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The duration of a bit within a character is termed the elementary time unit (etu). The etu is set by programming the SCIBAUD and SCIVALUE registers.

The value in the SCIBAUD register is used to define a clock which is a multiple of the baud rate. This is known as the Value X BAUD rate clock. The Value X BAUD rate clock is generated by dividing the reference clock by 1 + SCIBAUD. The SCIVALUE register defines the number of Value X BAUD rate clock periods which make up an etu.The etu is programmable and has different values dependent upon the stage of card processing.

During the ATR, the bit duration is known as the initial etu and is given by the following equation:

where f is the Smart Card clock frequency in Hertz.

Following the ATR (and establishment of the global parameters F and D), the bit duration is known as the current etu, and is given by the following equation:

where F and D are the clock rate conversion and bit rate adjustment parameters returned by the card, and f is the clock frequency applied to the Smart Card.

The etu is set by programming the SCIBAUD and SCIVALUE registers.

The SCIVALUE defines the number of baud rate clock periods that define the etu.

Therefore:

Thus the following equation must always be satisfied:

See ISO 7816-3 for the possible values of F and D that can be returned by the card.

The EMV standard specifies that f must be in the range 1–5 MHz. ISO 7816‑3 merely specifies a lower bound of 1 MHz.