AN1978 STMicroelectronics, AN1978 Datasheet - Page 3

• Manufacturer Part Number: AN1978
• Description: ST8004: SMART CARD INTERFACE
• Manufacturer: STMicroelectronics

Figure 3: Deactivation sequence
3. CLOCK TO THE CARD
The clock signal to the card is present on the CLK pin when the ST8004 is activated; ; its frequency can
be of the same value as one of its ratios compared to the input one (crystal or signal) as shown in the
following table (Table 1: CLK division).
Table 1: CLK Division
According to the EIA/ISO7816 specifications, the CLK duty cycle must be enclosed between 45% and
55% even if the clock division changes. In this case, at CLKDIV change, the ST8004 waits for the first
falling edge to ensure the duty cycle accuracy, as shown in Figure 4.
CLKDIV 1
0
0
1
1
CLKDIV 2
0
1
1
0
AN1978 - APPLICATION NOTE
1/8 f
¼ f
½ f
CLK
f
XTAL
XTAL
XTAL
XTAL
3/14