URGENT********** ATD sampling frequency ************PLEASE SOMEONE REPLY - zigbee Mc1321x

[bitasobhani]

Hi all
I am working on zigbee Mc1321x.
I need to send ATD samples from a node to coordinator. I encountered these limitations in zigbee mesh networks:

1-    ATD conversion clock can vary only between 500Khz to 2Mhz.

2-   Rate of sending packets can not be exaclty determined. It depends on network traffic. This is because the next packet can not be send until sending confirm of the previous packet is received.

3-   Maximum payload length is 98 bytes (without fragmentation).

4- timers can not be less than 4 milliseconds (250Hz)

Since ATD conversion clock is too high, I can't send samples as they are produced (because of packet rate limitation).
Buffering of samples does no work as well, because of payload length limitation. (for example if ATD frequency is 500Khz, buffering requeires a packet rate of 98*2=196 microseconds. This packet rate is high for even a very small network. Also fragmentation can not do anything.
If I use a timer for ATD sampling, frequency can not be more than 250Hz.

Despite these limitations, how can I reach high sampling frequencies?

THANKS ALOT
   

Added p/n to subject.

Message Edited by NLFSJ on 2008-09-24 01:36 PM

[Andrey]

You do actually achieve high sampling frequency. What you need is to somehow manage what you've got and then send it over the air whenever you have a chance. Let's say you get 5 ATD samples for every available time slot in the air + buffering. It would be wise to average those values, or you could send the mean, the median, low, high values and the number of samples taken. Try to look from a standpoint of what you really need to accomplish.

[Andrey]

Also, you could send the values in hex format, and if you only have a specific range of values you can compress it even more by stripping parts of the byte that is always the same.

[Lundin]

You must also have in mind that it is impossible to demand real-time transmission when using any kind of 2.4GHz radio, as the band is intended for commercial applications and is wide open for others. Apart from other zigbee nodes there will also be Bluetooth, WLAN, DECT etc etc in the air. None of these techniques are suitable for "hard" real time, especially as the number of radio nodes on the band is increasing rapidly.

I would expect to get the data delivered with something like +-100ms accuracy as the worst case.