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RM0008
Table 216. Flexible SWJ-DP pin assignment
Full SWJ (JTAG-DP + SW-DP) - Reset State
Full SWJ (JTAG-DP + SW-DP) but without NJTRST
JTAG-DP Disabled and SW-DP Enabled
JTAG-DP Disabled and SW-DP Disabled
Note:
When the APB bridge write buffer is full, it takes one extra APB cycle when writing the
AFIO_MAPR register. This is because the deactivation of the JTAGSW pins is done in two
cycles to guarantee a clean level on the nTRST and TCK input signals of the core.
●
Cycle 1: the JTAGSW input signals to the core are tied to 1 or 0 (to 1 for nTRST, TDI
and TMS, to 0 for TCK)
●
Cycle 2: the GPIO controller takes the control signals of the SWJTAG IO pins (like
controls of direction, pull-up/down, Schmitt trigger activation, etc.).
31.4.3
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 IOs 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 IOs 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.
Available debug ports
Doc ID 13902 Rev 12
Debug support (DBG)
SWJ IO pin assigned
PA13 /
PA14 /
PA15 /
JTMS/
JTCK/
JTDI
SWDIO
SWCLK
X
X
X
X
X
X
X
X
Released
PB3 /
PB4/
JTDO
NJTRST
X
X
X
1053/1096
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