3 ms·

The A-to-C cable often does not work because the resistors are supposed to be in there. So if you are having complete charge failures, try a different cable.

megous 4mo ago
A-C cable assembly always works, CC signal is connected within the cable to Vbus via 56kOhm resistor, but that's only relevant to the downstream port, not to the upstream USB-A power sourcing port which does not have access to the CC signal. Upstream port provides power unconditionally within some limits depending on port type (CDP/DCP/USB3.0/2.0 data port/...).
- exmadscientist 4mo ago
  That's how it's supposed to work, yeah. But there is some trash out there in the world. A lot of it, actually. Some naughty cables work with some naughty chargers work with some naughty devices. Postel's Law in action, I guess? Usually the best place to fix it is by getting rid of the bad cables. Usually.
  - mschuster91 4mo ago
    > Usually the best place to fix it is by getting rid of the bad cables. Usually. No. There is no USB-C to C cable that will charge a badly implemented device with a standards compliant charger. That is the entire point. An USB A to C cable is completely standards-compliant and safe, even if it always supplies 5V on the C end - any standards compliant USB-C device should not activate the MOSFET on its Vbus line unless it successfully negotiates via CC.
    - seba_dos1 4mo ago
      They mean bad USB-A to C cables with no resistor on CC line. Of course this is broken junk which will work with some devices and won't with others. I've also seen cables with resistors on both CC lines, which is also broken but in a slightly subtler way.
      - exmadscientist 4mo ago
        Right. That phrase "standards-compliant" in the above comments is doing a lot of heavy lifting. A lot of devices are not actually standards-compliant. Some are close. (This may actually be worse.) My experience has been that if the source and sink are broken, they are often hilariously badly broken and it is pretty easy to figure out that they are the problem, if not quite exactly what they've done wrong. But if things are flaky and weird and don't really make sense, it's probably the cable. Try a known-really-seriously-actually-standards-compliantly-good cable and many problems go away, even if the source and sink aren't perfect. (Many sources and sinks aren't standards-compliant because, even though they easily could be, they're trying to work around the other end not being standards-compliant itself, because that's what you've got to do to sell a product. So they're close but not quite there. This is not always ideal.)
      - stephen_g 4mo ago
        But it’s not what anyone was talking about. Such a cable should be really quite rare because it’s unlikely to work at all in most situations, whereas devices with USB-C ports that don’t work with PD chargers (due to a cent’s worth of missing Rd resistors) are irritatingly common, because they do work with USB-C to A cables!
        seba_dos1 4mo ago
        Huh? It seems clear to me that this is what exmadscientist was talking about, any other interpretation just doesn't make sense. And no, such cables would still work in plenty of cases. You usually get them by having them bundled with devices they do work well with. In fact, they always work fine with the kind of devices you mention. These cables aren't as common as USB-C-shaped junk that's missing resistors on the receptacle, but I stumbled upon them anyway and I didn't really try to.
    - megous 4mo ago
