While Mayhem firmware is running, a short press of the DFU button cycles through two diagnostic overlays and then hides them again:

DFU press	Result
1st	Show Performance overlay
2nd	Show Radio Settings overlay
3rd	Hide overlay

The overlay is drawn on top of the current screen without leaving the active app. Values update approximately once per second.

Note

The fields shown in both overlays are defined directly in the firmware source (ui_dfu_menu.hpp) and depend on the shared memory structure between the two processors (portapack_shared_memory.hpp). They may change between firmware versions if either file is modified. The table below reflects the current source code.

Overlay 1 — Performance

Displays CPU and memory diagnostics for both processors.

Field	Description
M0 core	M0 free core memory (bytes)
M0 heap	M0 free heap memory (bytes)
M0 frags	M0 heap fragment count
M0 stack	M0 free stack space (bytes)
M0 cpu %	M0 CPU utilisation (%)
M4 heap	M4 heap usage (bytes)
M4 stack	M4 stack usage (bytes)
M4 cpu %	M4 CPU utilisation (%)
M4 miss	M4 baseband buffer miss count
Uptime	Device uptime (seconds)

Overlay 2 — Radio Settings

Displays the current live radio configuration for both receiver and transmitter models.

Field	Description
RX Freq	Receiver target frequency (Hz)
RX BW	Receiver baseband bandwidth (Hz)
RX SampR	Receiver sampling rate (Hz)
RX Satu%	M4 saturation percentage
Modulatn	Modulation index
AM cfg	AM configuration index
NBFM cfg	Narrowband FM configuration index
WFM cfg	Wideband FM configuration index
TX Freq	Transmitter target frequency (Hz)
TX BW	Transmitter baseband bandwidth (Hz)
TX SampR	Transmitter sampling rate (Hz)

Using the DFU button to flash firmware

The DFU button serves a second, separate purpose: entering DFU (Device Firmware Upgrade) mode for flashing the HackRF base firmware from a PC. This only works before the Mayhem firmware has fully loaded — pressing DFU while the firmware is already running will only toggle the overlay described above.

To enter DFU mode for flashing, try one of these methods while the device is not yet booted into Mayhem:

• Hold DFU, plug in the USB cable, then release DFU.
• Hold DFU and RESET simultaneously, release RESET first, then release DFU.
• Hold DFU, power on the device (press the knob on H1/H2, or use the power switch on H4M), release DFU, then plug in USB.

When in DFU mode, the LEDs will be on and the screen will remain blank.

See Full-reset and Update-firmware-troubleshooting for full flashing instructions.