title: STM32 Nucleo F103RB: LD1 slow blinking with external supply
body: <p>I'm trying to use a Nucleo F103 RB (stm32) with an external supply (5V) but it seems it can't work because the led LD1 is blinking slowly between RED and OFF. According to the manual, it means "At power-on before USB initialization". I'm afraid I don't know what it means.
  Before connecting the Nucleo to the external supply, I had flashed the corresponding software using Keil uVision 4 by connecting the Nucleo to a PC via USB. There, I debugged the program and it ran correctly on the Nucleo. LD1 remained RED, not blinking. 
  I've tested other Nucleo F103RB samples with my program and also with other programs and I've had the same problem: working fine in debug mode with PC supply but not working with external supply. When I connect the Nucleo to the PC, I switch the jumper on U5V. When I connect an external supply, I switch the jumper on E5V.</p>
  
  <p>Has anybody faced this problem?
  Thank you in advance!</p>
  
  <p>Best regards</p>
  
  <p><strong><em>EDIT: Finally, it worked despite that LED blinking. The problem seemed to be an app which worked through Modbus. Thank you both for your help anyway ;)</em></strong></p>