Datasheet

M41ST87Y, M41ST87W Operating modes
Doc ID 9497 Rev 10 27/54
2.9 Internal tamper pull-up/down current
Depending on the capacitive and resistive loading of the tamper pin input (TP
XIN
), 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/TCLO
X
) determines the sizing of the internal pull-up/-down. TCHI/TCLO
X
= '1' uses a
1 MΩ pull-up/-down resistor, while TCHI/TCLO
X
= '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 TCM
X
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 f
c
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) f
c
1/f
c
(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
f
c
12PiRC⋅⋅⋅()⁄=
TP
IN
R
C
To Tamper Detect Switch
AI11185