MICRF302 datasheet(7/15 Pages) MICREL

7 / 15 page

Micrel, Inc.

MICRF302

October 2008

M9999-100908-A

MICRF302

ENCODER

DOUT

TRANSMITTER

RECEIVER

MCU

TXEN

Figure 4. Parallel Encoder / Decoder Configuration

PREAMBLE

DEAD TIME

SYNC

ADDRESS

DATA

CRC

Figure 5. Data Transmission Packet Format

Operation Overview

The Figure 4 shows the basic operation of a parallel

encoder/decoder configuration. When a button is pushed

(known as a push event), the MICRF302 Encoder sends

packets of data to the transmitter. Each packet contains

encoded data bits, suitable for transmission across an ASK

FSK

communications

channel.

The

receiver

demodulates the baseband information from the RF carrier,

which is then decoded by the MCU.

Data Transmission

In any communications link we must be sure that the

decoder puts out what the encoder puts in. Lost data is

acceptable when the encoder and decoder are out of range,

but incorrect data is completely unacceptable when the

encoder and decoder are within range. Micrel’s MICRF302

uses an error management hierarchy to prevent bad data

getting through the link:

Data

encoded

using

receiver-friendly

Manchester encoding

An industry-standard CRC (Cyclic Redundancy

Check) ensures that data is good before being

accepted by the decoder

Data is sent in packets. Each packet has a

preamble, sync field, and a payload. Packets are

sent in groups of four. So even though four identical

packets are transmitted, a single valid packet

received by the Decoder is sufficient to change the

Decoder’s outputs. Please see operating manual of

the MICRF302 for details.

Packet Format

Each data packet consists of a number of fields, shown in

Figure 5. A packet consists of six fields:

Preamble (32 bits, all zero) is for receiver and

decoder wakeup and synchronization

Dead Time (3 bit-times) allows the receiver’s AGC

to increase its sensitivity

Sync (four bits, 1111) identifies the end of the

preamble and the start of the payload

DEVADR—20 bits of Device Address—identifies

one unique Encoder that’s transmitting. decoders

compare the DEVADR field against their own value

and only accept the packet if a match is found. The

20-bit device address is programmed at the factory

to a unique value for each part.

Data (8 bits) carries the “real” information within the

packet.

CRC—Cyclic Redundancy Check—(8 bits) lets the

Decoder check for errors in the packet

Data Format

Manchester-coded data has two distinguishing features that

make it an excellent choice for low-cost RF data exchange:

Its 50% duty cycle is very friendly to RF receivers.

It always has a transition at the center of every bit

(Figure 6). This certainty of a transition simplifies the

decoder’s task of recovering the encoder’s clock

rate and then actually decoding the data stream.

Manchester-coded data is shown here:

Figure 6. Manchester Coded Data

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