- allowed in the specification. If D+ is open, the voltage will be logic high. If closed, D+ will read logic low regardless of the port type. If no data pin contact is sensed after the 1-second timeout period, the end device assumes that an SDP is present.
- Primary charger detection In this step, the end device differentiates the > 500mA-capable ports with the charging label (CDP and DCP) from the < 500mA port type (SDP). After disabling the current source from the DCD phase, the end device must then enable a 0.5V to 0.7V voltage source on D+ and a 25μA to 175μA current sink on D-. If a DCP or CDP is present, the 0.5V to 0.7V level will appear on D-. If a SDP is present, the voltage on D- will drop to zero. The end device switches in a comparator that compares D- to 0.25V to 0.4V. If the D- voltage is above 0.4V but less than the logic-low threshold of 0.8V, then the end device concludes that a charging port is present.
- Secondary charger detection After turning off the voltage source and current sink from the previous step, the end device needs to discern a CDP from DCP. To accomplish this, it performs the previous test in reverse. Thus a 0.5V to 0.7V voltage source is enabled on D-, and a 50μA current sink is enabled on D+. If a DCP is present, the 0.5V to 0.7V test voltage will appear on D+. If a CDP is present, the voltage on D+ will be zero.
- CDP charge current limit Since a CDP supports both data and high-current charging, one last distinction must be made. Because of the large amount of current in the USB cable, the host ground and device ground can only tolerate a ground offset of 375mV.
Figure 2. Summary of BC1.2 charger detection procedure.
While non-BC1.2-compliant chargers differ among manufacturers, many of