      You may wish to re-read the type-c specification, especially 4.5.2.2.3.2: "A USB 2.0 only Sink that doesn’t support accessories and is self-powered or requires only default power and does not support USB PD may transition directly to Attached.SNK when V BUS is detected." or 4.5.2.2.5: "A port that entered this state directly from Unattached.SNK due to detecting V BUS shall not determine orientation or availability of higher than Default USB Power and shall not use USB PD." or 4.5.2.2.11.2: "The port shall transition to Attached.SNK after tCCDebounce if or when V BUS is detected." CC detection, let alone PD negotiation is not needed. You can draw up to 2.5W right away from Vbus and be standard compliant without wiring anything to CC signals. Of course if you try this with a DRP device like a smartphone, you'll get no power. But that's not really an issue for type-c chargers or USB A-C cable assemblies.
deleted 4mo ago
[deleted]
ssl-3 4mo ago
That's close, but it's not quite complete. There seems to be lots of confusion here, and that's natural: It is confusing. For just-getting-power from a USB A port into a USB C peripheral: There are supposed to be 2 resistors in the peripheral device [always], and also 1 resistor within the cable for USB-to-legacy cables[1]. That's 3 resistors, total, to get a relatively dumb USB-C equipped peripheral device to reliably charge from both USB A and USB C hosts/chargers/whatevers: The cable itself: It gets an internal 56k pullup resistor between Vbus and USB C pin A5 -- which is the CC line [yes singular]). This resistor signifies the capabilities of the host/charger/whatever for devices that care (some do care, some do not care). The peripheral: This minimally needs two pulldown resistors [commonly 5.1k], between each of CC1 and CC2 [yes a plurality] and ground[2]. This tells a compliant USB C host/charger/whatever "It's OK! Send the juice juice!" regardless of connector orientation. [1]: https://www.usb.org/sites/default/files/USB%20Type-C%20Spec%20R2.0%20-%20August%202019.pdf https://www.usb.org/sites/default/files/USB%20Type-C%20Spec%... section 3.5 [bleh]: Again, it is a confusing thing. Nobody said that dealing with such flexible, ambidextrous connections would be simple. CC performs a lot of different tasks: It can be a bidirectional serial bus for active PD negotiations, and/or a resistor network for passively dealing with power, and it's the bit that performs detection of cable orientation for applications where that matters, and it probably does other stuff too. That single little wire is clever AF. It'd be simpler to use multiple wires instead of just one, but that would take more copper. Copper is expensive, and we each save a tiny bit of money (or a large pile of money globally) by using less copper instead of more of it.
- exmadscientist 4mo ago
  My point, which matches what you've written out in more detail, is that there's supposed to be stuff (resistors) in the cable, stuff which is often not there. There are plenty of these abhorrent cables in the wild. And when you come across one, things get weird, because some sources and sinks deviate from the standard in ways that make them work with these bad cables. Others don't do that. (I believe there were a lot more abhorrent cables in the early days, and thus began the chain of accommodation....) So unless your cable is known-good, if you are having trouble, trying a different cable should be the first thing you do. It really does often get things working. Contrarily, if you have identified a naughty cable, it should be immediately widlarized.

