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And where the document talks about SSI, it essentially means SPI (just with differential signalling etc.)? In other words, is it this SSI?: <https://en.wikipedia.org/wiki/Synchronous_Serial_Interface>

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It's a "synchronous serial interface" similar to SPI, but that isn't what the Wikipedia article you're linking to is about.

It's the DW_apb_ssi peripheral from Synopsys (https://www.synopsys.com/dw/ipdir.php?c=DW_apb_ssi).

It's always a bit funny to me how much of the Synopsis IP ends up in different chips. As an example a decade+ ago I implemented a from-scratch USB Peripheral stack on an STM32 microcontroller, basically because the vendor SDK wasn't capable of doing what I needed or being readily modified to do what I needed. A couple of years ago I was debugging some firmware for a chip from a completely different vendor and noticed that the USB registers looked... familiar. Looked back at the original project and was somewhat surprised to discover that it was exactly the same registers in the same order just mapped to a different spot in memory.

The same thing has happened in the Linux kernel - there are multiple semi-duplicate SoC USB drivers for the Synopsys core.

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