AN1978 STMicroelectronics, AN1978 Datasheet - Page 8

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

AN1978 - APPLICATION NOTE
5. FAULT DETECTION
ST8004 provides a fault detection circuitry as shown in Figure 1 monitoring the following conditions:
- Over-temperature detection
- Fault on card removing
- VDD under-voltage
- Vcc Short circuit protection
5.1. VDD under-voltage
The ST8004 logic circuitry is supplied by VDD; in order to avoid voltage spikes causing bad behaviors of
device and card, a voltage supervisor block is embedded as shown in Figure 1.
This check block allows the deactivation of the card when the VDD voltage falls below the VTH2 or VTH3
threshold, depending on the VTHSEL selection.
The VTH3 threshold can be used when the microcontroller I/Os continue to work down to 2.85V. In order
to provide the correct CLK signal to the card until time t12, the application circuitry should ensure,
through filter capacitors, that the VDD voltage falls more slowly than 1.6V/ms. The VDD capacitors used
in the evaluation board are 10µF and 100nF.
When the VTHSEL is high or floating and the VDD voltage falls below VTH2, the supervisor starts the
deactivation sequence immediately as shown in Figure 13 and the OFF pin goes low (Figure 11).
When the VTHSEL is tied low and the VDD voltage falls below VTH3 for more than about 20µs, the
supervisor starts the deactivation sequence as shown in Figure 10.
Figure 10: Deactivation sequence when V
8/14
DD
falls down the V
TH3
voltage
Conditions:
V
V
V
V
5/3V=L.
When the V
the Vth3 value (about 3.0V in this
case) a deactivation sequence
starts; after 20µs the RST pin
goes low and then the other
signals follow the deactivation
sequence.
THSEL
DD
DD
DDP
=4V to 0V;
S.R.=0.015V/µs
=5V;
= GND;
DD
voltage falls below