I’ve been learning how to repair Dell laptops for the past two weeks. I previously wrote about the DC power supply input circuit and listed all the components I found, including MOSFETs like SI7121DN and FDV301N, as well as various capacitors and resistors.
At the time, I didn’t mention it, but I want to record now that I did not see any inductors in the schematic. I’ve watched several laptop repair videos on YouTube, and they often mention inductors as an important part of the DC input power section. However, in this Dell Inspiron laptop, I couldn’t find any inductor in the DC input circuit on Page 32 of the Dell laptop schematic. The download link for this schematic is at the end of this diary note.
While studying the DC input circuit, I followed the signal labels shown on the schematic and discovered that they connect to the NPCE885PA0DX I/O controller chip. That led me to ask: how laptops and PCs start up?
To remind myself:
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When the laptop receives power from the adapter or battery, various power rails are activated: +19V, +5V, +3V, +1V, etc.
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The embedded controller (NPCE885PA0DX I/O controller) powers on first.
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The I/O controller generates system-level power signals such as +3V_SUS, +5V_SUS, and SUS_ON.
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It then activates the PCH (Platform Controller Hub) chip.
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The PCH begins the BIOS boot process by reading from the SPI Flash chip.
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After that, the CPU wakes up and continues with system initialization.
Given what I’ve learned so far, I think it’s time to dig deeper into how the PCH works, especially by following the power sequence described above.
From studying the schematic, it’s clear that the PCH is the central communication hub that connects multiple key components in the laptop, acting almost like a traffic controller between the CPU and peripheral devices.
Once the PCH is activated by the embedded controller, it begins handling various system-level tasks. It enables communication with the BIOS (SPI Flash chip) to start the boot process, and then it coordinates initialization and data flow between other system components like:
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LCD Display
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Hard Drive (HDD) or SSD
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USB ports
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Audio Codec
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Camera module
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CPU (via DMI interface)
Each of these devices is connected to the PCH via specific buses or interfaces, and the PCH uses its internal logic to manage power sequencing, clock generation, signal routing, and bus arbitration.
In general, the PCH is responsible for:
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Managing I/O interfaces such as USB, SATA, and PCIe.
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Handling power state transitions (S5, S4, S3, S0) with help from the embedded controller.
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Acting as a bridge between the CPU and the rest of the system using the DMI (Direct Media Interface).
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Starting and supervising the BIOS boot process.
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Enabling connectivity with peripheral devices and managing signals like SUSCLK, PLT_RST#, and PM_SLP_S3#, which are important for system wake/sleep behavior.
To keep things organized, I plan to write separate diary notes describing how the PCH connects to and communicates with each of the following components, as shown in the Dell schematic:
- PCH to LCD (LVDS/eDP interface)
- PCH to HDD/SSD (SATA interface)
- PCH to USB ports (USB 2.0 / 3.0 lines)
- PCH to Audio Codec (HD Audio interface)
- PCH to Camera (USB 2.0)
- PCH to CPU (DMI and FDI links)
Each entry will include a summary of the connection type, purpose, and any important power or signal lines involved, as well as what happens during the boot process related to that component.
By documenting this, I hope to create a clear map of how the laptop system initializes and operates—starting from the DC input, through the embedded controller, into the PCH, and finally across all connected hardware.
