This is the actual boot sequence a phone follows from 0V to CPU running.

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⭐ STEP 0 → Phone is OFF (Standby State)

• Only RTC LDO (Real-Time Clock) is ON
• PMIC monitors:
  • Power key line (PWRKEY)
  • USB VBUS (charging cable)
  • Battery voltage
  • Thermistor (NTC)
• All main rails are OFF
• CPU is in reset state

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⭐ STEP 1 → Power Key Press / Auto Boot Trigger

Trigger signals that can start the phone:

• PWRKEY pressed
• USB charger inserted
• Battery plugged in (auto-boot devices)

PMIC detects one of these and enters BOOT INIT MODE.

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⭐ STEP 2 → Pre-Power Rails (Always First)

PMIC brings up the basic rails required to “wake” the CPU.

Pre-power rails:

1. VRTC / VPH_PWR – RTC domain
2. VDD_MAIN – main battery line distributed everywhere
3. PMIC internal regulators ON

If any of these rails fail → phone dead / no reaction.

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⭐ STEP 3 → Reset & Clocks Initialization

PMIC signals:

• PS_HOLD low
• PMIC_RESET_N low
• BOOT_CONFIG pins read
• Xtal oscillator starts (19.2 MHz typical)

This provides the CPU with basic timing.

If PMIC_RESET_N never goes high → device cannot boot.

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⭐ STEP 4 → Primary Power Rails ON (CPU Power Start)

PMIC now turns ON the essential CPU rails.

Qualcomm Primary rails:

1. VDD_MX – internal logic domain
2. VDD_CX – CPU core voltage
3. VDD_PX / MDM rail – modem block
4. VDD_GFX – GPU rail
5. VDD_S1/S2 – sub power rails

If any rail is missing →
✔ 0.05–0.1A hang on DC supply
✔ no boot / no vibration

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⭐ STEP 5 → Secondary Rails ON (Memory & Interfaces)

When primary rails stabilize:

PMIC enables:

• VDDQ – RAM I/O
• VDD1 / VDD2 – LPDDR supply
• VDD_SDIO – SD / eMMC I/O
• VDD_USB – USB transceivers
• VREG_Lxx – multiple LDO rails

If RAM rails fail → 0.2–0.3A current hang
If VDDQ missing → boot loop or stuck logo

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⭐ STEP 6 → eMMC / UFS Storage Rails ON

Now the PMIC powers the storage chip.

For eMMC:

• VCC (2.8/3.0V) – NAND core
• VCCQ (1.8V/1.2V) – I/O interface
• RST_N toggles
• CMD/CLK/DATA lines active

For UFS:

• VCC (2.8/3.0V)
• VCCQ (1.2V/1.8V)
• REFCLK supplied
• RESET_N toggles

If these do not come up →
❌ dead storage, PMIC fault, or short
❌ 0.06A hang (Qualcomm common symptom)

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⭐ STEP 7 → Modem, WiFi, Bluetooth Rails

PMIC now brings up:

• RF_Power rails
• WTR / RFFE rails (modem)
• WCN rails (WiFi/Bluetooth/GPS)

Failure here →
Device boots OK but no network / no WiFi.

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⭐ STEP 8 → Display, Touch, Camera, Audio Rails

Finally, PMIC enables:

• Backlight boost (VBAT to 5–20V)
• Display LDO
• Touch IC I2C power
• Camera AVDD / DVDD / IOVDD
• Audio codec rails

If these fail →

• No display
• No backlight
• No touch
• No camera
• No audio

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⭐ STEP 9 → PS_HOLD from CPU (Very Important)

CPU sends PS_HOLD HIGH to PMIC.
This tells the PMIC:

“I am alive, keep giving me power.”

If PS_HOLD fails:
✔ Phone boots for 0.5 sec → dies
✔ Classic blink issue
✔ Classic 0.15–0.25A drop

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⭐ STEP 10 → System Running Normally

All regulators ON
Device running Android
Charging monitors active
Thermal system active

This is the full power-on ecosystem.

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🔥 COMPLETE RAIL ACTIVATION TIMELINE (IMPORTANT)

1️⃣ VRTC / VPH_PWR  
2️⃣ VDD_MAIN  
3️⃣ PMIC_RESET_N  
4️⃣ Clock (Xtal)  
5️⃣ VDD_MX / VDD_CX (CPU)  
6️⃣ VDDQ / VDD1 / VDD2 (RAM)  
7️⃣ VCC / VCCQ (eMMC/UFS)  
8️⃣ MODEM / RF Rails  
9️⃣ Display / Touch rails  
🔟 PS_HOLD High (CPU OK)