MRF49XA-ISLASHT datasheet(57/102 Pages) MICROCHIP

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Preliminary

DS70590C-page 57

MRF49XA

FIGURE 3-9:

FSK MODULATED DEVIATION – MAXIMUM TX TO RX OFFSET

The baseband filtering type can also be selected

between an analog filter and a digital filter.

3.10.1

ANALOG FILTERING MODE

For analog filtering, a simple RC low-pass filter is used,

along with a Schmitt Trigger circuit. The demodulator

output is fed to the RCLKOUT/FCAP/FINT pin over a

10 k

Ω resistor. The filter cut-off frequency is set by the

external capacitor connected to this pin and VSS. A

10 k

Ω resistor and the Schmitt Trigger are integrated

on the chip. An external capacitor for the RC filter has

to be chosen in accordance with the required bit rate.

The receiver can handle up to 256 kbps of data rate in

analog

operation.

The

receive

data

rate

programmable from 337 bps to 256 kbps. An internal

prescaler can be used to give better resolution when

setting up the receive data rate. The prescaler is

optional and can be disabled through DRSREG. The

analog filtering does not use the FIFO and the clock.

The clock is not provided for the demodulated data,

and hence, there is no need for setting the correct bit

rate.

3.10.2

DIGITAL FILTERING MODE

A digital filter is used with a clock frequency at 29 times

the data rate. For digital filtering, the synchronized

clock to the data is provided by the CLKRC. By using

this clock, the received data can fill the FIFO. If the

FIFO is not used, the recovered clock can be accessed

through RCLKOUT/FCAP/ FINT pin.

The CLKRC operates in three modes: Automatic mode,

Slow mode and Fast mode. All three modes are

configurable through BBFCREG. Each mode is

dependent on the type of signals it uses to determine

the valid data and also the number of incoming

preamble bits present at the beginning of the packet. In

Automatic mode, the CR CLKRC automatically

switches between the Fast and Slow mode. The noise

immunity of the CLKRC is very high in Slow mode;

however, it has slower settling time and requires more

accurate data timing than in Fast mode.

The registers associated with baseband filtering are:

•STSREG (see Register 2-1)

• RXCREG (see Register 2-7)

• BBFCREG (see Register 2-8)

• PMCREG (see Register 2-13)

10 kHz

BBBW

Programmable

RX Center

Freq.

Baseband Filter Characteristic

2 x deviation

Amplitude

10 kHz + Data Rate

Data Rate

TX-RX Offset

TX Center

Freq.

Frequency

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