Is it possible to write directly from JTAG to an eMMC connected to the SD/MMC controller on the Hard Processor System (HPS) side of the Cyclone® V SoC, similar to QSPI / NAND?

As of tool version v17.1, the HPS Flash Programmer available for writing QSPI / NAND for HPS does not support SD/MMC.
No other manufacturer-provided tools exist, so you will need to use another method, such as using DS-5.

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Category: SoC
Tools: SoC EDS
Device: Cyclone® V