megous 4mo ago

A-C cable assembly always works, CC signal is connected within the cable to Vbus via 56kOhm resistor, but that's only relevant to the downstream port, not to the upstream USB-A power sourcing port which does not have access to the CC signal. Upstream port provides power unconditionally within some limits depending on port type (CDP/DCP/USB3.0/2.0 data port/...).

exmadscientist 4mo ago
That's how it's supposed to work, yeah. But there is some trash out there in the world. A lot of it, actually. Some naughty cables work with some naughty chargers work with some naughty devices. Postel's Law in action, I guess? Usually the best place to fix it is by getting rid of the bad cables. Usually.
- mschuster91 4mo ago
  > Usually the best place to fix it is by getting rid of the bad cables. Usually. No. There is no USB-C to C cable that will charge a badly implemented device with a standards compliant charger. That is the entire point. An USB A to C cable is completely standards-compliant and safe, even if it always supplies 5V on the C end - any standards compliant USB-C device should not activate the MOSFET on its Vbus line unless it successfully negotiates via CC.
  - seba_dos1 4mo ago
    They mean bad USB-A to C cables with no resistor on CC line. Of course this is broken junk which will work with some devices and won't with others. I've also seen cables with resistors on both CC lines, which is also broken but in a slightly subtler way.
    - exmadscientist 4mo ago
      Right. That phrase "standards-compliant" in the above comments is doing a lot of heavy lifting. A lot of devices are not actually standards-compliant. Some are close. (This may actually be worse.) My experience has been that if the source and sink are broken, they are often hilariously badly broken and it is pretty easy to figure out that they are the problem, if not quite exactly what they've done wrong. But if things are flaky and weird and don't really make sense, it's probably the cable. Try a known-really-seriously-actually-standards-compliantly-good cable and many problems go away, even if the source and sink aren't perfect. (Many sources and sinks aren't standards-compliant because, even though they easily could be, they're trying to work around the other end not being standards-compliant itself, because that's what you've got to do to sell a product. So they're close but not quite there. This is not always ideal.)
    - stephen_g 4mo ago
      But it’s not what anyone was talking about. Such a cable should be really quite rare because it’s unlikely to work at all in most situations, whereas devices with USB-C ports that don’t work with PD chargers (due to a cent’s worth of missing Rd resistors) are irritatingly common, because they do work with USB-C to A cables!
      - seba_dos1 4mo ago
        Huh? It seems clear to me that this is what exmadscientist was talking about, any other interpretation just doesn't make sense. And no, such cables would still work in plenty of cases. You usually get them by having them bundled with devices they do work well with. In fact, they always work fine with the kind of devices you mention. These cables aren't as common as USB-C-shaped junk that's missing resistors on the receptacle, but I stumbled upon them anyway and I didn't really try to.
  - megous 4mo ago
    You may wish to re-read the type-c specification, especially 4.5.2.2.3.2: "A USB 2.0 only Sink that doesn’t support accessories and is self-powered or requires only default power and does not support USB PD may transition directly to Attached.SNK when V BUS is detected." or 4.5.2.2.5: "A port that entered this state directly from Unattached.SNK due to detecting V BUS shall not determine orientation or availability of higher than Default USB Power and shall not use USB PD." or 4.5.2.2.11.2: "The port shall transition to Attached.SNK after tCCDebounce if or when V BUS is detected." CC detection, let alone PD negotiation is not needed. You can draw up to 2.5W right away from Vbus and be standard compliant without wiring anything to CC signals. Of course if you try this with a DRP device like a smartphone, you'll get no power. But that's not really an issue for type-c chargers or USB A-C cable assemblies.

deleted 4mo ago

[deleted]

ssl-3 4mo ago

That's close, but it's not quite complete. There seems to be lots of confusion here, and that's natural: It is confusing. For just-getting-power from a USB A port into a USB C peripheral: There are supposed to be 2 resistors in the peripheral device [always], and also 1 resistor within the cable for USB-to-legacy cables[1]. That's 3 resistors, total, to get a relatively dumb USB-C equipped peripheral device to reliably charge from both USB A and USB C hosts/chargers/whatevers: The cable itself: It gets an internal 56k pullup resistor between Vbus and USB C pin A5 -- which is the CC line [yes singular]). This resistor signifies the capabilities of the host/charger/whatever for devices that care (some do care, some do not care). The peripheral: This minimally needs two pulldown resistors [commonly 5.1k], between each of CC1 and CC2 [yes a plurality] and ground[2]. This tells a compliant USB C host/charger/whatever "It's OK! Send the juice juice!" regardless of connector orientation. [1]: https://www.usb.org/sites/default/files/USB%20Type-C%20Spec%20R2.0%20-%20August%202019.pdf https://www.usb.org/sites/default/files/USB%20Type-C%20Spec%... section 3.5 [bleh]: Again, it is a confusing thing. Nobody said that dealing with such flexible, ambidextrous connections would be simple. CC performs a lot of different tasks: It can be a bidirectional serial bus for active PD negotiations, and/or a resistor network for passively dealing with power, and it's the bit that performs detection of cable orientation for applications where that matters, and it probably does other stuff too. That single little wire is clever AF. It'd be simpler to use multiple wires instead of just one, but that would take more copper. Copper is expensive, and we each save a tiny bit of money (or a large pile of money globally) by using less copper instead of more of it.

exmadscientist 4mo ago
My point, which matches what you've written out in more detail, is that there's supposed to be stuff (resistors) in the cable, stuff which is often not there. There are plenty of these abhorrent cables in the wild. And when you come across one, things get weird, because some sources and sinks deviate from the standard in ways that make them work with these bad cables. Others don't do that. (I believe there were a lot more abhorrent cables in the early days, and thus began the chain of accommodation....) So unless your cable is known-good, if you are having trouble, trying a different cable should be the first thing you do. It really does often get things working. Contrarily, if you have identified a naughty cable, it should be immediately widlarized.