Switching from USB 2.0 OTG to USB Type-C

By working through the over 100 pages of the USB Type C specification, it can seem like an almost indomitable task, especially since the type C spec takes on more specifications such as USB power delivery (PD) with again over 500 pages reference. Electronics industry describes the minimum requirements developers must meet when converting a USB 2.0 OTG product to a USB-type-C compliant product.

 

Cables, sockets and connectors are designed for USB type-C to be more robust and user-friendly than current USB cables (micro-A, type-A and type-B). Currently, type a connectors can only be inserted into a type a socket in one position. It is different with type C connectors that fit into the corresponding sockets in both orientations. This rather simple modification avoids frustrations when trying to plug a USB connector into a USB port in a wrong position.

 

USB products are getting more and more power from Vbus, and there is also a desire for shorter loading times. The USB Type C standard is therefore geared to the performance requirements of today’s products and also supports the requirements to the future. However, to take advantage of the benefits of USB Type-C, companies need to upgrade their existing products to meet the requirements of the Type C standard. This changeover process can be a challenging task, but it is easier to access the information contained in this post.

 

USB 2.0 OTG

 

A product with an interface of type USB 2.0 OTG is a portable device (for example, a mobile phone or a tablet) that can be used as a USB host or USB peripheral. All USB 2.0 OTG products must be equipped with a micro-A/b jack. A product that implements USB 2.0 OTG determines its respective role based on the state of the ID pin. If the ID pin is on ground potential (GND), the OTG product acts as a USB host and provides vbus to the connected USB peripheral. In the other case, the OTG device works as a USB peripheral.

 

Table 1 shows the connection assignments of the socket and the plug. The ID pin of the connector is either ground or it remains not to select the host or peripheral.

 

If a device works as a USB periphery, the portable USB 2.0 OTG product monitors the Vbus line. This can determine whether it is connected to a USB host or an external charger, such as a mobile phone to a power supply. With USB battery charging (BC 1.2) or any other suitable method, a portable product has the ability to request more than the 2.5 W predetermined by USB 2.0 to achieve shorter loading times.

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USB Type-C

 

The USB Type C specification defines a plug-in socket with the corresponding cable in such a way that the user does not have to pay attention to the orientation of the plug when plugged in (Figure 1). The cable used may have a Type C connector on both ends, or it will have a USB connector of an older type (micro-A, type-a, or type-B, and others) at one end. To be equipped for applications with larger bandwidths, the USB Type C specification complements the plug-in connection with several USB-3.1 pairs.

 

A fully wired type C cable supports both USB 2.0 and USB 3.1. However, when upgrading a USB 2.0 OTG product to a Type C product, the USB-3.1 signals are not required. Instead, these signals should be left not (electrically insulated) on the printed circuit board. Therefore, in Figure 3, the contacts belonging to USB 3.1 are provided with a type C socket with the name NC (no connection).

 

The connection assignment in Figure 2 shows two pairs with contacts of the types D +-and D-. However, this does not mean that there are two independent USB 2.0 paths, because a type C cable only has one line for D + and D-. However, the provision of two contact pairs for these two lines ensures that the plug can be inserted in both layers. For this reason, both wires (d + and D-) should be connected to the printed circuit board. The connection of these contacts on a printed circuit board inevitably results in a puncture line. Developers must make sure that the length of this stitch line does not exceed 2.5 mm, otherwise there may be a problem with the USB 2.0 interface.
Striking is the lack of the ID pin on the USB type-C connector. The choice between host and peripheral functionality is handled differently in type-C. Whether it is a host or a peripheral device, the system uses the CC (channel configuration) pins to determine cc1 and CC2, switching between pull-up and pull-down resistance at a given interval. Whether a device is given the definition “host” or “periphery” depends on the voltage level that is present after a given time on the CC pins.
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Dual-Role Port type-C
In the Type C ecosystem, the USB 2.0 OTG device is named dual-role-port (DRP). A DRP is a device that can function either as a USB host or as a USB peripheral. In Type C terminology, a USB host is referred to as a downstream facing port (DFP), but a USB peripheral is considered an upstream-facing port (UFP).
When choosing between DFP and UFP, a DRP device must switch between the DFP and UFP roles until a connection can be established. Figure 4 Replays the operating principle of a DRP according to the USB Type C specification.
While a DFP has a pull-up resistor (RP), there is a pull-down resistor (RD) on the UFP. As shown in Figure 4, RP and Rd are controlled by a switch to CC1/CC2. When connecting to a selected RP, the DRP device behaves like a DFP (host) and provides vbus to the connected peripheral. However, if the connection is made with selected Rd, the DRP device behaves like a UFP (peripheral) and monitors vbus to establish a data connection and/or to supply the own circuits with power.
It is also possible that a user has two DRP-capable devices together. It is possible that one of the devices is able to act as a host or periphery. For example, a mobile phone and a tablet can both be DRP-capable. However, if they are both connected, the mobile phone becomes the peripheral and the tablet to the host. It should not happen that the mobile phone becomes the host for the tablet and supplies it with Vbus. The phone should not charge the tablet.
Preferred roller for Type C
The Type C specification therefore provides optional paths for the DRP that a device can use to specify a particular preference role. These option paths carry the labels try. Src and try. SNK. In the case of portable devices, it is important that these two optional features are implemented. In a tablet, the implementation of try. SRC can be desirable so that it becomes the host when connecting to another DRP device. For a mobile phone, the implementation of try. SNK is useful so that it can work as a peripheral when connected to another DRP device.
A VCONN switch can also be seen in Figure 4. VCONN (5 V with at least 1 W) is intended for the supply of cables with active circuits. In type C parlance, these are called “active cables”. Typically, VCONN is used to supply a USB-3.1-to-the-wire-integrated device. However, VCONN is not required for products that support only USB 2.0.
However, at the end of this post, try. SNK has not yet been released as an engineering change notice (ECN) for the USB type-C 1.1 specification.
Single Chip Solution
One way to convert a USB 2.0 OTG product with a micro-A/b plug-in socket on a Type C connector is to use the TUSB32X product series from Texas Instruments. Depending on the state of a PIN or the value in an I ² c register, this product family can act as UFP, DFP or DRP. TI’s building blocks assume all aspects of the type C connection process. You provide an ID pin that mimics the behavior of the ID pins on the micro-A/b connector. This allows the function to be set as host or peripheral without difficulty. When connected as peripheral, the TUSB32x family signals the Vbus current supplied by the connected host via I ² c registers or Gpio pins. On the other hand, when connecting as a host, the available Vbus current is signalled to the connected peripheral.

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