SWD and JTAG

Both protocols are supported. Select one at connect time, or set the default with the --protocol server option.

connect { "protocol": "swd" }    # default
connect { "protocol": "jtag" }

list_probes reports which protocols the connected probe supports. Most CMSIS-DAP probes support both.

Which one to use

• SWD is the default and works on any Cortex-M with a debug port. It uses two wires (SWDIO, SWCLK) plus reset.
• JTAG requires the target to expose a JTAG TAP and the four/five JTAG pins. Many small Cortex-M0/M0+ devices do not bring out JTAG.

The server was verified on hardware over SWD. If a target does not support JTAG, connect returns ConnectFailed with a protocol error; the toolset still fully supports JTAG for targets that expose it.

Speed

Pass speed_khz in connect, or set --speed-khz at startup. The probe selects the highest supported speed at or below the request.

Connect under reset

For locked or non-responsive targets, hold the reset line during attach:

connect { "protocol": "swd", "under_reset": true }

This requires the probe’s reset pin to be wired to the target’s reset.