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9.5. SW-DP

This section describes the Serial Wire Debug Port (SW-DP) interface. In particular, it describes the Serial Wire Debug (SWD) protocol and how this protocol provides access to the debug port registers. These registers are described in detail in DAP programmer’s model.

The SW-DP operates with a synchronous serial interface. This uses a single bidirectional data signal and a clock signal.

Each sequence of operations on the wire consists of two or three phases:

Packet request

The external host debugger issues a request to the debug port. The debug port is the target of the request.

Acknowledge response

The target sends an acknowledge response to the host.

Data transfer phase

This phase is only present when either:

  • a data read or data write request is followed by a valid (OK) acknowledge response

  • the ORUNDETECT flag is set to 1 in the CTRL/STAT Register, see Control/Status Register, CTRL/STAT.

The data transfer is one of:

  • target to host, following a read request (RDATA)

  • host to target, following a write request (WDATA).


If the Overrun Detect bit in the CTRL/STAT Register is set to 1, then a data transfer phase is required on all responses, including WAIT and FAULT. For more information, see Sticky overrun behavior.

For details of the CTRL/STAT Register see Control/Status Register, CTRL/STAT.

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