Hardware debug for the Alveo Data Center accelerator cards typically uses the XVC-over-PCIe connection due to the inaccessibility of the physical card, and the JTAG connector on the card. While XVC-over-PCIe allows you to remotely debug your application running on the target platform, certain conditions such as AXI interconnect system hangs can prevent you from accessing the hardware debug functionality that depends on these PCIe/AXI features. Being able to debug these kinds of conditions is especially important for platform designers.
The JTAG Fallback feature is designed to provide access to debug networks that were previously only accessible through XVC-over-PCIe. The JTAG Fallback feature can be enabled without having to change the XVC-over-PCIe-based debug network in the platform design.
On the host side, when the Vivado hardware manager user
connects through the hw_server to a JTAG cable that is
connected to the physical JTAG pins of the accelerator card, or device under test (DUT),
the hw_server disables the XVC-over-PCIe pathway to the hardware. This lets you use the
XVC-over-PCIe cable as your primary debug path,
but enable debug over the JTAG cable directly when it is required in certain situations.
When you disconnect from the JTAG cable, the hw_server
re-enables the XVC-over-PCIe pathway to the
hardware.