3.3.3. Pin Features and Connections for HPS JTAG, Clocks, Reset and PoR
GUIDELINE: With the HPS in use (powered), supply a free running clock on HPS_CLK1 for SoC device HPS JTAG access.
Access to the HPS JTAG requires an active clock source driving HPS_CLK1.
GUIDELINE: When daisy chaining the FPGA and HPS JTAG for a single device, ensure that the HPS JTAG is first device in the chain (located before the FPGA JTAG).
Placing the HPS JTAG before the FPGA JTAG allows the ARM DS-5 debugger to initiate warm reset to the HPS. However, in case of cold reset the entire JTAG chain is broken until the cold reset completes, as discussed in the next section.
GUIDELINE: Consider board design to isolate HPS JTAG interface
The HPS Test Access Port (TAP) controller is reset on a cold reset. If the HPS JTAG and FPGA JTAG are daisy-chained together, the entire JTAG chain is broken until the cold reset completes. If access to the JTAG chain is required during HPS cold reset, design the board to allow HPS JTAG to be bypassed.
GUIDELINE: HPS_nRST is an open-drain, bidirectional dedicated warm reset I/O.
HPS_nRST is an active low, open-drain-type, bidirectional I/O. Externally asserting a logic low to the HPS_nRST pin initiates a warm reset of the HPS subsystem. HPS warm and cold reset can also be asserted from internal sources such as software-initiated resets and reset requests from the FPGA fabric. When the HPS is internally placed in a warm reset state, the HPS component becomes a reset source and drives the HPS_nRST pin low, resetting any connected board-level components.