ST STM32F301 6 Series Reference Manual page 846

Debug support (DBG)
Address A(3:2) value
0x8
0xC
28.8 SW debug port
28.8.1 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 kΩ
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.
28.8.2 SW protocol sequence
Each sequence consists 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
Bit
0
1
2
846/874
Table 116. 32-bit debug port registers addressed
through the shifted value A[3:2] (continued)
DP SELECT register: Used to select the current access port and the
active 4-words register window.
– Bits 31:24: APSEL: select the current AP
10 – Bits 23:8: reserved
– Bits 7:4: APBANKSEL: select the active 4-words register window on the
current AP
– Bits 3:0: reserved
DP RDBUFF register: Used to allow the debugger to get the final result
11 after a sequence of operations (without requesting new JTAG-DP
operation)
Table 117. Packet request (8-bits)
Name
Start Must be "1"
0: DP Access
APnDP
1: AP Access
0: Write Request
RnW
1: Read Request
RM0366 Rev 5
Description
Description
RM0366