CC2520 datasheet(89/133 Pages) TI

89 / 133 page

CC2520 DATASHEET

2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER

SWRS068 – DECEMBER 2007

WWW.TI.COM

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

Channel

filter I

Channel

filter Q

GPIO

SPI

LSB

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.

-3

-2

-1

-80

-70

-60

-50

-40

-30

-20

-10

Frequency [rad]

Figure 32: FFT of the random bytes

100

150

200

250

6.05

6.1

6.15

6.2

6.25

6.3

6.35

6.4

6.45

6.5

x 10

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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