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Rf12, Data filtering and clock recovery, Data validity blocks – Rainbow Electronics RF12 User Manual

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RF12

Data Filtering and Clock Recovery

Output data filtering can be completed by an external capacitor or by using digital filtering according

to the final application.

Analog operation: The filter is an RC type low-pass filter followed by a Schmitt-trigger (St). The

resistor (10 kOhm) and the St are integrated on the chip. An (external) capacitor can be chosen

according to the actual bit rate. In this mode, the receiver can handle up to 256 kbps data rate. The FIFO

can not be used in this mode and clock is not provided for the demodulated data.

Digital operation: A digital filter is used with a clock frequency at 29 times the bit rate. In this mode

there is a clock recovery circuit (CR), which can provide synchronized clock to the data. Using this clock

the received data can fill a FIFO. The CR has three operation modes: fast, slow, and automatic. In slow

mode, its noise immunity is very high, but it has slower settling time and requires more accurate data

timing than in fast mode. In automatic mode the CR automatically changes between fast and slow mode.

The CR starts in fast mode, then after locking it automatically switches to slow mode (Only the digital data

filter and the clock recovery use the bit rate clock. For analog operation, there is no need for setting the

correct bit rate.)

Data Validity Blocks

RSSI

A digital RSSI output is provided to monitor the input signal level. It goes high if the received signal

strength exceeds a given preprogrammed level. An analog RSSI signal is also available. The RSSI

settling time depends on the external filter capacitor. Pin 19 is used as analog RSSI output. The digital

RSSI can be monitored by reading the status register.

P1

-65 dBm

1300 mV

P2

-65 dBm

1000 mV

P3

-100 dBm

600 mV

P4

-100 dBm

300 mV

DQD

The Data Quality Detector is based on counting the spikes on the unfiltered received data. For

correct operation, the “DQD threshold” parameter must be filled in by using the Data Filter Command.

AFC

By using an integrated Automatic Frequency Control (AFC) feature, the receiver can minimize the

TX/RX offset in discrete steps, allowing the use of:

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