Packet Sniffing And Radio Test Output Signals; Histogram Of 20 Million Bytes Generated With The Random Instruction - Texas Instruments CC253x User Manual

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f – Frequency – rad
Figure 23-21. FFT of the Random Bytes
For the first 20 million individual bits, the probability of a one is P(1) = 0.500602 and P(0) = 1 – P(1) =
0.499398.
Note that to fully qualify the random generator as true random, much more elaborate tests are required.
There are software packages available on the internet that may be useful in this respect.

23.13 Packet Sniffing and Radio Test Output Signals

Packet sniffing is a nonintrusive way of observing data that is either being transmitted or received. The
packet sniffer outputs a clock and a data signal, which should be sampled on the rising edges of the clock.
The two packet sniffer signals are observable as GPIO outputs. For accurate time stamping, the SFD
signal should also be output.
Because the radio has a data rate of 250 kbps, the packet sniffer clock frequency is 250 kHz. The data is
output serially, with the MSB of each byte first, which is opposite of the actual RF transmission, but more
convenient when processing the data. It is possible to use a SPI slave to receive the data stream.
When sniffing frames in TX mode, the data that is read from the TXFIFO by the modulator is the same
data that is output by the packet sniffer. However, if automatic CRC generation is enabled, the packet
sniffer does NOT output these 2 bytes. Instead, it replaces the CRC bytes with 0x8080. This value can
never occur as the last two bytes of a received frame (when automatic CRC checking is enabled), and
thus it provides a way for the receiver of the sniffed data to separate frames that were transmitted and
frames that were received.
When sniffing frames in RX mode, the data that is written to the RXFIFO by the demodulator is the same
data that is output by the packet sniffer. In other words, the last two bytes are either the received CRC
value or the CRC OK, RSSI, correlation, or SRCRESINDEX value that may automatically replace the CRC
value, depending on configuration settings.
To set up the packet sniffer signals or some of the other RF Core observation outputs (in total maximum
3; rfc_obs_sig0, rfc_obs_sig1, and rfc_obs_sig2), the user must perform the following steps:
Step1: Determine which signal (rfc_obs_sig) to output on which GPIO pin (P1[0:5]). This is done using
the OBSSELx control registers (OBSSEL0–OBSSEL5) that control the observation output to pins P1[0:5]
(overriding the standard GPIO behavior for those pins).
Step2: Set the RFC_OBS_CTRL control registers (RFC_OBS_CTRL0–RFC_OBS_CTRL2) to select the
correct signals (rfc_obs_sig); for example, for packet sniffing one needs the rfc_sniff_data for the
packet sniffer data signal and rfc_sniff_clk for the corresponding clock signal.
SWRU191F – April 2009 – Revised April 2014
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G001
Copyright © 2009–2014, Texas Instruments Incorporated
Packet Sniffing and Radio Test Output Signals
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Figure 23-22. Histogram of 20 Million Bytes Generated
With the RANDOM Instruction
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Bin Number
G002
CC253x Radio
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