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Mark_T
Posts: **1,627**

From what I understand Gold codes are sets of equal length pseudo-random bit patterns that have low cross-correlation between any pair of distinct codes from the set. Or more importantly a guaranteed bound on the maximum cross-correlation between any two codes.

This makes them useful for communications as different codes can share a channel. For instance the GPS system uses Gold codes for the navigation data-stream,

each satellite using a different code. And these are documented.

There are two LSFRs of ten bits each (thus each has a 1023 cycle length). The two are combined (XORed) with different relative phases to generate different

codes. The phase offset is done by picking 1 fixed tap from the first LFSR and 2 variable taps from the 2nd - changing the variable tap points has the

effect of shifting the pattern.

In PASM I've brought this down to 5 instructions, due to the handy parity function of the carry flag in the TEST instruction and MUXC. Both LFSRs are

in the same register so the shift is shared and a single TEST instruction can combine the 3 output taps in one instruction:

I found information on these sites valuable: https://archive.org/details/ADiyReceiverForGpsAndGlonassSatellites, https://natronics.github.io/blag/2014/gps-prn/

I continue to marvel at how PASM can capture quite complex pieces of hardware in a few carefully chosed instructions, I thought this was a good example.

This makes them useful for communications as different codes can share a channel. For instance the GPS system uses Gold codes for the navigation data-stream,

each satellite using a different code. And these are documented.

There are two LSFRs of ten bits each (thus each has a 1023 cycle length). The two are combined (XORed) with different relative phases to generate different

codes. The phase offset is done by picking 1 fixed tap from the first LFSR and 2 variable taps from the 2nd - changing the variable tap points has the

effect of shifting the pattern.

In PASM I've brought this down to 5 instructions, due to the handy parity function of the carry flag in the TEST instruction and MUXC. Both LFSRs are

in the same register so the shift is shared and a single TEST instruction can combine the 3 output taps in one instruction:

step_lfsrs test val, H03A6_0000 wc ' feedback taps for G2 muxc val, H0000_8000 ' inject ready for the left shift test val, H0000_0204 wc ' feedback taps for G1 rcl val, #1 ' inject at LSB as well as shift both LFSRs test val, selector wc ' XOR the three output taps to put output bit in C flag step_lfsrs_ret ret H0000_0204 long $0000_0204 ' G1 poly H0000_8000 long $0000_8000 ' inject point for top 16 bits (RCL used for G1) H03A6_0000 long $03A6_0000 ' G2 poly in top 16 bits selector long $0022_0200 ' combine taps for G2 with G1's output

I found information on these sites valuable: https://archive.org/details/ADiyReceiverForGpsAndGlonassSatellites, https://natronics.github.io/blag/2014/gps-prn/

I continue to marvel at how PASM can capture quite complex pieces of hardware in a few carefully chosed instructions, I thought this was a good example.

## Comments

5 Commentssorted by Date Added Votes1,6270Vote UpVote DownI've been experimenting generating spread-spectrum signals (see other post of mine) using this to setup my

chip-tables. Even with 5 ASM instructions its not fast enough to feed bits out at the 5MHz I want so I store

the 1023 bits in 32 longs in the cog at start up, passing in the selector value. Pushing out 32 chips at a time

with WAITVID gives lots of time to modulate the signal, but generating the bits on the fly doesn't seem possible at

more than about 2.5MHz

8840Vote UpVote DownThe GPS L1 C/A gold codes are pretty clever since they can be implemented with a trivial amount of hardware (think 1970's technology) and have pretty good properties. Talking about having to store the codes to get speed - for fun check out the Galileo "memory" codes which were developed with genetic algorithms. There are no equations, just predetermined codes that you have to store.

21,3500Vote UpVote Downhttp://forums.parallax.com/discussion/146879/one-sensor-to-discriminate-walsh-modulated-ireds

It was made easy, since the transmitting and receiving were done in the same micro, so I didn't have to worry about phase locking.

-Phil

Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away.-Antoine de Saint-Exupery1,6270Vote UpVote Downhow I'm going to transmit (baseband with LED perhaps, having seen your Walsh function demo)

and how to mix/demodulate on receive side

8840Vote UpVote Downe.g. see page 36 in the book preview here for how this works with GPS:

https://www.amazon.com/GPS-Assisted-GPS-GNSS-SBAS/dp/1596933747

Sorry I couldn't find a better reference right now.