Using low cost STM32F103C8T6 (Blue Pill) boards with mbed.

Dependencies:   mbed-STM32F103C8T6 mbed

STM32F103C8T6 board, alias Blue Pill

It provides an affordable (about $2 on eBay) and flexible way for users to try out new ideas and build prototypes. The board is equipped with an STM32F103C8T6 microcontroller compatible with the NUCLEO-F103RB platform.

Microcontroller features

  • STM32F103C8T6 in LQFP48 package
  • ARM®32-bit Cortex®-M3 CPU
  • 72 MHz max CPU frequency
  • VDD from 2.0 V to 3.6 V
  • 64 KB Flash
  • 20 KB SRAM
  • GPIO (32) with external interrupt capability
  • 12-bit ADC (2) with 10 channels
  • RTC
  • Timers (4)
  • I2C (2)
  • USART (3)
  • SPI (2)
  • USB 2.0 full-speed
  • CAN

Board features

  • Small foot-print
  • Flexible board power supply: USB VBUS or external source (3.3V, 5V)
  • User LED: LED1
  • One push button: RESET
  • Programming/Debug port
  • Micro-B USB connector

Board pinout

Zoom in

Maximum allowed I/O voltage levels (next to pin names) are courtesy of Thor Sten to help you avoid board damage. For more details on pin definitions see Table 5 in the Datasheet.


Only the labels printed in blue/white or green/white (i.e. PC_13, PB_9, A0, D14 ...) must be used in your code. The other labels are given as information (alternate-functions, power pins, ...). You can also use these additional pin names:

            SERIAL_RX=PA_3  I2C_SDA=PB_9  SPI_MISO=PA_6
                                          SPI_SCK =PA_5
                                          SPI_CS  =PB_6

Please notice that in order to fit the small size board, the leading 'P' and the '_' characters are omitted from labels indicated on the board (e.g. Instead of 'PA_1' you can find the label 'A1' on the board). Arduino (green/white) and the additional naming labels are not indicated on the board.
Also notice that the on-board LED is connected to pin PC_13 and, via a resistor, to +3.3V. So to turn the LED on or off you have to set the DigitalOut to 0 or 1 respectively.


Zoom in /media/uploads/hudakz/stm32f103c8t6_schematic.png

Eagle library


Download: Eagle library

NOTE: Because neither lbr nor zip files can be uploaded to mbed wiki pages anymore, once downloaded, change the file extension from png to lbr.

Using the mbed online compiler to build programs for the STM32F103C8T6 board

  • Create a program as if it was for a NUCLEO-F103RB board (select NUCLEO-F103RB as target platform for the online compiler).
    Or click here to import this demo into your online compiler (then you can skip the following two steps).
  • Add #include "stm32f103c8t6.h" to main.cpp before #include "mbed.h" (the position matters!).

Blinking on-board LED:

#include "stm32f103c8t6.h"
#include "mbed.h"
int main() {
    confSysClock();     //Configure system clock (72MHz HSE clock, 48MHz USB clock)
    Serial      pc(PA_2, PA_3);
    DigitalOut  myled(LED1);
    while(1) {
        // The on-board LED is connected, via a resistor, to +3.3V (not to GND). 
        // So to turn the LED on or off we have to set it to 0 or 1 respectively
        myled = 0;      // turn the LED on
        wait_ms(200);   // 200 millisecond
        myled = 1;      // turn the LED off
        wait_ms(1000);  // 1000 millisecond



Keep in mind that the online compiler is checking for 128kB maximum flash size. However, the STM32F103C8T6 is equipped with only 64kB. So once the compilation is complete (started by clicking on the Build only button in the Compile drop list or by pressing Ctrl+B) you have to visually check the size of used flash memory in the Program details - Build tab. In order to fit into an STM32F103C8T6 board the used Flash must not exceed 64kB! Try to optimize your program until it's using less than 64kB flash memory. Have a look at mbed-STM32F030F4 and Andy's hints for some good tips.

