The dwDefaultClock field is a number value from the USB CCID descriptor:
Default ICC clock frequency in KHz. This is an integer value.
Example: 3.58 MHz is encoded as the integer value 3580. (00000DFCh)
This is used in ETU and waiting time calculations. It is the clock frequency used when reading the ATR data.
| dwDefaultClock | # | % |
|---|---|---|
| 4.000 MHz | 123 | 48.43 % |
| 4.800 MHz | 43 | 16.93 % |
| 3.580 MHz | 34 | 13.39 % |
| 3.700 MHz | 10 | 3.94 % |
| 3.686 MHz | 8 | 3.15 % |
| 4.615 MHz | 5 | 1.97 % |
| 1.500 MHz | 4 | 1.57 % |
| 3.600 MHz | 4 | 1.57 % |
| 2.000 MHz | 3 | 1.18 % |
| 3.570 MHz | 3 | 1.18 % |
| 3.750 MHz | 3 | 1.18 % |
| 1.000 MHz | 2 | 0.79 % |
| 3.000 MHz | 2 | 0.79 % |
| 3.571 MHz | 2 | 0.79 % |
| 3.685 MHz | 2 | 0.79 % |
| 3.692 MHz | 2 | 0.79 % |
| 1024.000 MHz | 1 | 0.39 % |
| 3.850 MHz | 1 | 0.39 % |
| 4.714 MHz | 1 | 0.39 % |
| 5.000 MHz | 1 | 0.39 % |
The most common default clock frequencies are:
- 4.0 Mhz (48% of readers)
- 4.8 Mhz (17%)
- 3.58 Mhz (13%)
Note that 3.57 Mhz (used by 3 readers) was the default speed when the reader-host communication was at 9600 bauds using a serial communication port (9600 * 372 = 3,571,200).
Now that the readers are using the USB protocol the 4 Mhz clock speed may be easier to use at the hardware level and not too far from the classic 3.57 Mhz supported by old smart cards.
If we draw the number of reader per clock frequency we have:
The value 1024 Mhz (1.024 GHz) is clearly from a bogus reader.