An etu (Elementary Time Unit) is the length of a single bit in a character frame.
ISO7816-3 defines it as (F/D) * (1/f) where F and D are the so-called 'transmission factors' and f is the frequency of the clock signal applied to the card.

The default values for F and D are 372 and 1, respectively, so on power-up or reset, the card will start sending its ATR using an etu that is f/372 in length.
As Russel already indicated, using an f of 3.579545MHz (don't worry, that's a standard frequency) will yield an etu of 1/9600s, which is equivalent to a transmission speed of 9600 bps.

Of course, you are free to choose another frequency, to give you a different transmission speed.

The unfortunate thing is the value 372, which is not a multiple of 16; the bitrate division factor used by most standard UARTs. This makes it difficult to use the same clockfrequency for both the microcontroller and the card.



Regards,
Rob.