USB-C board dead on a C-to-C cable? It is missing 5.1 kΩ on the CC pins

Plenty of low-cost ESP32 boards and modules have a USB-C port, yet stay completely dead on a C-to-C cable from a phone or laptop charger, while an old A-to-C cable works fine. The cause is a missing 5.1 kΩ resistor on each of the two CC pins: the USB-C charger cannot tell anything is plugged in, so it never turns the power on.

This lesson covers how that works, how to measure it and how to fix it. Everything stays at 5 V USB power. If you are unsure about powering a board in general, read powering an ESP32 correctly alongside this.

What the CC pins are

A USB-C plug has two Configuration Channel pins, CC1 and CC2 (two because the plug is reversible). The two ends use them to identify each other before any power flows.

• The power source, such as a charger, has a pull-up resistor Rp on CC.
• The power sink, such as our board, has a pull-down resistor Rd = 5.1 kΩ to ground on CC.

When they connect, Rp and Rd form a voltage divider. The charger sees the CC voltage drop, knows a sink is attached, and only then switches VBUS on.

On a spec-compliant USB-C port with nothing attached, VBUS must be 0 V. It applies 5 V only after it detects a sink through the CC pin.

Why A-to-C works but C-to-C does not

	A-to-C cable	C-to-C cable
Power source	USB-A port, 5 V all the time	USB-C port, waits to see Rd
Where Rp is	56 kΩ to VBUS inside the cable’s plug	inside the charger
Board without Rd	still powered	0 V
Board with both Rd	powered	powered

A legacy A-to-C cable is specified to carry a 56 kΩ Rp in its plug, which tells the device it is on an ordinary USB port. A USB-A port has no CC pin and supplies power all the time anyway. So a board without Rd looks fine, right up until the day you switch to a C-to-C cable.

Worked example: how the charger reads the CC voltage

The Rp value pulled up to 5 V advertises how much current the charger offers: 56 kΩ = default USB power, 22 kΩ = 1.5 A, 10 kΩ = 3 A. With our board’s 5.1 kΩ Rd, the CC voltage is a plain divider:

V_CC = 5 V × 5.1 ÷ (Rp + 5.1)

1. Rp 56 kΩ: 5 × 5.1 ÷ 61.1 = 0.42 V, inside 0.20–0.66 V = default USB power.
2. Rp 22 kΩ: 5 × 5.1 ÷ 27.1 = 0.94 V, inside 0.66–1.23 V = up to 1.5 A.
3. Rp 10 kΩ: 5 × 5.1 ÷ 15.1 = 1.69 V, above 1.23 V = up to 3 A.
4. No Rd: CC has no path to ground, the voltage never drops, and the charger decides nothing is attached. VBUS = 0 V.

The voltage bands above come from a commonly used summary table; below 0.20 V means no source is connected. A typical ESP32 board does not need to read this at all: having Rd is enough to get 5 V. But if a board will draw more than the default current (500 mA for USB 2.0), it should read the CC voltage first to see whether the charger can supply it.

One resistor per pin, never shared

The spec gives CC1 and CC2 their own separate Rd. The Raspberry Pi 4 (2019) tied CC1 and CC2 together into a single 5.1 kΩ resistor. It worked with simple cables, but with e-marked cables (those rated for fast data or 5 A), compliant chargers could refuse to supply power. A write-up on kernel.org called this a common design mistake and advised copying the spec’s sink circuit exactly.

The correct sink circuit for a USB 2.0 board:

• CC1 → 5.1 kΩ → GND
• CC2 → 5.1 kΩ → GND (a second resistor)
• The two D+ pins (A6, B6) tied together, and the two D− pins (A7, B7) tied together, so data works in either orientation.
• SuperSpeed pins left unconnected; ESP32 chips only support USB 2.0.

Measure your board with a multimeter

Unplug the board first and set the multimeter to resistance (if that is new, see multimeter basics).

Measure between	Expected	If not
CC1 and GND	about 5.1 kΩ	open circuit = no Rd
CC2 and GND	about 5.1 kΩ	same
CC1 and CC2	about 10.2 kΩ (two in series via ground)	near 0 Ω = tied together, Pi 4 style

On a small board, find the CC test points from its schematic, or plug a USB-C breakout in to reach them. An even easier test: plug a C-to-C cable into a USB-C charger. If nothing lights up but an A-to-C cable works, the board almost certainly lacks Rd.

How to fix it

1. Use an A-to-C cable from a USB-A port. The simplest fix, no rework.
2. Add Rd. If you solder well, add 5.1 kΩ from CC1 to ground and another from CC2 to ground. The USB-C pads are tiny, so watch for bridges to neighbouring pins.
3. Designing your own board? Always fit two 5.1 kΩ 1% resistors.
4. Buying a new board? Check its schematic for separate 5.1 kΩ resistors on CC1 and CC2 before you buy.

Common mistakes

• One resistor shared by both CC pins. Works with some cables, fails with e-marked ones.
• Using 10 kΩ because it was in the drawer. The charger misreads the voltage. Use 5.1 kΩ.
• Blaming the C-to-C cable. Another cable fails the same way; the problem is the board, not the cable.
• Wiring only one D+/D− pair. Flashing works with the plug one way round, and the computer cannot see the board the other way.
• Expecting 9 V or 20 V. Rd alone gets you 5 V. Higher voltages need a PD chip, and this lesson stays at 5 V low voltage.

Sources

• How to design a proper USB-C power sink (hint, not the way Raspberry Pi 4 did it) — people.kernel.org
• USB Type-C Cable and Adapter Tips and Tricks — Chromium OS
• USB Type-C Hardware Design Guide — Espressif ESP-IoT-Solution
• AN064: USB Type-C introduction — Richtek
• Simple way to use USB Type-C to get 5V at up to 3A — DigiKey TechForum