SWRU626 December 2025 CC3501E , CC3551E
Debug connections to the device are supported through an Arm serial wire debug (SWD) compliant interface.
The SWD interface requires two connections:
The SWD interface uses the standard logic levels of the device for SWD communication. See the device-specific data sheet for input and output logic levels for a given supply voltage (VIO). A SWCLK frequency of up to 20MHz is supported by the DEBUGSS.
During SWD operation, the SWDIO line can be driven high or driven low by either the target device or the debug probe. As either device can drive the line, when ownership of the shared SWDIO line is switched between the device and the debug probe, undriven time slots are inserted as a part of the SWD protocol. The primary purpose of the pull-up resistor on the SWDIO line, and the pull-down resistor on the SWCLK line, is to place the SWD pins into a known state when no debug probe is attached. A minimum resistance of 100 kΩ is recommended by Arm. The internal pull-up/pull-down resistors fulfill this requirement and external resistors are not required for correct operation of the SWD interface.
Upon physical connection of a debug probe, a configuration sequence must be sent from the debug probe to the target device to initiate a valid SWD connection with the SW-DP. Once the sequence is transmitted and the SWD connection is established, communication with enabled debug access points is possible and the bootcode is alerted through assertion of DBGSS.DBGCTL[1] SWDSEL bit which is continuously monitored in bootcode. The debug probe must be disconnected by sending disconnection sequence from the debug probe to target device.