add option to skip n SPI data bits

[danielkucera]

This is useful when you have no CS signal and want to offset the decoding by a few bits

[kamocat]

Hi Daniel, do you have a signal capture to give this some more context?
Have you considered looking for natural breaks in transmission to automatically guess where CS should be?

[danielkucera]

Hi @kamocat , you can understand the context from here: https://blog.danman.eu/updating-rns315-maps-for-fun-and-profit/
What do you mean by "looking for natural breaks" ? How can I align the decoder by that?

[kamocat]

PulseView doesn’t have data decoding for SD card in SD mode so I had to improvise to get the password data. I have used SPI decoder, and since we don’t have chip-select signal here to properly align bits into bytes, I extended it with an option to skip bits on start. So I was able to get the password bytes decoded.

So you're using the SPI decoder to read SD card data, instead of extending the SD card decoder to output data?

[danielkucera]

I know it's a shame but it's true. This was just a few lines, instead of implementing SD data decoding which would be 🤯

[kamocat]

It looks pretty simple to add this same functionality to the SD decoder. From the simplified SD standard,

Data Read

The DAT bus line is high by the pull-up when no data is transmitted. A transmitted data block consists of start bits (1 or 4 bits LOW), followed by a continuous data stream.
...

Data Write

The data transfer format is similar to the data read format.

So all you have to do is look for a falling edge on DAT0, and stop when all the data lines are high for 512 bits in a row.
True, it's not the full standard, but it gets you at least as far as the SPI bit offset, without guessing what the offset should be.

[danielkucera]

And handle 1-bit, 2-bit, 4-bit, 8-bit mode, CRC, and, and, and...
Sorry, I am not going into this.