© NXP Laboratories UK 2012
JN-DS-JN5142 1v0
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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
Tx
Bitstream
Rx
Bitstream
Protocol Timing Engine
Supervisor
Radio
Status
Control
Processor
Bus
Protocol
Timers
Security Coprocessor
Decrypt
Port
Encrypt
Port
AES
Codec
Figure 20: 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 without processor intervention including retries and random
backoffs.
When the transmission begins, the header of the frame is constructed from the parameters programmed by the
software and sent with the frame data through the serialiser to the Modem. At the same time, the radio is prepared
for transmission. During the passage of the bitstream to the modem, it passes through a CRC checksum generator
that calculates the checksum on-the-fly, and appends it to the end of the frame.
8.3.2 Reception
During reception, the radio is set to receive on a particular channel. On receipt of data from the modem, the frame is
directed into the Tx/Rx Frame Buffer where both header and frame data can be read by the protocol software. An
interrupt may be provided on receipt of the frame header. As the frame data is being received from the modem it is
passed through a checksum generator; at the end of the reception the checksum result is compared with the
checksum at the end of the message to ensure that the data has been received correctly. An interrupt may be
provided to indicate successful packet reception. During reception, the modem determines the Link Quality, which is
made available at the end of the reception as part of the requirements of IEEE802.15.4.
