how laptops and PCs start up?

While learning how to repair my laptop I started with the DC power input jack. Then I studied this power supply circuit and the components used described in my note Dell Inspiron Power Supply Revisted For Repair. I collected the information on the MOSFETs, capacitor and resistors used in that power supply circuit. But I realized that I should know how to power is applied to the various chips, which chip turns on first, and the sequence in which the various chips are activated. I think one has to know this before starting to repair a laptop motherboard and this note is about that.

Laptop Motherboard Power-Up Block Diagram

When power is first applied to a laptop or desktop motherboard, the power-up sequence is carefully coordinated, and yes—the I/O controller (Embedded Controller or EC) is often the first active chip in the chain.

🧠 Power-Up Sequence Overview (Modern Intel-Based System)

🔋 Step 1: Power applied (via adapter or battery)

Power rails like +3V, +5V, +19V appear.
Always-on power rail (e.g., +3V_ALW) powers the EC (Embedded Controller / I/O Controller).

⚙️ Step 2: EC/I/O Controller powers up

The Embedded Controller (like NPCE885PA0DX) comes alive.
It checks power button, battery status, charger status, lid state, etc.
It generates system power signals like +3V_SUS, +5V_SUS, and SUS_ON.

🔄 Step 3: EC enables PCH power

If conditions are okay (e.g., power button pressed), EC signals the PCH to begin its power-up.
The EC sends SLP_S3#, SLP_S5# and other power state signals to the PCH.
EC may also enable buck converters or linear regulators that supply power to the PCH and CPU.

🧠 Step 4: PCH starts

Once powered, the PCH asserts PLTRST#, initializes internal logic, and starts communication with the EC.
The PCH then begins the system firmware (BIOS/UEFI) boot process by accessing the SPI flash chip.

🔥 Step 5: CPU wakes up

After BIOS is read and initialized, the PCH sends signals (PWRGOOD, INIT#, etc.) to wake the CPU.
The CPU begins executing instructions from the BIOS (typically at reset vector address 0xFFFFFFF0).

📌 Summary of Signal/Power Flow

Power-Up Step	Target Chip	Notes
Power applied	EC / I/O Controller	+3V_ALW powers EC immediately
EC powers system rails	EC → PMIC → regulators	Enables +VCC_CORE, +1.05V, etc.
PCH wake-up	EC → PCH via `SLP_S5#`	PCH gets power and reset released
CPU activation	PCH → CPU	BIOS/UEFI starts executing
System boots	CPU + PCH	Bootloader loads OS

🔄 Step-by-Step Explanation

Step	Component	Role
1️⃣	+3V_ALW Rail	Always-on rail powers EC as soon as power is available.
2️⃣	EC / I/O Controller	First logic chip to wake up. Monitors power button, lid, charger.
3️⃣	EC Enables Main Rails	Signals power management to turn on main voltages: +VCC_CORE, +1.05V, etc.
4️⃣	PCH Powers Up	Receives power and reset, starts reading BIOS firmware from SPI flash.
5️⃣	PCH Wakes CPU	After init, PCH deasserts reset to CPU. CPU begins executing code.
6️⃣	CPU + RAM	BIOS initializes memory, peripherals. OS boot begins.

🔧 Key Signals and Rails

+3V_ALW: Always-on voltage for EC
SLP_S3#, SLP_S5#: Sleep state controls from EC to PCH
PLTRST#: Platform Reset (PCH to CPU)
PWRGOOD, VR_ON: Power-good signals from EC/PMIC to other devices
CLKREQ#: Clock request from PCH to clock generator

📘 More on EC Role

The EC (Embedded Controller) often handles:

Battery management
Fan speed control
Lid open/close detection
Keyboard scanning
Power sequencing (timing of power rails)
Communication with PCH (LPC, SMBus)
The Dell inspiron laptop uses NPCE885PA0DX - I/O controller EC chip.

🧠 Analogy to Microcontrollers

The EC is like a low-power microcontroller (MCU) with firmware, typically based on 8051 or ARM Cortex-M0/M3 cores.
It performs autonomous system management before CPU ever starts.
Similar to an ATmega328P, but dedicated to system-level coordination.