I'm having difficulties anlysing the USB Type-C Spec R2, section "USB Type-C Current" paragraph. It is said that "When a Source is advertising USB Type-C Default current, the Sink behavior is defined as follows: [...] It attaches as a USB Type-C Power Sinking Device (PSD), after which the Sink may draw up to 500 mA. ". I'm not sure if the USB Type-C default current advertised is referring to the USB 2.0/3.x:0.5/0.9A (which I think is the case given the previous paragraph) or the 1.5/3A usb type-c current.

The former interpretation would be a violation of the USB 2.0/3.x, since you can't draw more than 100mA without enumeration. The latter would mean that every website saying you only need to monitor the CC lines to draw up to 0.1/1.5/3A depending on the state of CC is wrong, and I would also not understand the whole point of the CC current advertisement.

Given the use of "default" in the previous and next paragraphs I believe it's the former interpretation. Am I wrong to believe there's a violation?


1 Answer 1


Type-C specifications define not only "USB devices" but also PSD - Power Sinking Devices. These are devices that don't have USB functionality, nor alternate mode functionality. The specs even has an explicit example - USB light (see P.27). Obviously the USB specifications do not apply to PSDs, and they cannot follow the limit of non-enumerated USB devices.

Therefore, there is no contradiction nor violation of anything.

  • \$\begingroup\$ Allow me to expand on that. There's 3 data paths inside USB-C that can all act independently. There's USB 2.0, the 4 "superspeed" lanes, and USB-PD. There's 3 means to manage power transfer, USB 2.0/3.x, USB-BC, and USB-PD. USB-C devices may use more than 1 to negotiate power. As they all start in the same place of the host sourcing 5 V the host can support any 1, 2, or all 3. These 3 data paths and 3 power protocols are not completely independent, only 1 power protocol can be active at once. \$\endgroup\$
    – MacGuffin
    Feb 22, 2021 at 9:51
  • \$\begingroup\$ @Ale..chenski So you're saying it's not a violation, because it's not a USB device but a PSD ? So for example a PSD can draw 0.5A on an old computer usb 2.0 port ? That also means that a PSD can draw 0.5A in any case without checking CC lines since it can draw 0.5A for legacy 2.0/3.x ports and 1.5A/3A on true usb-c ports depending on CC state. \$\endgroup\$ Feb 22, 2021 at 21:43

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