How do USB Type-C chargers support older USB devices?

tl;dr: All new USB-C dedicated chargers must also support USB Battery Charging 1.2 Dedicated Charging Port (BC 1.2 DCP) by shorting together Dp and Dn.

Ever run into a situation where you plug your older MicroB or Lightning port phone into a Type-C charger and nothing happens, or slow charging happens?

I was looking through some of the ECNs (Engineering Change Notices, or how the folks behind USB make changes to the specs) in the latest USB document bundle, and I noticed a document named, "USB Type-C ECN BC1.2 Clarification."

In red, there's the new requirement:
"A USB-based charger with a USB Type-C receptacle (Source) which is not capable of data
communication shall advertise Type-C current of at least 1.5A and shall short D+ and D- together
with a resistance less than 200ohms. This will ensure backwards compatibility with legacy sinks which
may use BC1.2 for charger detection."

Why did they make this change? Because prior to this requirement, BC 1.2 was completely optional and some dedicated chargers with Type-C receptacles chose to not implement it, instead implementing Apple's 2.4A BrickID, or nothing at all (floating Dp Dn). In some cases, this meant that completely valid cable combinations (using a C-to-Areceptacle adapter + A-to-B or A-to-Lightning, or using a C-to-uB, or a C-to-Lightning) would result in no charging or slow charging.

Going forward, all new dedicated chargers (those that lack any data functionality) must short Dp and Dn in accordance to BC 1.2 DCP. This will ensure that legacy devices will detect Type-C chargers as DCPs. This will include legacy iPhones and iPads as well.

Note: This requirement also means that it's no longer allowed to use Apple's BrickID method on Type-C receptacles anymore to advertise 2.4A on the port. This was covered in an earlier Type-C ECN which forbid all proprietary charging methods on the new Type-C connector, including Apple's. Now, if it's a receptacle dedicated charger, it MUST support BC 1.2 DCP.

This ECN, by the way, has also been rolled into the latest Universal Serial Bus
Type-C Cable and Connector Specification Revision 1.3.

+Nathan K. and +Hanpen for FYI, since they both ran into chargers that don't do this yet (various Anker and other chargers).
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