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SWD / JTAG Connectors and Pinout

JTAG was the traditional mechanism for debug connections for ARM parts, but with the Cortex-M family, ARM introduced the Serial Wire Debug (SWD) Interface. SWD is designed to reduce the pin count required for debug from the 5 used by JTAG (including GND) down to 3. In addition, one of the pins freed up by this can be used for Single Wire Viewing (SWV), which is a low cost tracing technology (which is used by the "Red State" functionality within Red Suite).

The SWD/SWV pins are overlaid on top of the JTAG pins as follows:


SWV Mode





Clock into the core

Use 10K or 100K Ohm pull-down resistor to GND



JTAG Test Data Input

Use 10K or 100K Ohm pull-up resistor to VCC



JTAG Test Data Output / SWV trace data output



JTAG Test Mode Select / SWD data in/out

Use 10K or 100K Ohm pull-up resistor to VCC



RESET - Connect this pin to the (active low) reset input of the target MCU

20-pin 0.1" JTAG/SWD Connector Pinout


  • This connector was originally defined for connection to ARM7/9 parts over JTAG, but is still sometimes found in Cortex-M systems (for connections over JTAG or SWD).
  • Dimensions of the ARM JTAG connector are 1.29" x 0.722" (33mm x 18.5mm).

The 10 pin Cortex debug connector

10-pin 0.05" JTAG/SWD Connector Pinout


Part number for the 10-pin 0.05" pitch connectors for target PCB

  • Harwin: M50-3500542
  • Mouser: 855-M50-3500542
  • Samtec shrouded header: FTSH-105-01-F-D-K

Cortex-M3 is the first ARM core to

There are special sequences defined to switch from JTAG mode (default) and SWD mode that can sent to the core through just the SW pins. Note that some MCUs (such as NXP LPC13xx) only support debugging via SWD.

Cortex-M0 also implements the Serial Wire Debug interface, but not JTAG. It also does not provide support for the SWV low cost tracing technology.

  • Note that RTCLK (Return clock) is only used on older ARM cores (ARM7TDMI and ARM9 family) before the debug was properly decoupled from the core clock domain.
  • Minimal pincount for SWD debug is 3 pins with SWDIO, CLK and GND
  • We would strongly recommend also including RESET in addition to SWDIO, CLK and GND. For debugging NXP LPC11xx, RESET is required.
  • Red Trace (Serial Wire Viewing) requires the SWV pin

HardwareDebugConnections (last edited 2013-10-22 13:38:23 by CrSupportAb)