Micrel, Inc.
MICRF302
March 2010
7
M9999-032610-A
MICRF302
ENCODER
DOUT
DO
D0
D1
D2
D3
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
or FSK RF 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:
1. Data is encoded using RF receiver-friendly
Manchester encoding
2. An industry-standard CRC (Cyclic Redundancy
Check) ensures that data is good before being
accepted by the decoder
3. 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:
1. Preamble (32 bits, all zero) is for receiver and
decoder wakeup and synchronization
2. Dead Time (3 bit-times) allows the receiver’s AGC
to increase its sensitivity
3. Sync (four bits, 1111) identifies the end of the
preamble and the start of the payload
4. 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.
5. Data (8 bits) carries the “real” information within the
packet.
6. 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:
1. Its 50% duty cycle is very friendly to RF receivers.
2. 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