TPS25751D datasheet(27/89 Pages) TI1

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USB-PD messages are transmitted in a USB Type-C system using a BMC signaling. The BMC signal is output

on the same pin (CC1 or CC2) that is DC biased due to the Rp (or Rd) cable attach mechanism.

8.3.1.1 USB-PD Encoding and Signaling

Figure 8-4 illustrates the high-level block diagram of the baseband USB-PD transmitter. Figure 8-5 illustrates the

high-level block diagram of the baseband USB-PD receiver.

4b5b

Encoder

BMC

Encoder

CRC

Data

to PD_TX

Figure 8-4. USB-PD Baseband Transmitter Block Diagram

BMC

Decoder

SOP

Detect

4b5b

Decoder

CRC

from PD_RX

Data

Figure 8-5. USB-PD Baseband Receiver Block Diagram

8.3.1.2 USB-PD Bi-Phase Marked Coding

The USB-PD physical layer implemented in the TPS25751 is compliant to the USB-PD Specifications. The

encoding scheme used for the baseband PD signal is a version of Manchester coding called Biphase Mark

Coding (BMC). In this code, there is a transition at the start of every bit time and there is a second transition in

the middle of the bit cell when a 1 is transmitted. This coding scheme is nearly DC balanced with limited disparity

(limited to 1/2 bit over an arbitrary packet, so a very low DC level). Figure 8-6 illustrates Biphase Mark Coding.

Data in

BMC

Figure 8-6. Biphase Mark Coding Example

The USB PD baseband signal is driven onto the CC1 or CC2 pin with a tri-state driver. The tri-state driver is slew

rate to limit coupling to D+/D– and to other signal lines in the Type-C fully featured cables. When sending the

USB-PD preamble, the transmitter starts by transmitting a low level. The receiver at the other end tolerates the

loss of the first edge. The transmitter terminates the final bit by an edge to ensure the receiver clocks the final bit

of EOP.

8.3.1.3 USB-PD Transmit (TX) and Receive (Rx) Masks

The USB-PD driver meets the defined USB-PD BMC TX masks. Because a BMC coded “1” contains a signal

edge at the beginning and middle of the UI, and the BMC coded “0” contains only an edge at the beginning, the

masks are different for each. The USB-PD receiver meets the defined USB-PD BMC Rx masks. The boundaries

of the Rx outer mask are specified to accommodate a change in signal amplitude due to the ground offset

through the cable. The Rx masks are therefore larger than the boundaries of the TX outer mask. Similarly, the

boundaries of the Rx inner mask are smaller than the boundaries of the TX inner mask. Triangular time masks

are superimposed on the TX outer masks and defined at the signal transitions to require a minimum edge rate

that has minimal impact on adjacent higher speed lanes. The TX inner mask enforces the maximum limits on the

rise and fall times. Refer to the USB-PD Specifications for more details.

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TPS25751

SLVSH93 – OCTOBER 2023

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