
Debug support (DBG)
RM0351
1778/1830
DocID024597 Rev 5
48.4.1
SWJ debug port pins
Five pins are used as outputs from the STM32L4x5/STM32L4x6 for the SWJ-DP as
alternate functions
of general-purpose I/Os. These pins are available on all packages.
48.4.2
Flexible SWJ-DP pin assignment
After RESET (SYSRESETn or PORESETn), all five pins used for the SWJ-DP are assigned
as dedicated pins immediately usable by the debugger host (note that the trace outputs are
not assigned except if explicitly programmed by the debugger host).
However, the STM32L4x5/STM32L4x6 MCUs offer the possibility of disabling some or all of
the SWJ-DP ports, and therefore the possibility of releasing the associated pins for general-
purpose I/O (GPIO) usage, except for NJTRST that can be left disconnected but cannot be
used as general purpose GPIO without loosing debugger connection. For more details on
how to disable SWJ-DP port pins, please refer to
Section 8.3.2: I/O pin alternate function
.
Table 310. SWJ debug port pins
SWJ-DP pin name
JTAG debug port
SW debug port
Pin
assign
ment
Type
Description
Type
Debug assignment
JTMS/SWDIO
I
JTAG Test Mode
Selection
IO
Serial Wire Data
Input/Output
PA13
JTCK/SWCLK
I
JTAG Test Clock
I
Serial Wire Clock
PA14
JTDI
I
JTAG Test Data Input
-
-
PA15
JTDO/TRACESWO
O
JTAG Test Data Output
-
TRACESWO if
asynchronous trace is
enabled
PB3
NJTRST
I
JTAG Test nReset
-
-
PB4
Table 311. Flexible SWJ-DP pin assignment
Available debug ports
SWJ IO pin assigned
PA13 /
JTMS/
SWDIO
PA14 /
JTCK/
SWCLK
PA15 /
JTDI
PB3 /
JTDO
PB4/
NJTRST
Full SWJ (JTAG-DP + SW-DP) - Reset State
X
X
X
X
X
Full SWJ (JTAG-DP + SW-DP) but without NJTRST
X
X
X
X
JTAG-DP disabled and SW-DP enabled
X
X
JTAG-DP disabled and SW-DP disabled
Released