How did the _initial_etu calcuated for 2DOT5Mhz?

[Bruce Lee]

Dear Friends,
How did the _initial_etu calcuated for 2DOT5Mhz?
in SCLib.cpp file. line 105.

case CLK_2MHZ:
  _initial_etu  = 186;   <<<< this one.
  _guardTime    = 2 * _initial_etu;
  _ocra1        = 3;
  break;
case CLK_2DOT5MHZ:
  _initial_etu  = 148;  <<<< this one.
  _guardTime    = 2 * _initial_etu;
  _ocra1        = 2;
  break;
case CLK_4MHZ:
  _initial_etu  = 93;
  _guardTime    = 2 * _initial_etu;
  _ocra1        = 1;
  break;

the default ETU for ISO7816-3 is 372 and the clock is 3.59Mhz
so in your code why clock runs at 2Mhz the ETU is 186. (372/2 = 186).

hope you can help me to calcuate the clock runs at 3.2534 Mhz and what the ETU should be .

Bruce lee

[angus71]

Hi Bruce,

As I understood the ISO7816-3 the "372" value is not a time, but a divider (D) of the used frequency.

So if you use for example a 1MHz clock you get the following values

fc / D = 1MHz / 372 = ~2688 bit/s, which leads to a time per bit of 372us

So for your example it should be

3.59Mhz / 372 = ~9650 bit/s -> etu = 103us

With the given frencecy of 3.59MHz one bit should take around 103us (micro seconds).

If you have any questions just let me know.

With kind regards

Angus

[caocongcong]

Dear friend,
I am a student from china. The default ETU for ISO7816-3 is 372 and the clock is 3.59Mhz, so I just want to communication in 3.59MHz/372 = 9600 bit/s, my smartcard has its own clock, which is 3.59MHz.
After reading your code, I can hardly find a way to initial the function with you code.
hope you can help me.
cao congcong