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Hot!AT86RF215 transmit times exceptionally high in non-legacy modes

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benpicco
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2019/10/17 08:42:26 (permalink)
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AT86RF215 transmit times exceptionally high in non-legacy modes

Hi,
When using the at86rf215 transceiver I experience some strange behavior when transmitting frames in non legacy (everything non 802.15.4-2006 O-QPSK, 250 kbit/s) modes.
 
E.g. according to the data sheet, O-QPSK, 2000 kChip/s with rate mode 4 should yield 1000 kbit/s. For this, I would expect frame times to be ¼ of the legacy mode. However, what I see is quite the opposite:
 
rate mode 4:
frame length | frame time
-------------+-----------
   20 bytes  |  5.6 ms
   30 bytes  |  5.7 ms
   40 bytes  |  5.8 ms
   50 bytes  |  5.9 ms
   60 bytes  |  6.0 ms
   80 bytes  |  6.2 ms
  
legacy:
frame length | frame time
-------------+-----------
   20 bytes  |  0.6  ms
   30 bytes  |  0.94 ms
   40 bytes  |  1.32 ms
   50 bytes  |  1.8  ms
   60 bytes  |  2.0  ms
   80 bytes  |  2.6  ms

I measured those timings by attaching a 10 Ω shunt resistor to the current measure pins of the ATREB215-XPRO module. The voltage graph gives a good sense as to when the module is transmitting and receiving. I did this for two modules so I can see when one is sending and the other is receiving. 
 
When I send a packet with a constant payload size, I see this:
 
The forum won't let me inline the images - please see the attachments.
 
It appears as if there is a constant overhead (preamble?) of about 5ms for all the non-legacy modes.
This is odd as I can only set a preamble length for FSK, not O-QPSK.
 
What is going on here?
 
post edited by benpicco - 2019/10/17 08:48:01

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