UM2191
USB
Type-C and Power Delivery
DocID030479 Rev 2
13/55
1.9
Power negotiation
When a connection is made and the respective roles have been assigned, the source and
the sink negotiate a contract for the power objects: the selected configuration channel (CC)
allows them to establish communication and negotiate the power according to the protocol
described in USB Power Delivery specification.
Originally, all the devices equipped with USB Type-
C™ are able to provide up to 15 W (5 V
and up to 3 A) power via the V
BUS
path, but every subsequent request for delivering or
receiving power from 15 W to 100 W (5 V at 3 A to 20 V at 5 A) must be negotiated
according to the USB Power Delivery protocol.
The messages exchanged between a source (provider) and sink (consumer) are illustrated
in
Figure 7: "Message flow during power negotiation"
.
1.
Initially, the source dispatches a
Source_Capabilities
message to inform the port
partner (sink) of its power capabilities.
2.
The sink then sends a
Request
for one of the advertised power profiles.
3.
The source accepts or rejects this request according to its power balance.
4.
If confirmed, the source sends an
Accept
to the sink
5.
The source then switches to the requested power profile and sends a
PS_Ready
confirmation message.
Each received message is acknowledged with a
GoodCRC
to confirm correct reception.
Incorrect reception should be ignored and persistent communication errors should trigger a
soft reset to reset protocol parameters and re-establish communication. If the error
persists, a hard reset is performed.