Squonk refers us to this 3-part tutorial on the ARM SWD debug protocol.
LPC ARM Cortex-based microcontrollers from NXP can be controlled via SWD, ARM’s Serial Wire Debug protocol and the Coresight register set which allow nonintrusive debugging. This blog post series summarizes use of the SWD protocol to do basic debugging functions on the Cortex-M0 and presents demonstration code and a simple hardware design to implement a USB-to-SWD bridge. With additional software the SWD-to-USB bridge could allow debugging or flash programming from a computer or other USB host device.
Via the forum.