As it turns out this can be done without too much fuss by using a digital to analogue (DAC) "resistor ladder". The two options I looked at were the R-2R ladder and the simpler binary weighted ladder. For two levels the binary weighted DAC is the simplest, it just needs two resistors (one double the other one's resistance).

Here is the circuit diagram

So one input goes through the 1k resistor and the other through a 2k resistor and you measure the output. With an DAC with only two levels there is tolerance for some noise on the digital lines.

I'm working at ~3 volts, so the levels I'm going to see are

Voltage | D0 | D1 |
---|---|---|

0v | 0 | 0 |

1V | 0 | 1 |

2V | 1 | 0 |

3V | 1 | 1 |

You can clearly see the three levels on the scope when measuring Vout

I kept the clock separate on channel 2 so that I could trigger on it. Here is the clock + data

With only two channels going through the DAC you could reconstruct the two digital data channels by eye. That is not much fun though. So I modified my F# script from the previous post to split the data into two values and then do the logical analysis and print out two channels of data. This proved very useful when using a SPI device that returns data (e.g. SD card). Here is the full F# script also available as a gist.