This is a measurement of the SPI traffic, courtesy of my new Rigol 1052E scope (and no, I have not hacked it yet, but I fully intend to). Yellow is MISO data and blue is SCK. This shows that the I2C clock is a pretty manageable 920 kHz, which just happens to be half the CPU frequency. Second, measurements on a wider timescale shows very little traffic. This is to be expected: the outdoor unit is essentially solar powered and you want to conserve that power as much as possible.
|The SPI Equivalent of "First Post"|