RF215 transmission time on MR-O-QPSK is much slower then expected
I had a project involving RF215v1 module with MR-O-QPSK, 100kchiprate, rate mode 3 in EU band. According to 15g, the bit rate is up to 50kb/s and my frame is about 35 bytes. Therefore, the time on the fly is about 5.6ms for PSDU and little extra for SHR, PHR, and other delays.
But when I use CNTC to count TX time with reset on TX command and read at TX end, the result is about 27ms. I confirmed the measurement with oscilloscope and toggled pins.
After that, I bought SAM4S Xpro with RF215 extend board just to check my register settings with performance analyzer. In my view point the setting checks out.
Eventually, I put chat application (AT13282) to the board, disabling auto ACK, change rate mode to 3 and measure the TX time the same way. I got the same result as before... for 35 byte of data, it takes about 27ms...
So, I tried 2.4G with application's default setting, the TX time is around 25ms.
Is it actually the correct TX time, if I misunderstanding the standard or datasheet? Or could it be something with the chip?