Connecting to the board AN4989
40/118 AN4989 Rev 3
4 Connecting to the board
The way IDEs get connected to the boards is not always known. In case of trouble, a basic
knowledge about this topic can save time in identifying and fixing the issue.
This chapter intends to provide the minimal set of information in order to prevent or quickly
fix issues related to connection.
4.1 SWD/JTAG pinout
On STMicroelectronics hardware kits, SWD must be made available for connection with ST-
LINK.
SWD is always mapped on PA13 (SWDIO) and PA14 (SWCLK). This is the default state
after reset.
Nothing specific is required in the application code to make SWD work.
Special attention must be paid to make sure that, voluntarily or accidentally, the SWD pins
are not switched to some alternate functions or affected by I/O settings modifications.
Hint: For instance, STM32Cube PWR examples switch all GPIO (including SWD) in an
analog state in order to minimize consumption. This disconnects the debugger. A
Connect Under Reset using NRST is required to take back the control of the
board. (Refer to Section 4.2).
When using STM32CubeMX at configuration stage, PA13 and PA14 can be in one of three
states upon selection of Serial Wire in SYS/Debug configuration list:
• Reset, shown by the pins colored in gray in Figure 24
• Reserved but inactive shown by the pins colored in orange in Figure 25
• Active shown by the pins colored in green in Figure 26
Figure 24. SWD pins PA13 and PA14 in Reset state under STM32CubeMX
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