RFM75 datasheet(9/27 Pages) ETC2

Part #	RFM75

Description	Low Power High Performance 2.4 GHz GFSK Transceiver

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Page 9 of 27

RFM75 V1.0

E‐mail:sales@hoperf.com website://www.hoperf.com

5.1.1

Preamble

The preamble is a bit sequence used to detect 0

and 1 levels in the receiver. The preamble is

one byte long and is either 01010101 or

10101010. If the first bit in the address is 1 the

preamble is automatically set to 10101010 and

if

the

first

bit

is

0

the

preamble

is

automatically set to 01010101. This is done to

ensure there are enough transitions in the

preamble to stabilize the receiver.

5.1.2

Address

This is the address for the receiver. An address

ensures that the packet is detected by the target

receiver. The address field can be configured

to be 3, 4, or 5 bytes long by the AW register.

The PRX device can open up to six data pipes

to support up to six PTX devices with unique

addresses. All six PTX device addresses are

searched simultaneously. In PRX side, the data

pipes

are

enabled

with

the

bits

in

the

EN_RXADDR register. By default only data

pipe 0 and 1 are enabled.

Each data pipe address is configured in the

RX_ADDR_PX registers.

Each pipe can have up to 5 bytes configurable

address. Data pipe 0 has a unique 5 byte

address. Data pipes 1-5 share the 4 most

significant address bytes. The LSB byte must

be unique for all 6 pipes.

To ensure that the ACK packet from the PRX

is transmitted to the correct PTX, the PRX

takes the data pipe address where it received

the packet and uses it as the TX address when

transmitting the ACK packet.

On the PRX, the RX_ADDR_Pn, defined as

the pipe address, must be unique. On the PTX

the TX_ADDR must be the same as the

RX_ADDR_P0 on the PTX, and as the pipe

address for the designated pipe on the PRX.

No other data pipe can receive data until a

complete packet is received by a data pipe that

has detected its address. When multiple PTX

devices are transmitting to a PRX, the ARD

can be used to skew the auto retransmission so

that they only block each other once.

5.1.3

Packet Control

When Dynamic Payload Length function is

enabled, the packet control field contains a 6

bit payload length field, a 2 bit PID (Packet

Identity) field and, a 1 bit NO_ACK flag.

Payload length

The payload length field is only used if the

Dynamic Payload Length function is enabled.

PID

The 2 bit PID field is used to detect whether

the received packet is new or retransmitted.

PID prevents the PRX device from presenting

the same payload more than once to the MCU.

The PID field is incremented at the TX side

for each new packet received through the SPI.

The PID and CRC fields are used by the PRX

device to determine whether a packet is old or

new. When several data packets are lost on the

link, the PID fields may become equal to the

last received PID. If a packet has the same PID

as the previous packet, RFM75 compares the

CRC sums from both packets. If the CRC

sums are also equal, the last received packet is

considered a copy of the previously received

packet and discarded.

NO_ACK

The NO_ACK flag is only used when the auto

acknowledgement feature is used. Setting the

flag high, tells the receiver that the packet is

not to be auto acknowledged.

The PTX can set the NO_ACK flag bit in the

Packet Control Field with the command:

W_TX_PAYLOAD_NOACK.

However,

the

function

must

first

be

enabled

in

the

FEATURE

register

by

setting

the

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