Rainbow Electronics ATmega128RFA1 User Manual
Page 136
136
8266A-MCU Wireless-12/09
ATmega128RFA1
Figure 10-1. SFD and Beacon Timestamp Generation
Note that Figure 10-1 contains no exact timing information; it is for visualization only.
The beacon timestamp register is updated with the SFD timestamp value at the end of
the frame (RX_END IRQ), if the received frame was a beacon frame with valid FCS and
expected source PAN identifier or { PAN_ID_1, PAN_ID_0} = 0xFFFF.
The register value is valid until a new beacon frame has been received or the beacon
timestamp is updated manually. A manual beacon timestamp can be generated by
writing “1” to SCMBTS of the SCCR0 register.
10.10.2 Relative Compare Mode for Superframe Access Timing
The IEEE 802.15.4 describes a superframe structure which contains different time slots
where a device can access the channel.
The Symbol Counter together with the three compare units provide support for waking
up the device at the right time to receive the beacon for superframe synchronization
and at certain times within the superframe.
A typical superframe timing scenario using the symbol counter relative compare mode
is shown in
page 137. The Symbol Counter values in the figure do not
reflect realistic time intervals but demonstrate the principle of operation.