When designing your board, you should also take the following signals into account for debug purposes. Note Cortex-M0/M0+ parts do not support SWV trace. Note in particular that if the MCU provides an internal pull-up on this signal, then adding an external pull-down on the board is not recommended. The main thing is that it does not float. Note for TCK/SWCLK, although a pull-down is recommended, you can alternatively use a pull-up. But if external resistor is provided in such cases, then it must match that provided internally by the MCU. If an internal resistor is provided for a pin by the MCU, then an external resistor is not required for that pin.

Failure to do this will lead to, at best, unreliable debug connections, or more likely no ability to debug at all. This will prevent the signals from floating when they are not connected to anything. You may use resistors between 10K and 100K for these signals. Where internal resistors are not provided, these should be added externally onto your board as detailed above. You will need to review the datasheet for the specific MCU being used to confirm. Some MCUs do not include internal pull-up or pull-down resistors on JTAG/SWD pins. JTAG Test Data Output / SWV trace data output (SWO) Use 10K-100K Ohm pull-down resistor to GND

Required pull-up / pull-down (if not implemented internally by MCU) The SWD/SWV pins are overlaid on top of the JTAG pins as follows: In addition, one of the pins freed up by this can be used for the low cost SWO tracing technology - for more details see the FAQ ' Overview of Trace support in LPCXpresso IDE'.

SWD is designed to reduce the pin count required for debug from the 5 used by JTAG (including GND) down to 3. JTAG was the traditional mechanism for debug connections for ARM7/9 parts, but with the Cortex-M family, ARM introduced the Serial Wire Debug (SWD) Interface.

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