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[aprssig] Aprx v2 beta, now with Viscous Digipeater

Scott Miller scott at opentrac.org
Thu Oct 22 16:07:15 UTC 2009


> Having said that, a typical 8 bit microcontroller probably doesn't
> have enough RAM to implement this.

The 512 bytes in the OT1+ would be a little tight.  With 2k of RAM in 
the MC9S08AW32 MCU I'm using for the OT1+ RT and a few other projects it 
would certainly be doable.

The T2 uses a list of 16-bit hashes for tracking dupes, with an 8-bit 
time-to-live for each, so that's 3 bytes per entry.  Currently the list 
is 15 entries long, so 45 bytes of storage total.

To implement this on the T2 shouldn't require more than one extra bit of 
storage per entry.  I don't see why it's necessary to count the actual 
number of times it's heard - either you get it once (which automatically 
results in an entry in the dupe list) or you get it more than once, and 
set the excess flag.  I can reduce the TTL timer to 7 bits and not 
increase the storage requirements, though it'll mean a bit more code in 
flash to process the bitfield.

The T2 can already defer digi processing of a packet after the APRS 
parsing, KISS output, MONITOR display, and everything else has been done 
(and in fact any one of those can be deferred) - as long as this is an 
on/off option and doesn't require more analysis of the path to decide 
whether to act on it, it shouldn't take more than a few lines of code to 
implement.

I'm tempted to go try it out now, but at the moment I've got to go fire 
up the forklift and make some space in the warehouse for weather 
stations that are due in today.

Scott
N1VG



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