Sunday, February 9, 2014

CCID descriptor statistics: bNumClockSupported

Article from the serie "CCID descriptor statistics"

The bNumClockSupported field is a number value from the CCID USB descriptor:
The number of clock frequencies that are supported by the CCID. If the value is 00h, the supported clock frequencies are assumed to be the default clock frequency defined by dwDefaultClock and the maximum clock frequency defined by dwMaximumClock.

The reader must implement the command PC_to_RDR_SetDataRateAndClockFrequency if more than one clock frequency is supported.

0 (will use whatever is returned)19074.80 %
13814.96 %
4187.09 %
041.57 %
520.79 %
210.39 %
610.39 %

This field is not use by my CCID driver. The vast majority of readers have the "Automatic ICC clock frequency change according to parameters" bit set in dwFeatures. So the clock selection is made by the reader itself.