Pages: [1]   Go Down
Author Topic: OOK transmission format decode problems  (Read 2484 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Newbie
*
Karma: 0
Posts: 20
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I am working with a medical sensor that transmits some data every five minutes.  I know some specs on the transmission based on FCC information and other published material.  (OOK, 8192 bits/sec, 300khz bandwidth, 76 bits in the packet, 5 ASCII Char Transmitter ID, 402.142mhz).  The problem is the format of the transmission.  I am
using an RFM22B module from HopeRF/Sparkfun, and it wants to know the format (preamble length, sync word, CRC, etc) so it can detect a legitimate transmission before it puts it in the buffer.  I have tried guessing, but there are too many variables to make that an option.  I have a radio (being a ham) that can RX on that freq, and tried using my soundcard as a spectrum analyzer to record the signal for manual decoding into binary.  I am pretty sure the sound card is not fast enough to get a sample to use.  Also, with the transmission being only 9ms long, it's hard to catch it completely.  I am going to try reading the analog output (GPIO0) with the Arduino board next, but I am running out of ideas on how to get the pulse stream so I can decode it and determine the transmission format.  Has anyone done something like this before and have some ideas on how to accomplish this?

Thanks!

Jay
Logged

Massachusetts, USA
Offline Offline
Tesla Member
***
Karma: 180
Posts: 8108
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Download this spreadsheet:

http://www.hoperf.com/upfile/RF22B%2023B%2031B%2042B%2043B%20Register%20Settings_RevB1-v5.xls

Use it to calculate the register settings for FIFO Mode (not PacketHeader+FIFO Mode).

Send those to the radio and see what data you get in.  I hope that by using FIFO mode it will pass you the data as it arrives and not look for a packet header.
Logged

Send Bitcoin tips to: 1L3CTDoTgrXNA5WyF77uWqt4gUdye9mezN
Send Litecoin tips to : LVtpaq6JgJAZwvnVq3ftVeHafWkcpmuR1e

Offline Offline
Newbie
*
Karma: 0
Posts: 20
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

If I read the document correctly, you still need the preamble and sync words to be correct for the FIFO mode to work.  I could be wrong though.....
Logged

Massachusetts, USA
Offline Offline
Tesla Member
***
Karma: 180
Posts: 8108
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

You can reduce the sync word to one byte and try them each until you get data.  :/
Logged

Send Bitcoin tips to: 1L3CTDoTgrXNA5WyF77uWqt4gUdye9mezN
Send Litecoin tips to : LVtpaq6JgJAZwvnVq3ftVeHafWkcpmuR1e

Offline Offline
Newbie
*
Karma: 0
Posts: 20
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Here's what I am trying currently:
Preamble Detection Threshold (Register 0x35) set to 0x7A (giving me the longest Threshold possible- 16 bits)
Preamble Length (Register 0x34) 0x01 (smallest) to 0xff (largest) I am starting at 1 and working my way up every 10 minutes
SyncWord 3 (Technically the "first" sync word) is the only one enabled. I have this set to 0xAA (Which should be the same as the preamble 10101010)

My hope is that I can confuse the receiver to take the preamble as the preamble and the sync, and then put the next 255 bits into the buffer. So far no luck. Problem is I have no clue what the sync word could be.  my fear is that it's all too small.  9ms transmission @ 8192 bits is a shade less then < 80 bits.

So the packet structure could look like this:

0xAA            0x??     0x30     0x43     0x54     0x41     0x33     0x??     0xYY
Preamble      Sync     |-------------Transmitter ID------------|    Data      CRC

Assuming the documentation is right, 76 bits = 9ms @ 8192 bits/sec.  72 bits with a nibble left over? Maybe the preamble is longer?
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 20
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I have switched to using a CC1101 RF module now.  I am shocked that I can't get anywhere with either of these.  I'm not even getting the board to see that there is a signal. 
Logged

Offline Offline
Full Member
***
Karma: 0
Posts: 199
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Did anyone else suddenly think of Terry Pratchett when reading the post title?
Logged

Pages: [1]   Go Up
Jump to: