Functional description

ST8024

22/31

detector are internally filtered, allowing spurious current pulses up to 200 mA with a duration

in the order of µs to be drawn by the card without causing deactivation. The average current

a 100 nF capacitor with an ESR < 100 m

nF capacitor with the same ESR should be tied to CGND near card reader contact C1.

5.10

Fault detection

The following fault conditions are monitored:

– Removal of a card during a transaction

–Overheating.

– There are two different cases (see Figure 8):

–CMDVCC HIGH outside a card session. Output OFF is LOW if a card is not in the

detected by the supply supervisor, this generates an internal Power-on reset pulse

but does not act upon OFF. No short-circuit or overheating is detected because the

card is not powered-up.

–CMDVCC LOW within a card session. Output OFF goes LOW when a fault condition

is detected. As soon as this occurs, an emergency deactivation is performed

automatically (see Figure 9). When the system controller resets CMDVCC to HIGH it

may sense the OFF level again after completing the deactivation sequence. This

distinguishes between a hardware problem or a card extraction (OFF goes HIGH

again if a card is present).

Depending on the type of card-present switch within the connector (normally-closed or

normally-open) and on the mechanical characteristics of the switch, bouncing may occur on

the PRES signals at card insertion or withdrawal.

There is a debounce feature in the device with an 8 ms typical duration (see Figure 8).

When a card is inserted, output OFF goes HIGH only at the end of the debounce time.

When the card is extracted, an automatic deactivation sequence of the card is performed on

the first true/false transition on PRES or PRES and output OFF goes LOW.

Figure 8.