ST8034AT datasheet(24/39 Pages) STMICROELECTRONICS

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ST8034T, ST8034AT, ST8034P, ST8034C

Functional description

6.3

Clock circuits

6.3.1

ST8034T and ST8034AT clock possibilities

The clock signal for the card (CLK output) is either provided by an external clock signal

connected to the XTAL1 pin or generated by a crystal connected between the XTAL1 and

XTAL2 pins. The ST8034 device automatically detects if an external clock is connected to

the XTAL1, which eliminates the need for a separate clock source selection pin. Automatic

clock source detection is performed on each activation command (falling edge of

CMDVCC). The presence of an external clock on the XTAL1 pin is checked during a time

window defined by the internal oscillator. If the external clock is detected, the crystal

oscillator is stopped. If the clock is not detected, the crystal oscillator is started. When the

external clock is used, the clock signal must be present on the XTAL1 pin before the

CMDVCC falling edge. If the external clock is used, connect it to XTAL1 input and leave the

XTAL2 pin floating. The XTAL1 pin cannot be left floating, either a crystal or an external

clock source needs to be connected.

Figure 11. External clock usage

The clock frequency is selected using the CLKDIV pin and is either fXTAL/2 or fXTAL/4 on the

ST8034T device or fXTAL or fXTAL/2 on the ST8034AT, as shown in Table 13. The frequency

change is synchronous, meaning that after transition on the CLKDIV input, the present clock

period is completed and after that the new whole clock period starts, therefore no clock

period is shortened during the frequency switchover.

If an external crystal is used, the duty cycle on the CLK pin should be between 45% and

55%. If an external clock is connected to the XTAL1 pin, its duty cycle must be between 48%

and 52% so that the CLK output duty cycle is between 45% and 55%.

Table 13. ST8034T and ST8034AT clock frequency selection

CLKDIV pin level

CLK frequency

ST8034T

ST8034AT

High

fXTAL/2

Low

fXTAL/4

fXTAL

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