CC2520 DATASHEET
2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER
SWRS068 – DECEMBER 2007
24 Random Number Generation
CC2520 can output random bits in two different ways. Common for these are that the chip should be in RX
when generation of random bits are required. One must also make sure that the chip has been in RX long
enough for the transients to have died out. A convenient way to do this is to wait for the RSSI valid signal to
go high.
•
Single random bits from either the I or Q channel (configurable) can be output on GPIO pins at a rate of
8MHz. One can also select to xor the I and Q bits before they are output on a GPIO pin. These bits are
taken from the least significant bit in the I and/or Q channel after the decimation filter in the demodulator.
•
CC2520 supports an instruction called RANDOM that allows the user to read randomly generated bytes
over the SPI. These bytes are generated from the least significant bit of the I channel output from the
channel filter in the demodulator.
ADC I
ADC Q
Decimator
I
Decimator
Q
LSB
LSB
GPIO
Channel
filter I
Channel
filter Q
LSB
SPI
Figure 31: Random bit generation in the demodulator
A simple test of the RANDOM instruction shows satisfactory performance for most practical uses. About 20
million bytes were read using the RANDOM instruction. When interpreted as unsigned integers between 0
and 255, the mean value was 127.6518, which indicates that there is a DC component.
The FFT of the 2
14
first bytes is shown in Figure 33. Note that the DC component is clearly visible. A
histogram (32 bins) of the 20 million values is shown in Figure 34.
0
-10
-20
6.5
6.45
6.4
6.35
x 10
5
-30
dB
-40
-50
-60
6.3
6.25
6.2
6.15
6.1
-70
-80
6.05
-3
-2
-1
0
Frequency [rad]
1
2
3
6
0
50
100
150
200
250
Figure 32: FFT of the random bytes
Figure 33: Histogram of 20 million bytes generated
with the RANDOM instruction
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.
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