Debug and trace

An external debugger can access the device via the DAP.

The debug access port (DAP) implements a standard ARM® CoreSight™ serial wire debug port (SW-DP), which implements the serial wire debug protocol (SWD). SWD is a two-pin serial interface, see SWDCLK and SWDIO in Debug and trace overview.

In addition to the default access port in CPU (AHB-AP), the DAP includes a custom control access port (CTRL-AP). The CTRL-AP is described in more detail in CTRL-AP - Control access port.

Access port protection blocks the debugger from read and write access to all CPU registers and memory-mapped addresses when enabled.

Access port protection is enabled and disabled differently depending on the build code of the device.

Access port protection controlled by hardware and software

This information refers to build codes Fxx and later.

By default, access port protection is enabled.

Access port protection is disabled by issuing an ERASEALL command via CTRL-AP. Read CTRL-AP.APPROTECTSTATUS to ensure that access port protection is disabled, and repeat the ERASEALL command if needed. This command will erase the flash, UICR, and RAM. CTRL-AP is described in more detail in CTRL-AP - Control access port. Access port protection will remain disabled until one of the following occurs:

• Pin reset
• Power or brownout reset
• Watchdog reset if not in Debug Interface Mode, see Debug Interface mode
• Wake from System OFF if not in Emulated System OFF

To keep access port protection disabled, the following actions must be performed:

• Program UICR.APPROTECT to HwDisabled. This disables the hardware part of the access port protection scheme after the first reset of any type. The hardware part of the access port protection will stay disabled as long as UICR.APPROTECT is not overwritten.
• Firmware must write APPROTECT.DISABLE to SwDisable. This disables the software part of the access port protection scheme.

Note: Register APPROTECT.DISABLE is reset after pin reset, power or brownout reset, watchdog reset, or wake from System OFF as mentioned above.

The following figure is an example on how a device with access port protection enabled can be erased, programmed, and configured to allow debugging. Operations sent from debugger as well as registers written by firmware will affect the access port state.

Access port protection is enabled when the disabling conditions are not present. For additional security, it is recommended to write Enabled to UICR.APPROTECT, and have firmware write Force to APPROTECT.FORCEPROTECT. This is illustrated in the following figure.

Note: Register APPROTECT.FORCEPROTECT is reset after any reset.

The control access port (CTRL-AP) is a custom access port that enables control of the device when other access ports in the DAP are disabled by the access port protection.

Access port protection is described in more detail in Access port protection.

Control access port has the following features:

• Soft reset - see Reset for more information
• Disabling of access port protection - device control is allowed through CTRL-AP even when all other access ports in DAP are disabled by access port protection

Before an external debugger can access either CPU's access port (AHB-AP) or the control access port (CTRL-AP), the debugger must first request the device to power up via CxxxPWRUPREQ in the SWJ-DP.

If the device is in System OFF when power is requested via CxxxPWRUPREQ, the system will wake up and the DIF flag in RESETREAS will be set. The device is in the Debug Interface mode as long as the debugger is requesting power via CxxxPWRUPREQ. Once the debugger stops requesting power via CxxxPWRUPREQ, the device is back in normal mode. Some peripherals behave differently in Debug Interface mode compared to normal mode. These differences are described in more detail in the chapters of the peripherals that are affected.

When a debug session is over, the external debugger must make sure to put the device back into normal mode since the overall power consumption is higher in Debug Interface mode than in normal mode.

For details on how to use the debug capabilities, read the debug documentation of your IDE.

The nRF52840 supports real-time debugging.

Real-time debugging allows interrupts to execute to completion in real time when breakpoints are set in Thread mode or lower priority interrupts. This enables developers to set breakpoints and single-step through the code without the risk of real-time event-driven threads running at higher priority failing. For example, this enables the device to continue to service the high-priority interrupts of an external controller or sensor without failure or loss of state synchronization while the developer steps through code in a low-priority thread.

The device supports ETM and ITM trace.

Trace data from the ETM and the ITM is sent to an external debugger via a 4-bit wide parallel trace port interface unit (TPIU), see TRACEDATA[0] through TRACEDATA[3] and TRACECLK in Debug and trace overview.

In addition to parallel trace, the TPIU supports serial trace via the serial wire output (SWO) trace protocol. Parallel and serial trace cannot be used at the same time. ETM trace is only supported in Parallel Trace mode, while ITM trace is supported in both Parallel and Serial Trace modes.

For details on how to use the trace capabilities, read the debug documentation of your IDE.

TPIU's trace pins are multiplexed with GPIOs. SWO and TRACEDATA[0] use the same GPIO. See Pin assignments for more information.

Trace speed is configured in register TRACECONFIG. The speed of the trace pins depends on the DRIVE setting of the GPIOs that the trace pins are multiplexed with. Only S0S1 and H0H1 drives are suitable for debugging. S0S1 is the default DRIVE setting at reset. If parallel or serial trace port signals are not fast enough with the default settings, all GPIOs in use for tracing should be set to high drive (H0H1). The DRIVE setting for these GPIOs should not be overwritten by firmware during the debugging session.