Debug support (DBG)
RM0365
1041/1080
DocID025202 Rev 7
33.4.3 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.