User Manual

Jennic
© Jennic 2009 JN-DS-JN5148-001 1v2 29
Preliminary
8.2 Modem
The modem performs all the necessary modulation and spreading functions required for digital transmission and
reception of data at 250kbps in the 2450MHz radio frequency band in compliance with the IEEE802.15.4 standard. It
also provides a high data rate modes at 500 and 667kbps.
Features provided to support network channel selection algorithms include Energy Detection (ED), Link Quality
Indication (LQI) and fully programmable Clear Channel Assessment (CCA).
The Modem provides a digital Receive Signal Strength Indication (RSSI) that facilitates the implementation of the
IEEE 802.15.4 ED function.
The LQI is defined in the IEEE 802.15.4 standard as a characterisation of the strength and/or data quality of a
received packet. The Modem produces a signal quality metric based upon correlation magnitudes, which may be
used in conjunction with the ED value to formulate the LQI.
The CCA capability of the Modem supports all modes of operation defined in the IEEE 802.15.4 standard, namely
Energy above ED threshold, Carrier Sense and Carrier Sense and/or energy above ED threshold.
8.3 Baseband Processor
The baseband processor provides all time-critical functions of the IEEE802.15.4 MAC layer. Dedicated hardware
guarantees air interface timing is precise. The MAC layer hardware/software partitioning, enables software to
implement the sequencing of events required by the protocol and to schedule timed events with millisecond
resolution, and the hardware to implement specific events with microsecond timing resolution. The protocol software
layer performs the higher-layer aspects of the protocol, sending management and data messages between endpoint
and coordinator nodes, using the services provided by the baseband processor.
Append
Checksum
Verify
Checksum
CSMA CCA
Backoff
Control
Deserialiser
Serialiser
Tx/Rx
Frame
Buffer
Protocol
Timers
Tx
Bitstream
Rx
Bitstream
Protocol Timing Engine
Supervisor
Radio
Status
Control
AES
Codec
Inline
Security
Decrypt
Port
Encrypt
Port
AES
Codec
Processor
Bus
Figure 17: Baseband Processor
8.3.1 Transmit
A transmission is performed by software writing the data to be transferred into the Tx/Rx Frame Buffer, together with
parameters such as the destination address and the number of retries allowed, and programming one of the protocol
timers to indicate the time at which the frame is to be sent. This time will be determined by the software tracking the
higher-layer aspects of the protocol such as superframe timing and slot boundaries. Once the packet is prepared and
protocol timer set, the supervisor block controls the transmission. When the scheduled time arrives, the supervisor
controls the sequencing of the radio and modem to perform the type of transmission required. It can perform all the
algorithms required by IEEE802.15.4 such as CSMA/CA, GTS without processor intervention including retries and
random backoffs.