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Updated 7 November 2026

Core8Bit power

Power rails, battery, wiring and the firmware power thresholds.

Safety. Solder only with the battery disconnected. Check polarity twice. Never short 5 V to 3.3 V. Do not use a damaged or swollen cell.

Contents

Rails

Rail Made by Feeds Switched by
VBAT_CELL (3.0–4.2 V) Li-Po 606090 1S through the charger / "battery driver" board boost #1 and boost #2 inputs —
5 V permanent boost #2 (step-up module) STM32 core board always on
3.3 V regulator on the core board MCU, SDRAM, QSPI, SD card, panel logic, sticks, buttons; header 3V3 (R20.1, R29.1) —
5 V switched boost #1 (step-up module) ESP32 VIN, PAM8406 VCC, 21 V backlight booster PE4: HIGH = on, LOW = off
~21 V backlight 21 V booster, fed from boost #1 panel backlight (21 V / 40 mA) follows PE4
USB 5 V USB cable charger input —

Do not use boost #2 EN as a reset or power switch. With EN LOW a plain asynchronous boost still passes about VBAT − 0.3 V to its output, so the core board stays powered. The hard reset shorts NRST to GND instead (see Mainboard plan).

PE4: the switched 5 V rail

Moment PE4
Reset LOW. Fit a 100 kΩ pull-down from boost #1 EN to GND so the rail stays off while the MCU is in reset
Cold-boot battery check LOW; the cell is measured unloaded
Audio start the DAC ramps to mid-scale first, then PE4 goes HIGH, so the amplifier does not pop
Backlight on DISP (PD5) HIGH → PE4 HIGH → picture
Sleep / shutdown picture off → DISP LOW → PE4 LOW → 200 ms → audio off

PE4 also feeds the ESP32 and the amplifier. Turning the backlight off turns them off too.

Battery

Item Value
Cell Li-Po 606090, 1S, 3800 mAh (4000 mAh is better), with a protection board (BMS)
Charger charger / "battery driver" board (see bom.md)
Fuel gauge none; the firmware measures voltage only

How the battery is measured

  • The firmware reads the MCU VBAT pin (core-board header BAT, L22.1) through the MCU's internal /4 divider, referenced to the internal VREFINT.
  • It subtracts a fixed 250 mV offset for the drop between the cell and the VBAT pin.
  • The VBAT pin is rated for 3.6 V maximum; a full cell is 4.2 V. Do not connect the cell straight to BAT. The circuit between the cell and BAT is TBD.

Power policy

Situation Threshold Behaviour
Cold boot 10 % below 10 % the console stays dark and measures again every 20 s, on a MENU press, or when the charger is plugged in
Wi-Fi 15 % below 15 % the ESP32 sleeps; it comes back by itself when the charge recovers
Low warning 10 % 5 s banner and a red battery icon
Blink 5 % the battery icon blinks
Auto-sleep 5 %, or 3050 mV, held for 5 s the console sleeps. Never while charging
Wake from sleep 6 % below 6 % the console does not wake

0 % = 3050 mV. The cold-boot check is there because starting the 5 V rail on a flat cell trips the cell's BMS.

Charging detection

  • On the wired build PA9 follows the 5 V rail and is not used. The firmware detects charging from a voltage rise of ≥ 180 mV over about 12 s.
  • On the mainboard PA9 measures the USB-C VBUS through a divider, see mainboard.md. In OTG mode PA9 also sees 5 V. The microphone moves from PA9 to another free ADC input (TBD).

Wiring

USB (charge) ─► charger / "battery driver" board ◄─► Li-Po 606090 1S
                          │ OUT+
                          ├─► boost #2 (always on) ─► 5 V ─► core board (5V pin or USB: TBD) ─► on-board 3.3 V
                          │                                     └─ 3V3 headers: panel logic, sticks, buttons
                          └─► boost #1 (EN ◄─ PE4, header R24.2) ─► 5 V switched ─┬─► ESP32 DevKit VIN
                                                                                 ├─► PAM8406 VCC
                                                                                 └─► 21 V booster ─► panel backlight
core-board header BAT (L22.1) = MCU VBAT (max 3.6 V)

Step-up module used for boost #1 and boost #2

From To Notes
Li-Po + / − charger board battery pads check polarity twice
charger OUT+ / − boost #1 IN+/IN−, boost #2 IN+/IN− 22–24 AWG, from one point (star)
boost #2 O+ / O− core board 5 V / GND always on
STM32 PE4 (R24.2) boost #1 EN active HIGH; 100 kΩ pull-down to GND. The module has no labelled EN pad: use the EN/SHDN pin of its SO-8 controller
boost #1 O+ / O− ESP32 VIN/GND, PAM8406 VCC/GND, 21 V booster input 22–24 AWG
GND common star point

Mainboard plan: single USB-C and hard reset

In development (mainboard.md). The wired build does not have this. Values and pins are TBD.

USB-C VBUS ─┬─► SS34 / ideal diode (P-MOSFET) ─► charger board input ◄─► Li-Po 606090 1S
            │                                      │ CE ◄─ charging off during OTG (TBD)
            ├─► divider ─► PA9 (VBUS sense)        ├─► boost #2 ─► STM32 (NRST ◄─ reset button → GND)
            │                                      └─► boost #1 (EN ◄─ PE4) ─► ESP32, PAM8406, 21 V
            └─◄─ P-MOSFET switch ◄─ 5 V of boost #1 (PE4)
                     ▲ OTG_EN (GPIO, TBD)
CC1/CC2 ─► TUSB320 (DRP) ─► INT/ID ─► GPIO (TBD)
Block Design
Charging input VBUS → SS34 (Schottky) or P-MOSFET ideal diode → charger board input. Stops 5 V flowing back into the connector
OTG power P-MOSFET switch from the boost #1 5 V rail to VBUS, driven by a GPIO. Works only while PE4 = HIGH
Charge disable the charger's CE is off during OTG, so the battery does not charge from its own boost
CC TUSB320 in DRP mode; INT/ID output to a GPIO. Do not use fixed 5.1 kΩ (sink-only) or 56 kΩ (host-only) resistors
VBUS sense PA9 through a divider
Hard reset SMD micro button on the back, under a paperclip hole in case-back. It shorts the core board's NRST (RST) pin to GND, with a 100 nF capacitor to GND against bounce. Releasing it restarts the STM32

PE4 LOW does not fully unpower the 5 V loads with a plain asynchronous boost (VBAT − 0.3 V passes through). For a full cut-off use a boost with load disconnect or a switch on its output.

Known issues

Symptom Cause Fix
Amplifier pop at reset PE4 floats HIGH during reset 100 kΩ pull-down on boost #1 EN
Console reboots at start-up on a flat battery 5 V inrush trips the cell's BMS charge the battery; cold boot needs 10 %
Charging icon unreliable PA9 follows the 5 V rail expected on the wired build; charging is detected from the voltage rise