M41ST87YMX6F datasheet(27/52 Pages) STMICROELECTRONICS

27 / 52 page

M41ST87Y, M41ST87W

Operating modes

Doc ID 9497 Rev 8

27/52

2.9

Internal tamper pull-up/down current

Depending on the capacitive and resistive loading of the tamper pin input (TPXIN), the user

may require more or less current from the internal pull-up/down used when monitoring the

normally closed switch mode. The state of the tamper current hi/tamper current low bit

(TCHI/TCLOX) determines the sizing of the internal pull-up/-down. TCHI/TCLOX = '1' uses a

1 M

Ω pull-up/-down resistor, while TCHI/TCLOX = '0' uses a 10 MΩ pull-up/-down resistor

(see Figure 18 on page 24).

2.10

Avoiding inadvertent tampers (normally closed

configuration)

In some applications it may be necessary to use a low pass filter to reduce electrical noise

on the tamper input pin when the TCMX bit = 0 (normally closed). This is especially true if

the tamper detect switch is located some distance (> 6”) from the tamper input pin. A low

pass filter can prevent unwanted, higher frequency noise from inadvertently being detected

as a tamper condition caused by the “antenna-effect” (produced by a longer signal wire or

mesh). This low pass filter can be constructed using a series resistor (R) in conjunction with

a capacitor (C) on the tamper input pin.

The cut-off frequency fc is determined according to the formula:

Figure 21.

Low-pass filter implementation for noise immunity

Table 6.

Calculated cut-off frequency for typical capacitance and resistance

values

R (

Ω)C (F)

1/fc (s)

1000

1.00E-09

15.9 MHz

6.28 µs

1000

1.00E-06

159.2 Hz

6.28 ms

5000

1.00E-09

31.8 kHz

31.4 µs

5000

1.00E-06

31.8 Hz

31.4 ms

10000

1.00E-09

15.9 kHz

62.8 µs

10000

1.00E-06

15.9 Hz

62.8 ms

12 Pi R C

⋅⋅ ⋅

()

⁄

TPIN

To Tamper Detect Switch

AI11185

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