Think of the SDP as the classic USB port. In addition to USB communication, it provides 100mA of current to peripheral devices when connected; the 100mA is negotiable up to 500mA. While most ports typically do not enforce this current limit, higher currents are not guaranteed. A DCP does not support USB communication, but can supply charge currents beyond 500mA without any negotiation.
A CDP supports both USB communication and high-current charging; it features internal circuitry that is switched on during the charger detection phase. Some electronics manufacturers build proprietary charger identification schemes in addition to the USB port types outlined in the specification. Those various schemes add another layer of charger detection technology that cannot be overlooked.
The Charger Detection Process
Figure 1. USB connector pins and data contact detection (DCD).
The charger detection phase outlined in BC1.2 has five basic steps.
- VBUS detection To ensure proper sequencing for any potential devices connected to a USB port, the VBUS and GND pins on the connector are intentionally made longer than the D+ and D- pins. This ensures that they make contact first (see Figure 1). Thus, before any detection can occur, the device must first sense that VBUS is present.
- Data contact detection (DCD) Once the voltage on VBUS is valid, the portable device must ensure that the data pins also make contact before any detection can occur. The end device might incorrectly identify what charger is present if it makes a premature decision before the data pins make contact.To perform DCD, the peripheral device must enable a 7µA to 13μA current source (referenced to +3.3V) on D+ and monitor its voltage. This current range is chosen to maintain proper logic levels across all voltage and resistance tolerances