UM0306 Debug support (DBG)
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In the SWJ-DP, the two JTAG pins of the SW-DP are multiplexed with some of the five JTAG
pins of the JTAG-DP.
The figure above shows that the asynchronous TRACE output (TRACESWO) is multiplexed
with TDO. This means that the asynchronous trace can only be used with SW-DP, not JTAG-
DP.
20.3.1 Mechanism to select the JTAG-DP or the SW-DP
By default, the JTAG-Debug Port is active.
If the debugger host wants to switch to the SW-DP, it must provide a dedicated JTAG
sequence on TMS/TCK (respectively mapped to SWDIO and SWCLK) which disables the
JTAG-DP and enables the SW-DP. This way it is possible to activate the SWDP using only
the SWCLK and SWDIO pins.
This sequence is:
1. Send more than 50 TCK cycles with TMS (SWDIO) =1
2. Send the 16-bit sequence on TMS (SWDIO) = 0111100111100111 (MSB transmitted
first)
3. Send more than 50 TCK cycles with TMS (SWDIO) =1
TRACESWO
JTDO
JTDI
JNTRST
nTRST
TDI
TDO
SWJ-DP
TDO
TDI
nTRST
TCK
TMS
nPOTRST
JTAG-DP
nPOTRST
From
power-on
reset
DBGRESETn
DBGDI
DBGDO
DBGDOEN
DBGCLK
SW-DP
SWCLKTCK
SWDOEN
SWDO
SWDITMS
SWD/JTAG
select
JTMS/SWDIO
JTCK/SWCLK
(asynchronous trace)
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