M41ST87W 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

M41ST87W 数据表(PDF) 27 Page - STMicroelectronics

M41ST87W 数据表(HTML) 27 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

数据表下载

链接网址

M41ST87W 数据表(PDF) 27 Page - STMicroelectronics

M41ST87W 数据表(HTML) 27 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

数据表 下载

链接网址

数据表下载