Gayathri Vasudevan, Cypress
embedded.com (May 15, 2017)
As described in the first part of this two-part series, USB Type-C is the newly introduced and powerful interconnect standard for USB. When paired with the new Power Delivery (PD) specification, Type-C offers enhancements to the existing USB 3.1 interconnect that lower the cost and simplify the implementation of power delivery over USB. In this article, we describe the USB Type-C power delivery protocol.
Figure 9 shows the PD message format. All PD messages are transmitted at 300KHz +/- 10% over the CC line. The first 64 bits (Preamble) are an alternating 1, 0 pattern so that the receiver can synchronize with the actual transmitted clock. The next 16-bit word (Address or Type), contains the message address, indicating the message recipient or other type information (SOP* communication explained below). The next 16-bit field contains the message header, which is encoded as shown in Figure 9. The header field includes a data object count (#Data Objects). If the data object count is 0, then the message type is “control.” If it is 1 through 7, then up to seven 32-bit data objects follow, and the message type is “data.” A 32-bit CRC is transmitted, followed by a 4-bit end of packet (EOP) token that completes the message. If the calculated CRC is the same as the received CRC, then the Type-C device physical layer passes the decoded message up to its protocol layer for decoding. Refer to the USB-PD specification for more details on the message structure.
Click here to read more ...