Capturing multiple channels of digital data on a two channel digital oscilloscope

In my last post I showed how I parsed the CSV data to do some basic logic analysis. The problem is that I have a two channel scope and need to capture three channels for SPI (i.e. clock + MOSI + MISO).

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.

Logic level parsing Rigol oscilloscope CSV data with F#

I was recently debugging a SPI communication but had no logic analyser. Fortunately my oscilloscope (rigol DS1052e) can export a capture as CSV (I assume that most digital oscilloscopes can export to CSV too).
Parsing the CSV is trivial, getting a logic level from it is also fairly easy. I realise that this is terribly quick & dirty but it worked 100% for what I needed. Here is the high level process

1. Ignore timings - SPI is clocked so actual timing is irrelevant for this analysis
2. Parse X and Y channels
3. Convert clock channel voltage into logic level 1 or 0
4. Discard all rows apart from rows where the clock goes high
5. Convert data channel to logic level
6. Group into bytes
7. Display hex value

As I said, very simple but it worked well and I was able to see exactly what was going on. Obviously this only works for synchronous (clocked) protocols like SPI/I2C but that is all I needed.

Here is the full F# script (also see the gist)