AN1978 STMicroelectronics, AN1978 Datasheet - Page 4

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

AN1978 - APPLICATION NOTE
Figure 4: Clock duty cycle on CLKDIV change
The clock signal can be obtained by a crystal connected between the XTAL1 and XTAL2 pins or by an
external signal applied to the XTAL1 pin; in this case the XTAL2 pin must be left floating. The external
signal voltage value must be enclosed between GND and VDD.
In the PCB design, in order to reduce the reflections especially for the high frequency, the Xtal should be
connected as close as possible to the XTAL pins, as shown in figure 5.
Figure 5: XTAL connection
Two compensation capacitors, lower than 22pF, can be used to improve the oscillator performance even
if the characterization tests remarked that the CLK Duty Cycle is enclosed between 45% and 55%,
4/14
XTAL2
XTAL1
GND
VDD
2MHz to 26MHz XTAL (Y1)
CH1: output CLK waveform.
CH1: output CLK waveform.
CH2: CLKDIV1 pin.
CH2: CLKDIV1 pin.
Conditions:
Conditions:
VDD=3.3V;
VDD=3.3V;
VDDP=5V;
VDDP=5V;
5/3V=H;
5/3V=H;
CMDVCC=L.
CMDVCC=L.
F
F
CLKDIV2=0V;
CLKDIV2=0V;
The output Duty Cycle is 50%±5%
The output Duty Cycle is 50%±5%
even if the Clock Division changes.
even if the Clock Division changes.
XTAL1
XTAL1
100 mils
Compensation capacitors
=10MHz;
=10MHz;