Technical Manual
Table Of Contents
- Wireless Sensor Interface A720 (addIT™)
- 1. About the A720
- 2. Hardware
- 4. Software
- 4.1. AMOS
- 4.2. Mode Check
- 4.3. A/D Task
- 4.4. The Terminal Task
- 4.5. The Radio Interface Task
- 4.5.1. Digital Squelch
- 4.5.2. Modulation Technique Used
- 4.5.3. Generic Format of a Radio Frame
- 4.5.4. Data Frames
- 4.5.5. Frame Types
- 4.5.5.1. Request
- 4.5.5.2. Broadcast Answer
- 4.5.5.3. Set I/O Request
- 4.5.5.4. Read I/O Answer
- 4.5.5.5. Broadcast Request
- 4.5.5.6. Ping
- 4.5.5.7. Pong
- 4.5.5.8. Memory Dump Request
- 4.5.5.9. Memory Dump Answer
- 4.5.5.10. Data
- 4.5.5.11. Set ID
- 4.5.5.12. Set Slot Time and Sample Rate
- 4.5.5.13. Set Frequency
- 4.5.5.14. Set Battery Charge Levels
- 4.5.5.15. Set Pulse Counters Parameters
- 4.5.5.16. General Acknowledge
43
The Radio Interface Task
pled RSSI is under a preset threshold, the unit will immediately go back to sleep. This
procedure needs under 20 mS, typically (from wake-up to the result).
If however, a RF level superior to the preset threshold is detected, the microprocessor
will try to detect a valid header, which is composed of a 2 kHz tone of at least 0.5 sec-
onds long. The tone detection is performed by the microcontroller in software, 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 immedi-
ately to sleep. The destination ID is situated very early in the frame header (see also
“Generic Format of a Radio Frame” on page 44).
From the above it becomes clear that in order to initiate a communication, a requester
must send first a header which is at least 0.5 seconds long: these are called long header
frames. Of course, after the communication is established the headers are short, of
only 16 bytes (i.e. 8 msec. – called short header frames). If a timeout occurs, the system
will restart by sending long header frames.
4.5.2. Modulation Technique Used
The communication via radio is made by using a special MSK (Minimum Shift Key-
ing) scheme; both the encoding and the decoding of the MSK frames is made in soft-
ware – there is virtually no hardware modem. A zero byte is transmitted as a
sequence of 250 µS one level followed by a 250 µs zero level, while a one byte is trans-
mitted as a 500 µS one level followed by a 500 µS zero level. A complete sequence of
one and zero level forms a bit cell.
The modulation scheme is self-clocked. The transmission speed is content-depen-
dent, varying from 1 kbps (when sending only ones) to 2 kbps (when sending only
zeroes). On the average, an 1500 bps transmission speed is reached.
The data interchange between stations is made by means of frames. The frames have
a header, a 16 bit-sync character a data block and a 16-bit CRC number. The bytes
forming the frame are send synchronously, with no start and/or stop bits. The data
block is assembled after the sync character was detected.
00
0
1
1
1