ST8024

11/23

Figure 4: Voltage supervisor

WITH AN EXTERNAL DIVIDER ON PIN PORADJ

If an external resistor bridge is connected to pin PORADJ (R1 and R2 in Fig.1), then the following occurs:

Input PORADJ is biased internally with a pull-down current source of 4 µA which is removed when the

voltage on pin PORADJ exceeds 1 V.

This ensures that after detection of the external bridge by the IC during power-on, the input current on pin

PORADJ does not cause inaccuracy of the bridge voltage.

The minimum threshold voltage should be higher than 2 V. The maximum threshold voltage may be up to

APPLICATION EXAMPLES

The voltage supervisor is used as Power-on reset and as supply dropout detection during a card session.

Supply dropout detection is to ensure that a proper deactivation sequence is followed before the voltage

is too low. For the internal voltage supervisor to function, the system microcontroller should operate down

to 2.35 V to ensure a proper deactivation sequence. If this is not possible, external resistor values can be

chosen to overcome the problem.

CLOCK CIRCUITRY

The card clock signal (CLK) is derived from a clock signal input to pin XTAL1 or from a crystal operating

at up to 26 MHz connected between pins XTAL1 and XTAL2.

via inputs CLKDIV1 and CLKDIV2 (see Table 20).