Programming the STM32F103C8T6 board

NUCLEO ST-LINK/V2-1 and drag & drop

You can use the NUCLEO virtual disk to program the STM32F103C8T6 board (drag and drop programming). To do that, an additional NUCLEO board is needed (any type equipped with ST-LINK/V2-1 will do).

  • Remove the two jumpers from the CN2 connector as illustrated in Figure 8: /media/uploads/hudakz/nucleo_prog.png

  • Connect the NUCLEO board CN4 connector to the STM32F103C8T6 board using flying wires as follows:
NUCLEO board
CN4 connector
debug connector
  • Provide power for the STM32F103C8T6 board through a 3.3V pin, 5V pin or over a USB cable. (The VDD_TARGET pin on the NUCLEO board CON4 does not work as source of power).

  • Connect the NUCLEO board to your PC over a USB cable.

  • To program the STM32F103C8T6 board, click on the Compile button and save the binary to the NUCLEO virtual disk .
    For more details have a look at the User Manual, chapter 6.2.4 Using ST-LINK/V2-1 to program and debug an external STM32 application.

ST-Link V2 USB dongle and STM32 ST-LINK utility

If you would like to use an ST-Link V2 USB dongle (aka ST-Link V2 Programming Unit) to program the board apply the same wiring as specified above. If not done yet install an ST-Link/V2 driver onto your PC. Plug the ST-Link V2 dongle into your PC. Then click on the Compile button and save the binary to your local disk. Install and run the STM32 ST-LINK utility. Once the program is running open the binary built with the online compiler and click on the Program verify button.


STM32 USART system memory bootloader and Flasher-STM32

Have a look at STM32 Embedded Bootloader.

Download: FLASHER-STM32
For more details read: Appliction Note AN2606.

Using serial port (not just for debugging)

  • Connect an FTDI or similar USB to Serial TTL converter to your PC and to an on-board serial port (for example PA_2, PA_3). Make sure you connect the on-board TX pin to the converter's RX pin and the on-board RX pin to the converter's TX pin.

/media/uploads/hudakz/stm32f103c8t6_hookup.jpg Zoom in

  • In your code, create a Serial object (using TX and RX pin names of the connected serial port).
  • Use printf function to send serial messages to the connected PC.

Sending debug messages over the ST-Link virtual com port

In case you would like to spare the external USB-Serial converter for other purposes then there is available an alternative solution proposed by X M (bitman). You can use the ST-Link virtual com port also for debugging of programs running on the STM32F103C8T6 board. However, that will require a soldering iron (and probably some soldering skills). According to the User Manual, chapter 6.8 "USART communication", solder bridges (on the back side of the NUCLEO board) SB62 and SB63 should be ON, SB13 and SB14 should be OFF. In such case it is possible to connect another USART to the NUCLEO (ST-Link) CN3 connector using flying wires. For instance on STM32F103C8T6 board it is possible to use USART2 available on PA_2 (TX) and PA_3 (RX). Two flying wires shall be connected as follows:

STM32F103C8T6 board, pin PA_2 (Serial2 TX)<=>NUCLEO board CN3 connector, pin RX
STM32F103C8T6 board, pin PA_3 (Serial2 RX)<=>NUCLEO board CN3 connector, pin TX

A smart trick proposed by Nothing Special makes even soldering needless.
The point is to redirect the UART on the NUCLEO board by software (without modifying the solder bridges on the back side of the NUCLEO board) and convert it into a "Debugger". On the NUCLEO board that you are going to use as programmer/ debugger, choose any Serial port other than Serial2 (other than the default port used for standard UART) to be initialized as standard UART. In the program below (using NUCLEO-F103RB as programmer/debugger) Serial1 (PA_9, PA_10) was selected.


#include "mbed.h" 

// declarations needed to change the parameters of stdio UART 
extern serial_t     stdio_uart; 
extern int          stdio_uart_inited; 

int main() {
    serial_init(&stdio_uart, PA_9, PA_10); // other than Serial2
    stdio_uart_inited = 1; 
    printf("Ready for debugging\r\n");

Once compiled (remember to select the NUCLEO board used for programing/debugging as target for the online compiler), download the "Debugger" program to the NUCLEO board. Please make sure you have the two jumpers in place on the CN2 connector when programming the NUCLEO board. Once the "Debugger" binary has been downloaded to the NUCLEO board, remove the two jumpers again.

