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| Signal Without Noise | Signal With Noise |
|---|---|
| Positive differential i2c line to ground: | |
V_min: 840 mV V_max: 1.72 V |
V_min: 720 mV V_max: 1.80 V Approximately 90 mVpp of noise has been introduced to this signal. |
| Inverted differential i2c to ground: | |
V_min: 1.32 V V_max: 2.16 V |
V_min: 1.24 V V_max: 2.28 V Approximately 100 mVpp of noise has been introduced to this signal. |
Positive diff i2c to inverted diff i2c: | |
V_min: -360 mV V_max: 1.28 V |
V_min: -400 mV V_max: 1.40 V Approximately 80 mVpp of noise is observed in this signal. |
| i2c data line to ground (on solar master board i.e. destination): | |
V_min: -120 mV V_max: 3.16 V |
V_min: -80 mV V_max: 3.12 V Noise is negligible. |
We can see from the scope readings of the differential i2c lines that there is some substantial noise introduced by the walkie talkies, which is not observed at the destination. One concern is that the noise is present when measuring between the two differential i2c lines. This is likely explained by the probing wires used picking up additional noise. When probing the i2c data line using test points on the solar master pcb, the probing wires were extremely short, and no substantial noise was observed. If this is the case (that the probing wires are picking up significant noise), then these measurements have even more noise than should be expected in regular operation. We should consider devising a better setup to minimize the noise introduced by the probing wires, and repeating these tests.
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In conclusion, differential i2c is quite effective in attenuating noise (at least, on a bench test with one slave board, using reasonably long /TODO: insert measurement/ wires). In theory, differential i2c's noise rejection should similar to that of CAN (both serialized data transmitted on differential lines). However, this test should be repeated 1. in the car under expected operating conditions, and 2. with a setup devised to minimize noise introduced by the probing wires.