Texas Instruments 3138 155 232931 Linear if and AGC Settings, Rssi / Energy Detection, Timing

CC2420

21.8 Timing

Table 8 shows some examples of the time used by the security module for different operations.

Mode	l(a)	l(m)	l(MIC)	Time
				[us]
CCM	50	69	8	222
CTR	-	15	-	99
CBC	17	98	12	99
Stand-	-	16	-	14
alone

Table 8. Security timing examples

22 Linear IF and AGC Settings

CC2420 is based on a linear IF chain where the signal amplification is done in an analog VGA (variable gain amplifier). The gain of the VGA is digitally controlled.

The AGC (Automatic Gain Control) loop ensures that the ADC operates inside its

23 RSSI / Energy Detection

CC2420 has a built-in RSSI (Received Signal Strength Indicator) providing a digital value that can be read from the 8

bit, signed 2’s complement RSSI.RSSI_VAL register.

The RSSI value is always averaged over 8 symbol periods (128 µs), in accordance with [1]. The RSSI_VALID status bit (Table 5) indicates when the RSSI value is valid, meaning that the receiver has been enabled for at least 8 symbol periods.

The RSSI register value RSSI.RSSI_VAL can be referred to the power P at the RF pins by using the following equations:

P = RSSI_VAL + RSSI_OFFSET [dBm]

where the RSSI_OFFSET is found empirically during system development from the front end gain. RSSI_OFFSET is approximately –45. E.g. if reading a value

dynamic range by using an analog/digital feedback loop.

The AGC characteristics are set through the AGCCTRL, AGCTST0, AGCTST1 and AGCTST2 registers. The reset values should be used for all AGC control and test registers.

of –20 from the RSSI register, the RF input power is approximately –65 dBm.

A typical plot of the RSSI_VAL reading as function of input power is shown in Figure

27.It can be seen from the figure that the RSSI reading from CC2420 is very linear and has a dynamic range of about 100 dB.

The RSSI register value RSSI.RSSI_VAL is calculated and continuously updated for each symbol after RSSI has become valid.

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