NXP Semiconductors freescale K51 Series Reference Manual

• recently plugged into a USB port, and• drawing no more than 2.5 mA total system current from the USB bus.There are many allowable precursors to this set of initial conditions. For example, thedevice could have been powered down and subsequently powered up upon being pluggedinto the USB bus. Alternatively, the device could have been in a low power state that wasexited due to the plugin event. Or, the device could have been operating in normal runmode, powered by a separate supply or non-rechargable battery.44.5.1.2 VBUS Contact DetectionOnce the device is plugged into a USB port, the VBUS_detect system interrupt istriggered. System software should do the following to initialize the module and start thecharger detection sequence:1. Restore power if the module is powered-off.2. Set the CONTROL[SR] bit to initiate a software reset.3. Configure the USBDCD module: Program the CLOCK register and the timingparameters as needed.4. Set the CONTROL[IE] bit to enable interrupts (by default), or clear the bit if using asoftware polling method.5. Set the CONTROL[START] bit to start the charger detection sequence.44.5.1.3 Data Pin Contact DetectionBecause the detection sequence depends upon the state of the USB D+, the module mustensure that the data pins have made contact. USB plugs and receptables are designedsuch that when the plug is inserted into the receptable, the power pins make contactbefore the data pins make contact. See the following figure.Functional DescriptionK51 Sub-Family Reference Manual, Rev. 6, Nov 20111108 Freescale Semiconductor, Inc.