Retiming USB4 over USB-C

Article By : Brian Holden

Retimers have never been more important for interconnect designs and are essential for the successful operation of USB4 and the other high-speed protocols that are defined for USB-C...

The USB-C connector is the one connector to rule them all. It has wonderful flexibility in its definition and has been widely adopted across different interconnects. That flexibility and diversity of use are also the source of many difficulties when trying to retime the signals that run over it, particularly in active cables.  A key source of those difficulties for retimers are the many interactions between all of the options and modes that USB-C supports.

USB-C supports USB, Thunderbolt, DisplayPort, HDMI and MHL as well as other emerging protocols via a series of alternative or “Alt” modes. Each has its own protocol and physical and layers that have to be dealt with separately by a retimer. They were defined by different groups at different times and for strongly different purposes. The protocol layers are different from each other. The physical layers were also defined in disparate ways with differing rates and significant backward compatibility modes and constraints. It is a feat of clever engineering that they could be fit together and operate over one connector definition.

Figure 1: Multiple protocols are defined to run over the USB-C connector using Alt modes.

USB-C also supports reversibility where the connector can be plugged in an arbitrary alignment in two dimensions. The first alignment dimension is that each end of a cable can be connected to either the host end or the peripheral end, and the alignment is not known until it is plugged in. USB-C has a fully symmetric plug and receptacle definition for these two roles, a major improvement over the USB-A connector. The various protocols all have host-driven configuration cycles, so the distinction of which side the commands will be coming from is important for active cables whose retimers must participate in those cycles. This means that a retimer must be flexible enough to accept, process and propagate the overhead packets from and to either direction.

Figure 2: USB-C active cables have to handle a configuration flow in either direction.

The second alignment dimension is that each end of the cable can be plugged in with either side of the plug on top. In order to allow the user from having to fumble with the orientation of the plug, USB-C has been defined to be allow the plug to be inserted either way up at both ends. This again is a major improvement over the USB-A connector and accomplished by defining each of the two pairs in each direction to be pinned out in opposite corners of the connector. All of the other connections including the power, ground and the sideband signals are also defined in a symmetric manner. There are some application scenarios where control of the reversing is important, so the ability to flip the connection between the wires internally is a requirement of a full-featured USB-C retimer.

Figure 3: The wire-wire connectivity of USB-C can be flipped depending on how it is plugged in.

USB-C retimers also have to interact with power delivery (PD) controllers. USB-C allows a negotiation between the producer and consumer of the power that is delivered over the link. Because of this, a USB-C retimer needs to support a two-wire interface (TWI) to a PD controller.

USB-C retimers must support more than one clocking mode. Modes are bit-level retiming (BLR) and separate reference clock with independent SSC (SRIS).

USB-C also allows certain multi-function modes. One example of this is where one high-speed lane is USB 3.2 and another lane is DisplayPort. The needed configuration cannot be determined in advance by a retimer. Instead, it is declared as needed by the host after a negotiation with the peripheral.

Because of the 20 Gbps SerDes rate, USB4 is the generation that needs retimers and not redrivers to do the job. The three main disadvantages of an analog redriver are:

  1. Redrivers amplify both the signal and their internal receiver noise.
  2. Redrivers only partially clean up inter-symbol interference (ISI).
  3. Redrivers do not restore the eye width and allow jitter to be propagated.

As a result, the full reach of the link both before and after the redriver cannot be utilized. Shorter trace lengths must be employed in each place to minimize the impact of the added noise, residual ISI, jitter and narrowed eye width. Due to these issues, a significant burden is placed on the system developer to understand and characterize the complex impact of the redriver on the end system across all envisaged usage scenarios. These issues multiply when more than one retimer is used in the data path.

Figure 4: Retimers completely restore the signal. Redrivers do not.

USB4 is also the generation that needs a retimer physically close to every port in all but the smallest form factors. This is because for a USB4 port to achieve its full reach, the loss budget cannot be used up crossing a printed circuit board (PCB) from the microprocessor to the USB-C connector. In most earlier generations, the loss across the PCB could be absorbed into the loss budget. In most host scenarios for USB4, a retimer is needed, particularly when a system does not want to use the most expensive types of PCB materials.  Of course, if it is possible to put the connector immediately next to the microprocessor, a retimer may not be necessary.

Figure 5: USB4 is the generation that will need retimers near the connector.

In some applications, the microprocessor only has a generic SerDes that does not support the full requirements of USB4 and the other protocols. In some of these cases, a second retimer is needed immediately next to the host.

In addition, USB4 is also the generation where active cables will come to the forefront. USB has been able to get by until now for the most part with passive cables. While passive cables are still an option for USB4 in the shortest reaches, active cables will be needed in many circumstances. This fact will interfere with USB4’s usefulness in some applications because of the increased power draw needed by retimers.

Several of the protocols have defined methods for the host to reach into each one of a series of retimers and adjust the equalizer parameters for each. There are multiple modes within these protocols defining symmetric and asymmetric methods. USB-C retimers must participate in all of these protocols.

Figure 6: Sequences of retimers will become common with USB4.

Because of the challenging signal integrity environment that surrounds USB4, a retimer may support eye scope functionality such as eye height and eye width measurements. Bit error measurement is also needed with the ability to generate and check PRBS patterns.

As if USB4 wasn’t hard enough to retime on its own, these additional features must be supported. The combination of all of these factors make multi-protocol retimers for USB4 over USB-C the most complex retimers ever produced.  A few available retimer solutions support USB4, DisplayPort and Thunderbolt protocols, allow direction reversal, support connector flipping, and external PD controllers, and offer extensive test & measurement capabilities. Kandou offers one such solution.

Retimers have never been more important for interconnect designs and are essential for the successful operation of USB4 and the other high-speed protocols that are defined for USB-C.

— Brian Holden is vice president of standards at Kandou.

Leave a comment