ST8024CTR datasheet(15/23 Pages) STMICROELECTRONICS

15 / 23 page

ST8024

15/23

Figure 7: Deactivation sequence

VCC GENERATOR

The VCC generator has a capacity to supply up to 80 mA continuously at 5 V and 65 mA at 3 V. An internal

overload detector operates at approximately 120 mA. Current samples to the 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 must stay below the specified maximum current

value. For reasons of VCC voltage accuracy, a 100 nF capacitor with an ESR < 100 mΩ should be tied to

CGND near to pin VCC, and 100 nF capacitor with the same ESR should be tied to CGND near card

reader contact C1.

FAULT DETECTION

The following fault conditions are monitored:

- Short-circuit or high current on VCC

- Removal of a card during a transaction

- VDD dropping

- DC/DC converter operating out of the specified values (VDDP too low or current from VUP too high)

- Overheating.

There are two different cases (see Fig.8):

- CMDVCC HIGH outside a card session. Output OFF is LOW if a card is not in the card reader, and HIGH

if a card is in the reader. A voltage drop on the VDD supply is 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 Fig.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 Fig.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.

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