Atypical Charger Detection
It is possible to detect ACA chargers, as well as certain chargers that do not comply with the USB specification, using additional resour-
ces on the device. Accessory charging adapters (ACA) chargers use a resistor to ground on a special ID pin and a specific voltage on
the USB D- pin to encode the type of ACA and its capabilities. If ACA detection is required, the ID pin signal should be connected to
any GPIO on the device which supports ADC input, and an external current source or pullup resistor must be provided. The ADC may
be used to measure the voltage on the ID signal and the voltage on D- to distinguish between different ACA options. Applications need-
ing to determine ACA ports should check for ACA after primary detection is complete and, optionally, after data contact detection is
complete.
Many dedicated charging units pre-date the USB Battery Charging Specification or do not comply with this specification for other rea-
sons, such as additional supply current capabilities . Most of these cases implement resistive voltage dividers to produce very specific
voltages on the D+ and D- pins. In this case, the D+ and D- pins may be measured directly using the ADC to determine the voltage
levels and whether such a charger is attached. Normally, this would be performed after VBUS is detected and before going through the
data contact detection sequence.
EFM8UB3 Reference Manual
Universal Serial Bus (USB0)
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