I'm talking about a USB type-C male to USB type-A female adapter, the ones you need to connect a USB flash drive to the latest MacBooks.

USB-C adapter

According to USB-C specification, this sort of adapter needs a 5.1 kΩ pull-down resistor on type-C plug's A5 pin (configuration channel). This is needed, because the host device won't supply power until it detects a connection on CC.

However, I have seen some such adapters boasting a 56 kΩ pull-up resistor, which would be needed for a type-C male to type-A male adapter, but as far as I know, not for the kind I'm talking about in this question. Why would they include this resistor? It was marketed as a safety feature.

  • 3
    \$\begingroup\$ Because you don't want to accidentally connect +5.1V from one host to +4.9V from the other. \$\endgroup\$ – Janka Jul 3 '19 at 10:31


Adapters with Type-C male to Type-A female connections advertising a 56 kΩ resistor value are wrong. The resistor tells the Type-C downstream-facing port (DFP) whether the connected device is sourcing or sinking current. The presence of a female Type-A receptacle means that the adapter should use a 5.1 kΩ pull-down resistor to indicate that the device (whatever is plugged into the Type-A receptacle) is going to sink current.

I realized as I was researching this, that I have completely gotten it backward in the first version of this answer (now deleted), which is also what I think also happened in the case of adapters advertising the incorrect resistance values! Re-reading the specification several times, I've come to the conclusion that my initial approach was wrong. I've had to work with USB Type-C a little bit in a current project, and it's still pretty new to me. Anyway, this answer should now be correct.

USB Type-C introduces the concept of dual-role-power so devices, such as a battery, that can deliver (source) as well as accept (sink) current, can use a single connector.

A USB 2.0 type battery pack, for example has both a type-A female and a micro-B female connector:

Legacy USB 2.0 battery bank

The USB Type-C specifications (available at USB.org as "Universal Serial Bus type-C Cable and Connector Specification") explain different resistor values for determining whether a device is sourcing or sinking current.

Sourcing Table 4-24 (page 235):

Values used for pull-up \$R_p\$:

USB-C Source CC Termination Resistor Value Table

Sinking Table 4-25 (page 236):

Values used for pull-down \$R_d\$:

USB-C Sink CC Termination Resistor Value Table

Page 85 specifically covers USB Type-C to USB 3.1 Standard-A Receptacle assemblies (the adapter shown in the question):

Note 1. Pin A5 (CC) of the USB Type-C plug shall be connected to GND through a resistor \$R_d\$ (5.1 kΩ ± 20%).

If you're not careful (as I wasn't originally, and I expect the adapter makers also screwed up), you could look at Page 77 or 78 which show the specifications for USB Type-C plug to USB 3.1 and 2.0 Standard-A plugs. For these, the resistor is a pull up of 56 kΩ.

  • \$\begingroup\$ @endolith The line wasn't meant to imply that the text above it was the first version, just to separate the current short answer from the longer narrative about the revisions I'd made getting to it. Thanks! \$\endgroup\$ – JYelton Apr 11 '20 at 21:45

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