Using Real Time Clock

It seems that RTC is not fully functional for the NUCLEO-F103RB and STM32F103C8T6 boards. Fortunately Maxim Ibragimov published an amendment here.

Additional demos for the STM32F103C8T6 board

Offline compilation and debugging with CooCox CoIDE

  • I do not recommend to use the Beta version but rather install CoIDE V1.7.8.
  • Keep the NUCLEO board ( ST-LINK) or ST-Link V2 USB dongle (aka ST-Link V2 Programming Unit) connected to the STM32F103C8T6 board as explained above in section Programming the STM32F103C8T6 board.
  • Export your program from the mbed online compiler to the CooCox CoIDE toolchain.


  • Save the file into a folder on you PC's hard drive and unzip.

  • Run CoIDE and open the project.

  • On the menu bar click on Project. Choose Select Toolchain Path and select the path to the GNU Arm Embedded Toolchain. Once completed the path will apply to all projects. (It is not necessary to repeat this for each project but rather do it only once after installing the toolchain.)



  • Go to Configuration and in the Link tab click on the Edit button to open the Scatter File for editing.


  • Once the stm32f103xb.ld file is opened change the length of available FLASH from 128K to 64K:

    /media/uploads/hudakz/stm32f103ct6_64k.png Save the change and close the file. From now on, the offline compiler will compare the size of used Flash memory with 64kB maximum Flash size and we'll get an error message in case it's exceeded.

  • To setup the debugger, open the Debugger tab and use the following settings:


  • To ensure that the execution of instructions during debugging is performed in the foreseen sequence, go to the Compile tab and select None (-O0) optimization:


  • To be able to use the Serial class in application programs built offline, in the Project side bar navigate to headers/mbed/TARGET_NUCLEO_F103RB/TARGET_STM/TARGET_STM32F1 and open the device.h header file for editing by double-clicking on it. Then add a new line saying #include "mbed_config.h" as below:

    /media/uploads/hudakz/stm32f103c8t6_device_h.png After such modification the program should compile with no errors.
  • Another option is to use the default serial port which is automatically created by the mbed library. In such case no Serial object (like Serial pc(PA_2, PA_3);) has to be created. Instead the global printf function is called as below. In this case also the binary code becomes smaller. The default settings are 9600 bits/s, one start bit, eight data bits, no parity bit, one stop bit.

#include "stm32f103c8t6.h"
#include "mbed.h"
int main() {
    confSysClock();     //Configure system clock (72MHz HSE clock, 48MHz USB clock)
    //Serial      pc(PA_2, PA_3);
    DigitalOut  myled(LED1);
    while(1) {
        // The on-board LED is connected, via a resistor, to +3.3V (not to GND). 
        // So to turn the LED on or off we have to set it to 0 or 1 respectively
        myled = 0;      // turn the LED on
        wait_ms(200);   // 200 millisecond
        myled = 1;      // turn the LED off
        wait_ms(1000);  // 1000 millisecond

  • Open main.cpp for editing and rebuild the project by clicking on the Rebuild button:


  • Reprogram the board by clicking on the Download Code To Flash button:
    NOTE: Any time you launch the debugger the program is getting recompiled and the board reprogrammed. So you can skip this step. I included it just to illustrate how easily you can (re)program the board offline.


  • Set up some debug points and launch the debugger by clicking on the Start Debug button:


  • You can execute the instructions step by step (or run to the next debug point) and observe the variables and outputs whether they are changing as expected...


    Happy coding and debugging :-)
Download repository: zip gz

Files at revision 11:c050fcabba46

Name Size Actions
main.cpp 628 Revisions Annotate
mbed-STM32F103C8T6.lib 67 Revisions Annotate
mbed.bld 66 Revisions Annotate