the used transfer protocol and the general data size of the measured values for BLE.

I hope this brought some insight.

Kind regards,
Franjo

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