DocID024597 Rev 3 1649/1693
RM0351 Debug support (DBG)
1678
44.4.3 Internal pull-up and pull-down on JTAG pins
It is necessary to ensure that the JTAG input pins are not floating since they are directly
connected to flip-flops to control the debug mode features. Special care must be taken with
the SWCLK/TCK pin which is directly connected to the clock of some of these flip-flops.
To avoid any uncontrolled IO levels, the device embeds internal pull-ups and pull-downs on
the JTAG input pins:
• NJTRST: internal pull-up
• JTDI: internal pull-up
• JTMS/SWDIO: internal pull-up
• TCK/SWCLK: internal pull-down
Once a JTAG IO is released by the user software, the GPIO controller takes control again.
The reset states of the GPIO control registers put the I/Os in the equivalent state:
• NJTRST: input pull-up
• JTDI: input pull-up
• JTMS/SWDIO: input pull-up
• JTCK/SWCLK: input pull-down
• JTDO: input floating
The software can then use these I/Os as standard GPIOs.
Note: The JTAG IEEE standard recommends to add pull-ups on TDI, TMS and nTRST but there is
no special recommendation for TCK. However, for JTCK, the device needs an integrated
pull-down.
Having embedded pull-ups and pull-downs removes the need to add external resistors.
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