A single USB-C port on a laptop can charge the battery, power accessories, carry USB data at several speeds, output video and, on some machines, run Thunderbolt. In the schematic, that one port fans out into several pages: the connector itself, protection, a Power Delivery controller, power switches for both directions, and a chain of signal switches and conditioners. This guide shows how those pages fit together and how to read them when a USB-C port will not charge or will not carry data.
The building blocks
| Block | Job | Typical location in the schematic |
|---|---|---|
| Connector | 24 contacts carrying power, configuration and data | USB-C connector page |
| Protection | ESD and surge protection on every line | Next to the connector |
| PD controller | Detects attachments, negotiates power, controls switches | Its own page |
| Sink switches | Let power in from a charger | Near the PD controller or charger |
| Source switch | Supplies power out to accessories | Near the connector |
| Mux / redriver / retimer | Routes and conditions high-speed lanes, handles alternate modes | Its own page |
| Thunderbolt controller (some laptops) | Thunderbolt and USB4 functions | Its own pages |
The connector type is described under usb c.
The connector page
A USB-C receptacle has two rows of contacts, A and B, so the plug works either way up. The schematic shows them all:
- VBUS — several pins, the power line;
- GND — several pins;
- CC1 and CC2 — configuration channels: attachment detection, orientation and Power Delivery messages, described under cc pins;
- D+ and D− — the USB 2.0 pair, on both rows, usually joined on the board;
- TX and RX pairs — high-speed lanes for USB 3, USB4 or DisplayPort;
- SBU1 and SBU2 — sideband use, for example the DisplayPort AUX channel in alternate mode.
The connector page usually also shows the protection parts: TVS diodes on VBUS, ESD arrays on the data lines, and sometimes dedicated protection chips for CC and SBU, which must survive accidental contact with VBUS on a damaged plug.
The PD controller page
The PD controller is the brain of the port. Its page shows:
- supply pins — often both a system supply and a supply derived from VBUS, so the controller can work with a flat battery;
- CC1 and CC2 connections;
- gate-drive outputs for the VBUS sink and source switches;
- a host interface — I²C to the EC and sometimes to the chipset;
- a firmware source — a small SPI flash on some designs, internal memory on others;
- GPIOs controlling muxes, retimers and other parts of the port.
The controller is described under pd controller.
The power switches
Sink path
When a charger is connected, power flows from VBUS through one or two MOSFETs into the laptop's input rail. They are usually back-to-back N-channel MOSFETs driven by the PD controller, or a dedicated switch chip. Their output joins the charger IC's input, often alongside the barrel-jack input on laptops that have both. The whole path from port to charger is followed in The USB-C Power Path on Laptops: From Port to Charger IC.
Source path
When the laptop powers an accessory, a smaller switch connects a 5 V rail to VBUS. It usually includes current limiting and a fault output.
High-speed signal pages
The TX and RX pairs go through one or more of:
- a mux that selects between USB and DisplayPort and handles the plug's orientation;
- a redriver or retimer that restores signal quality over the board's traces, described under retimer redriver;
- a Thunderbolt or USB4 controller on laptops with those features.
These chips have their own supplies, enables, resets and configuration — often controlled by the PD controller or the EC — and some have their own firmware.
Power roles and the dead-battery case
A USB-C port on a laptop can act as a power sink (charging), a power source (powering accessories) or both at different times. The PD controller decides which, based on what is attached and what the laptop needs. One situation deserves special attention in the schematic: the dead-battery case. With a flat battery and no other power, the PD controller must start from VBUS alone, negotiate a contract and switch the sink path on before anything else on the board has power. The schematic shows how this is done — usually a small regulator or a dedicated pin fed from VBUS, and sometimes a hardware default that enables the sink switches before firmware runs. If that path is broken, the laptop charges when the battery has some charge but never revives a flat one.
Reading the section for a charging fault
- On the connector page, note the VBUS, CC1 and CC2 nets and their protection parts.
- On the PD controller page, check its supplies — including the one derived from VBUS — and its CC connections.
- Follow the sink-switch gate drives to the MOSFETs and their output to the input rail.
- Note the I²C link to the EC; some designs need it before enabling charging.
- Check whether the controller loads firmware from a flash chip.
That gives you the measurement list for USB-C Laptop Not Charging: PD Controller Diagnosis.
Reading the section for a data or video fault
- Trace the D+ and D− pair from the connector to the chipset or a hub.
- Trace the TX and RX pairs through the mux and retimer.
- Check those chips' supplies, enables and reset lines.
- Note which signals the PD controller drives to select modes.
Firmware in the USB-C chain
Several parts of the chain may run firmware: the PD controller, a Thunderbolt or USB4 controller, and some retimers. The schematic shows where each loads it from — a dedicated flash chip, the main BIOS flash or internal memory. When a port misbehaves after an update, or behaves differently from an identical port, firmware is a real suspect alongside hardware.
Multiple ports
When a laptop has two or more USB-C ports, the schematic may use one PD controller per port or one controller for two ports. Shared parts mean shared failures: if both ports stop charging at once, look at what they share.
Example: a port that charges but carries no data
A laptop's USB-C port charges normally but no device connected to it is ever detected. On the connector page, the D+ and D− pairs from both contact rows join before an ESD array and continue to the chipset. With power removed, diode-mode readings on D+ and D− at the connector are OL, while the same lines on the other USB-C port read normally. Following the net in the boardview, continuity stops at the ESD array, which is cracked — the port had been levered sideways. A new ESD array and a reflowed connector restore data.
FAQ
Why does the PD controller need a supply from VBUS?
So it can negotiate power when the battery is flat and no other supply exists.
What are SBU pins for?
Sideband signals, for example the DisplayPort AUX channel when the port outputs video.
Why are there two sets of D+ and D− pins?
So the plug works either way up. On the board they are usually joined.
Does every USB-C port have a retimer?
No. Retimers and redrivers appear where the signal path is long or the speed is high, such as Thunderbolt ports.
Can one PD controller handle two ports?
Some can. The schematic shows whether ports share a controller.
Bottom line
A USB-C port in a schematic is a connector, protection, a PD controller, sink and source switches and a chain of signal parts. For charging faults follow VBUS, CC, the controller's supplies and the sink switches; for data and video faults follow the D± pair and the high-speed lanes through muxes and retimers. Shared parts between ports explain faults that affect more than one.