[SOLVED] SWD multidrop?

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  • [SOLVED] SWD multidrop?

    The wiki info on the current J-LINK models lists a feature named "SWD multidrop".
    Where we can find more about this? It sounds like a cheap alternative to JTAG chain for systems with multiple targets.

    -- dd
  • How about to get to the vendor?
    It is a feature specified by ARM:
    developer.arm.com/documentatio…ulti-drop-support?lang=en

    developer.arm.com/documentation/ka001195/latest

    The tricky part:
    It is an option, the device vendor MAY license, so you need to check for your specific chip.
    Even more tricky:
    Many vendors have licensed it but screwed up the implementation.
    The selection for multi-drop is based on a <DevId> + <InstId>. This is needed because all devices from a family, from a vendor, usually share a <DevID> (e.g. all STM32L4 MCUs may report the same DevID)
    The <InstId> must be somewhat configurable (device’s vendor job) because the chip has no idea in what context it may be placed/soldered later on. Some ideas for configuring the <InstId> are: Pins, option bytes, …
    Many vendors have messed up the <InstId> to be connected to all 0s or all 1s internally. For such devices, there is no way of having multiple ones of these in a multi-drop setup.
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  • Thanks Alex. All I understood from these articles is that this is not just a 'passive' fan-out of the SWD lines to multiple MCUs, and should be licensed by MCU maker. So not a cheap JTAG alternative. Ok.