Information

VBUS
D+
GND
VBUS
D
D+
GND
ReceptaclePlug
VBUS
D
Figure 43-10. Relative Pin Positions in USB Plugs and Receptacles
As a result, when a portable USB device is attached to an upstream port, the portable
USB device detects VBUS before the data pins have made contact. The time between
power pins and data pins making contact depends on how fast the plug is inserted into the
receptable. Delays of several hundred milliseconds are possible.
43.5.1.3.1 Debouncing the Data Pin Contact
When system software has initiated the charger detection sequence, as described in Initial
System Conditions the USBDCD module turns on the I
DP_SRC
current source and enables
the R
DM_DWN
pulldown resistor. If the data pins have not made contact, the D+ line
remains high. Once the data pins make contact, the D+ line goes low and debouncing
begins.
Once the D+ line goes low, the module continuously samples the D+ line over the
duration of the T
DCD_DBNC
debounce time interval.T
DCD_DBNC
defaults to 10 ms but can
be programmed in the TIMER0[TDCD_DBNC] field. See the description of the TIMER0
Register for register information.
When it has remained low for the entire interval, the debouncing is complete. However, if
the D+ line returns high during the debounce interval, the module waits until the D+ line
goes low again to restart the debouncing. This cycle repeats until either:
• the data pin contact has been successfully debounced (see Success in Detecting Data
Pin Contact (Phase Completion)), or
• a timeout occurs (see Charger Detection Sequence Timeout).
43.5.1.3.2 Success in Detecting Data Pin Contact (Phase Completion)
After successfully debouncing the D+ state, the module does the following:
Chapter 43 USB Device Charger Detection Module (USBDCD)
K20 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc. 1105