[TangerineSDR] Better news: I2C Extension at 30 m using Odroid N2 seems OK

David Witten wittend at wwrinc.com
Sat Feb 29 13:01:07 EST 2020


I got tired of fooling with my Xavier and went back to the Odroid N2.  The
Xavier is a bit complicated and I could not figure out in a reasonable
amount of time how the kernel module supporting I2C is configured.
Everything on the Xavier is about moving data fast.

But now it seems to work.  Maybe the kittens chewed on something in the
night.  Or maybe I altered the kernel driver setup.

On the N2 I now see the following output using a 30 m (100 foot) shielded
Cat 6a (allegedly) cable strung across the lower level of my house:

dave at n2:~/projects/SWx-C/i2c-c$ gcc simplei2c.c -o simplei2c
dave at n2:~/projects/SWx-C/i2c-c$ ./simplei2c
i2c_init OK!
RM3100 Detected Properly: REVID: 22.
Polling I2C device: 20.

Time: 1582997418307, Temp Near: 25.00, Temp Far: 20.62 C   X:   -84914,  Y:
   -1881,  Z:    -7275
Time: 1582997419869, Temp Near: 25.06, Temp Far: 20.69 C   X:   -85104,  Y:
   -1880,  Z:    -7277
Time: 1582997421431, Temp Near: 25.06, Temp Far: 20.69 C   X:   -84916,  Y:
   -1880,  Z:    -7277
Time: 1582997422992, Temp Near: 25.00, Temp Far: 20.69 C   X:   -85037,  Y:
   -1879,  Z:    -7276
Time: 1582997424554, Temp Near: 25.00, Temp Far: 20.69 C   X:   -84927,  Y:
   -1879,  Z:    -7277
Time: 1582997426115, Temp Near: 25.06, Temp Far: 20.62 C   X:   -85020,  Y:
   -1880,  Z:    -7278
Time: 1582997427677, Temp Near: 25.00, Temp Far: 20.62 C   X:   -84876,  Y:
   -1880,  Z:    -7277
Time: 1582997429238, Temp Near: 25.00, Temp Far: 20.62 C   X:   -85027,  Y:
   -1879,  Z:    -7277
Time: 1582997430799, Temp Near: 25.06, Temp Far: 20.69 C   X:   -85216,  Y:
   -1880,  Z:    -7277
Time: 1582997432361, Temp Near: 25.00, Temp Far: 20.69 C   X:   -84861,  Y:
   -1880,  Z:    -7278
Time: 1582997433922, Temp Near: 25.06, Temp Far: 20.62 C   X:   -84931,  Y:
   -1879,  Z:    -7276
Time: 1582997435484, Temp Near: 25.00, Temp Far: 20.62 C   X:   -85045,  Y:
   -1880,  Z:    -7277
^C
dave at n2:~/projects/SWx-C/i2c-c$

It is a good bit chillier at the unfinished end of my house.

The voltage at the far end seems to be about 4.93 V on my ultra-cheapie
multimeter.  The near end shows exactly 5v. It is powered from the N2 40
pin bus. The N2 itself is powered by the 12v 2A power supply purchased from
Ameridroid.

I believe that I may have changed the default I2C rate on this N2 by
editing the device-tree database (dtb).  I'm not clear whether these kernel
drivers support modifying the data rate dynamically from userspace code. I
will investigate that all further and report back.

Dave Witten, KD0EAG
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