Debug support (DBG)
RM0365
1047/1080
DocID025202 Rev 7
33.8
SW debug port
33.8.1 SW
protocol
introduction
This synchronous serial protocol uses two pins:
•
SWCLK: clock from host to target
•
SWDIO: bidirectional
The protocol allows two banks of registers (DPACC registers and APACC registers) to be
read and written to.
Bits are transferred LSB-first on the wire.
For SWDIO bidirectional management, the line must be pulled-up on the board (100 k
Ω
recommended by ARM
®
).
Each time the direction of SWDIO changes in the protocol, a turnaround time is inserted
where the line is not driven by the host nor the target. By default, this turnaround time is one
bit time, however this can be adjusted by configuring the SWCLK frequency.
33.8.2 SW
protocol
sequence
Each sequence consist of three phases:
1.
Packet request (8 bits) transmitted by the host
2. Acknowledge response (3 bits) transmitted by the target
3. Data transfer phase (33 bits) transmitted by the host or the target
Refer to the Cortex-M4
®
F r0p1
TRM
for a detailed description of DPACC and APACC
registers.
The packet request is always followed by the turnaround time (default 1 bit) where neither
the host nor target drive the line.
Table 185. Packet request (8-bits)
Bit
Name
Description
0
Start
Must be “1”
1
APnDP
0: DP Access
1: AP Access
2
RnW
0: Write Request
1: Read Request
4:3
A(3:2)
Address field of the DP or AP registers (refer to
5
Parity
Single bit parity of preceding bits
6
Stop
0
7
Park
Not driven by the host. Must be read as “1” by the target because of
the pull-up