Information

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.
41.5.1.3.1 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. After the data pins make contact, the D+ line goes low and debouncing
begins.
After the D+ line goes low, the module continuously samples the D+ line over the
duration of the T
DCD_DBNC
debounce time interval.By deafult, T
DCD_DBNC
is 10 ms, but it
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 of the following
happens:
• The data pin contact has been successfully debounced (see Success in detecting data
pin contact (phase completion)).
• A timeout occurs (see Charger detection sequence timeout).
41.5.1.3.2 Success in detecting data pin contact (phase completion)
After successfully debouncing the D+ state, the module does the following:
• Updates the STATUS register to reflect phase completion (See Table 41-18 for field
values.)
• Directly proceeds to the next step in the sequence: detection of a charging port (See
Charging port detection.)
Functional description
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
942 Freescale Semiconductor, Inc.