2.7.1 Debugging and Programming Pins

In-Circuit Programming and Debugging Pins

The SWDIO/TMS, TDO/TRACESWO, SWCLK/TCK, and TDI pins are used for In-Circuit Programming and debugging purposes. It is recommended to keep the trace length between the debug external connector and the debug pins on the device as short as possible to minimize ESD/EMI vulnerabilities. If the debug external connector is expected to experience an ESD event, a series resistor is recommended, with the value in the range of a few tens of Ohms, not to exceed 100 Ohms with protection using Transient Voltage Suppressors (TVS), at the user’s discretion. See image below for more information.

Important: The SWDIO/TMS, SWCLK/TCK, and TDI pins are not internally pulled-up after reset by default but a pull-up resistor on these pins is critical for reliable operations. A recommended value for this pull-up resistor must be comprised between 15 kΩ and 50 kΩ.

Test Pin

The TST pin is used for JTAG Boundary Scan Manufacturing Test or Fast Flash Programming mode. The TST pin integrates a permanent pulldown resistor of about 15 kΩ to GND, so that it can be left unconnected for normal operations.

Note:
  1. The TST pin must be tied low along with JTAGSEL to VDDIO, and PD0 must be tied to GND to enter JTAG Boundary Scan.
  2. The TST pin must be tied low along with PA3 and PA4 pins to enter Fast Programming when FFPI is enabled.
  3. In harsh environments, it is strongly recommended to tie this pin to GND if not used or to add an external low-value resistor such as, 10 kΩ.

Trace Pins

When present on select pin counts, the trace pins can be connected to a hardware trace-enabled programmer/debugger to provide a compressed real-time instruction trace. When used for trace, the TRACED0, TRACED1, TRACED2, TRACED3 and TRACECLK pins should be dedicated for this use. The trace hardware requires a 22Ω series resistor between the trace pins and the external trace connector.