Learn the Fundamentals of Mechatronics!

We put together a test circuit to test the proper functioning of our ARM development boards. During the testing we ran into a problem which allows us to present the opportunity to show how we solved the problem.

The circuit works by turning on and off an LED for each pin individually so we can determine whether a pin is working or not. This lets us know if there are any bridges between pins on the microcontroller or if there are any cold solder joints.

The circuit works as expected, but the problem is the actual programming pins (SWDIO and SWCLK) are set to output to the LEDs as well, even though they don’t have any LEDs. You can always ensure the functionality of these pins just by programming the chip. So how do you get these two pins out of that function so you can reprogram the chip again?

Since those pins have been programmed as a function of digital output rather than SWDIO and SWCLK for programming, there is no way for the STM32 ST-Link utility to actually connect to the chip to communicate with it. The only thing you can get is voltage to the circuit, but no communication can happen.

If you try to connect to the microcontroller you will get a “Cannot connect to target” error and generally the recommendations in the error message aren’t going to be very helpful.

First we need to find the reset pin. On this development board the reset pin is located on pin 7. So we locate pin 7 and put one end of a 500ohm resistor on the pin and pull it down to ground to put the chip into reset mode.

Next we go to the ST-Link utility and select “Full chip erase” (dynamite button). Notice it won’t do anything but it will lock up. Now we will remove the resistor and we see now that the full chip erase progresses successfully.

Now if we click on “Connect to Target”, it will connect perfectly and we can go ahead and program it again and it will program successfully.