User's Manual

58
Software
AA440. In addition, full source routing of frames for other destinations is implement-
ed.
Figure 24. State Transition Diagram of the radio communication task.
4.4.1. Digital Squelch
In order to reduce the power consumption during receive mode, the unit uses a pro-
tocol originally developed by Adcon Telemetry for the A730 family, but with some
additional refinements to further reduce the power consumption. The receiver is
pulsed at a 0.5 seconds interval. At wake-up, the receiver samples first the RF chan-
nel for a carrier, by measuring the RSSI. The RSSI output is extremely stable due to
the wide dynamic range that the IF chip exhibits (over 100 dB, temperature compen-
sated). If the sampled RSSI is under a preset threshold, the unit will immediately go
back to sleep. This procedure takes under 20 mS, typically (from wake-up to result).
If, however, an RF level superior to the preset threshold is detected, the micropro-
cessor will try to detect a valid header, which is composed of a 2 kHz tone of at least
0.5 seconds long. The tone detection is performed by the microcontroller in soft-
ware, and takes at most 6 additional milliseconds. If no valid tone is detected, the
unit goes back to sleep, otherwise it tries to decode the frame.
Based on the destination ID, the frame will be identified. If it is not for that particular
unit (own ID), then the microcontroller will cease decoding it and will go immediately
Idle
Measure RF
Check
Destination
No Signal
0.5 sec
Interr
upt
Header Detected
Not for us
Get Frame
We are called
New Frame Possible
Wait
Nhops + 1 s
and
Hunt Syncs
Timeout Expired
Answer
Appropriately
Ack
Resend
Frame with
Long
Header
Frame Sent Twice
No Ack
Payload is for us
Must route
Update
Header
and Send
Ack
No Ack