USB ports and cables allow interconnection of a variety of compatible electronic devices, such as desktop computers, automobile dashboard consoles and battery-powered portable devices such as laptop computers, tablets, mobile phones, e-readers and MP3 players. USB ports are accessed using standardized USB cable connections to provide serial communications between devices, as well as electrical power transfer for charging and operating battery-powered peripheral devices. USB compatible systems often include interface integrated circuits (ICs) mounted to an internal circuit board to interface USB data and power connections to host system circuitry such as power circuits and host processors. Dedicated USB charging devices are also available having multiple USB ports for charging various portable devices, which may include circuitry for fast charging certain peripheral devices. Many desktop and laptop computers include multiple USB ports for data transfer and/or peripheral device charging. USB power delivery (USB-PD) and Type-C (USB-C) specifications describe delivery of higher power over USB cables and connectors to provide a universal power plug for devices that may accommodate more than 5V charging power, for example, for fast or quick-charging capabilities. The USB-PD specification defines communications for negotiating voltage and current levels for power transfer from a source port to a sink port, where the power negotiation communications is independent from the normal USB communications.
USB-PD defines four kinds of USB compatible devices: Provider-Only, Provider/Consumer, Consumer/Provider, and Consumer-Only. Devices that are Provider-Only, Provider/Consumer, or Consumer/Providers may sometimes be in a Source role—meaning they are providing DC voltage on the Vbus wire for the far-end device to consume or sink. USB-C recognizes three kinds of USB devices: downward facing port (DFP), upward facing port (UFP), and dual-role port (DRP). In the absence of USB PD messaging, the DFP is the source of power and the UFP is sinking the power. USB-C cables and connectors include configuration channel (CC) lines for power configuration as well as for baseband communications. USB-PD specifications provide baseband communications using Biphase Mark Coding (BMC) for message exchange over a configuration channel (CC) wire or line of the USB cable. USB-C systems use a Type-C plug with two configuration channel lines CC1 and CC2. The USB-PD specification defines a half-duplex packet-based communication link between ports connected via a USB-PD cable and connectors to exchange information that enables the two ports to communicate and negotiate the voltage and current provided from a Source port to a Sink port. The ports can negotiate to switch roles (Source to Sink and vice versa). The BMC communications on the CC lines is independent from the normal USB communications that go through D+ and D− lines of the USB cable.
The CC line or lines may also be used for negotiating power transfer configurations of connected devices by way of analog signal levels. For example, up to 15 W of power can be delivered for USB Type-C cables without USB-PD messaging by controlling the DC voltage on the CC pin. The nominal voltage of the CC line is determined by pull up current from the DFP device (e.g., using a pull up resistor RP or a current source) and a pull down resistor RD (or pull down current source) from the UFP device. The CC line voltage value can thus vary from 0.3V to 2.4V in many instance due to combinations of the pull up and pull down levels. However, the baseband communication signals on the CC lines for typical BMC data packet exchange range from 0 to 1.1V. Consequently, the power supplies used for USB-C transmitters require pull down current to avoid having RP/RD combination charge up the baseband transmitter power supply node when a BMC output logic “1” is transmitted by the baseband transceiver. This is due to the CC line connection through the baseband transmitter to the supply that powers the transmitter. If inadequate pull down current is provided, and particularly if long duration transitions occur, the baseband transmit output can eventually be charged up and the transmit signals can go outside acceptable levels. For example, reverse current can be as high as 350 uA when a 47 k Ω pull up resistor and 3.3V supply are used. Accordingly, improved USB port controllers and techniques are desired to ensure robust baseband communications without excessive power consumption.