hwtest RCM

A read-only RCM payload that probes every BPMP-side IC the Switch boot ROM hands off to, then reports identity / status / faults to the LCD and UART_B (Joy-Con right rail, 115200 8N1). Output is also captured into a heap buffer that gets written to backup/<emmc_serial>/hwtest.txt on the SD card.

The intended audience is repair techs working on Erista or Mariko consoles (handheld, V2, OLED, Lite). The output is colour-coded so a unit with a borderline reading or a real fault stands out without reading every line.

Building

The Hekate source tree is vendored as a git submodule at third_party/hekate. Initialize it before the first build:

git submodule update --init
export DEVKITARM=/opt/devkitpro/devkitARM   # or wherever yours lives
make                                        # produces both variants

make runs twice. The first pass uses DEBUG_UART_PORT=1 and produces the default build at build/hwtest.bin. The second pass uses JC_PROBE=1 and produces build-jc/hwtest_jc.bin. The two builds are mutually exclusive on the wire because the Joy-Con stack needs UART_B at 1 Mbaud with hardware flow control while the debug log uses 115200 8N1, so only one variant can run at a time.

Variant	UART_B owner	Use it when
hwtest.bin (default)	debug log @ 115200 8N1	tech is capturing UART output for diagnosis
hwtest_jc.bin	Joy-Con stack @ 1 Mbaud	tech needs Joy-Con detection / FW version

The default build is what you want 90% of the time.

Running

Inject like any other RCM payload (fusee-launcher, TegraRcmGUI, modchip, RCM jig). The unit boots into the diagnostic dump within ~3 seconds, writes the SD report, and lands on the on-LCD pager.

If the SD slot is empty / faulty / the unit was injected without a card, the payload still boots and runs. The save step is skipped cleanly with an [save] SD not ready line on UART; the LCD pager remains usable.

Key	Action
n / space / VOL+	Next page
p / b / VOL-	Previous page
r	Refresh current LCD page
a / A	Refresh ALL probes (re-emit full UART dump)
G<name>\n	Refresh just the named group (host viewer uses this)
s / S	Re-write the SD report file
q / POWER	Power off the console

What's reported

Nine logical pages (some span multiple LCD-sized sub-pages):

Page	Probes
Verdict	top-level cross-checks across SoC / fuses / charger / display / touch / battery / eMMC / thermal / USB-PD / fuel-gauge
SoC	HIDREV chip ID + major/minor
Fuses	identity + speedo / IDDQ + lot/wafer + public key + SBK/DK + KFUSE
Power & charging	MAX77620 + GPIOs + regulators + 5V + battery + charger + USB-PD + thermal + fan
Memory & clocks	LPDDR4 mode regs + CLK_RST raw + decoded rates
Storage	SD + eMMC + Switch serial + partitions + health + GPT + BOOT0/pkg1 + AutoRCM + SD content scan
Display	DSI panel ID + backlight PWM + GPIO state
Inputs	touch FW + Joy-Con rails + buttons + AC adapter
Raw state	GPIO pin census + UART debug port + reset reason + PMC scratch

Host viewer (host_tools/hwtest_viewer.py)

ImGui front-end that connects to the same UART, parses the line-oriented dump, and shows it as a clickable page list with green, yellow, or red severity colouring. The toolbar exposes the pager keys (n, p, r, s, q) plus a "Refresh group" button that sends a G<group>\n command. That refreshes just the selected group on the host side without touching the LCD.

pip install -r host_tools/requirements.txt
python host_tools/hwtest_viewer.py --port /dev/ttyUSB0
# or replay a saved capture (no console needed):
python host_tools/hwtest_viewer.py --replay path/to/uart_capture.txt

The Switch UART runs at 1.8 V. Use a level-shifter between the Tegra rail and any USB-TTL cable that doesn't natively support 1.8 V. For wiring the Joy-Con right rail as a UART tap, see this gbatemp guide: https://gbatemp.net/threads/how-to-make-a-joycon-rail-uart-to-capture-boot-log.625784/.

Layout

main.c               - probe sequence + sinks + pager + UART command loop
emmcsn.c             - Hekate-style backup/<emmc_serial>/<sub>/<file> path
diskio.c, ffconf.h   - FatFS glue + config (vendored from the Hekate bootloader)
exception_handlers.S - boot relocator (vendored from the Hekate bootloader)
gfx/                 - LCD driver (vendored from the Hekate bootloader)
stubs.c              - minerva_deinit no-op
link.ld              - link script
Makefile             - devkitARM build for both variants
host_tools/          - ImGui + pyserial UART viewer (parser, GUI, replay)

Acknowledgements

Heavy reliance on the work of:

• Hekate / BDK
• Lockpick_RCM
• switchbrew.org