Function: 1. All IO mouth leads (with screen printing, convenient to connect various modules) 2. A serial port to elicit alone, there are multiple sets of 3.3 v to 5 v power supply 3. A reset button 4. The BOOT option 5. SWD downloads interface (by jLINK STLINK download) 6. A power indicator light 7. The onboard 8 MHZ crystal vibration PCB size: 30 mm * 42.5 mm Location hole size: 2.1 mm (diameter)