Datasheet

Revision 1.5 Page 31 of 41
nRF905 Product Specification
12 nRF905 features
12.1 Carrier Detect
When the nRF905 is in ShockBurst™ RX, the Carrier Detect (CD) pin is set high if a RF carrier is present
at the channel the device is programmed to. This feature is very effective to avoid collision of packets from
different transmitters operating at the same frequency. Whenever a device is ready to transmit it could first
be set into receive mode and sense whether or not the wanted channel is available for outgoing data. This
forms a very simple listen before transmit protocol. Operating Carrier Detect (CD) with Reduced RX Power
mode is an extremely power efficient RF system. Typical Carrier Detect level (CD) is typically 5dB lower
than sensitivity, that is, if sensitivity is –100dBm then the Carrier Detect function senses a carrier wave as
low as –105dBm. Below –105dBm the Carrier Detect signal is low, that is, 0V. Above –95dBm the Carrier
Detect signal is high, that is, Vdd. Between approximately -95 to -105 the Carrier Detect Signal toggles.
12.2 Address Match
When the nRF905 is in ShockBurst™ RX mode, the Address Match (AM) pin is set high as soon as an
incoming packet with an address that is identical with the device’s own identity is received. With the
Address Match pin the controller is alerted that the nRF905 is receiving data actually before the Data
Ready (DR) signal is set high. If the Data Ready (DR) pin is not set high, that is, the CRC is incorrect then
the Address Match (AM) pin is reset to low at the end of the received data packet. This function can be
very useful for an MCU. If Address Match (AM) is high then the MCU can make a decision to wait and see
if Data Ready (DR) is set high indicating a valid data packet has been received or ignore that a possible
packet is being received and switch modes.
12.3 Data Ready
The Data Ready (DR) signal makes it possible to largely reduce the complexity of the MCU software pro-
gram.
In ShockBurst™ TX, the Data Ready (DR) signal is set high when a complete packet is transmitted, telling
the MCU that the nRF905 is ready for new actions. It is reset to low at the start of a new packet transmis-
sion or when switched to a different mode, that is, receive mode or standby mode.
In ShockBurst™ TX Auto Retransmit the Data Ready (DR) signal is set high at the beginning of the pre-
amble and is set low at the end of the preamble. The Data Ready (DR) signal therefore pulses at the
beginning of each transmitted data packet.
In ShockBurst™ RX, the signal is set high when nRF905 has received a valid packet, that is, a valid
address, packet length and correct CRC. The MCU can then retrieve the payload through the SPI. The
Data Ready (DR) pin is reset to low once the data has been clocked out of the data buffer or the device is
switched to transmit mode.
12.4 Auto Retransmit
One way to increase system reliability in a noisy environment or in a system without collision control is to
transmit a packet several times. This is easily accomplished with the Auto Retransmit feature in nRF905.
By setting the AUTO_RETRAN bit to “1” in the configuration register, the circuit keeps sending the same
data packet as long as TRX_CE and TX_EN is high. As soon as TRX_CE is set low the device finishes
sending the packet it is currently transmitting and then returns to standby mode.