Looking for a small, inexpensive ($25-30) ARM development board (say 120-180 MHz ARM Cortex-M4 with FPU, 512kB-1MB of FLASH and 256 KByte of RAM? Then have a look at the Teensy 3.5 and Teensy 3.6 by PJRC/Paul Stoffregen:
Teensy 3.5 with NXP K64F ARM Cortex-M4F
The only problem? it is not possible to debug it :-(. At least not in the traditional sense. This article is about how to change the board to use it with any normal SWD debugging tool e.g. Eclipse and the Segger J-Link :-).
For a project I want to use the Teensy 3.1 board (see “Hacking the Teensy V3.1 for SWD Debugging“) in USB CDC mode: that way the Teensy board can connect to the host and exchange data or attach a console to the Teensy board: that way I can connect the Teensy over USB to the host use the USB as communication interface: