STM8T141 datasheet(26/50 Pages) STMICROELECTRONICS

26 / 50 page

Design guidelines

STM8T141

26/50

Doc ID 15699 Rev 7

Design guidelines

8.1

Shield function

The STM8T141 offers a built-in shielding function. This function provides the following

advantages for designing the end-application:

●

Sensing electrode separated from sealed electronics.

●

Sensing wire shielded from unwanted environmental interferences.

●

Enhanced proximity detection when used with battery (DC) applications.

The shield principle consists in actively driving the shield plane or element with the same

signal as that of the electrode. The parasitic capacitance between the electrode and the

shield does not need to be charged anymore and its effect on the sensitivity is cancelled.

Note:

Grounding the shield reduces the sensitivity of the keys and may render the system

unusable.

8.1.1

Shield application example

Ideally, a coaxial cable is used for the shield. A RX (typically 2 k) resistor should be

connected to the CX pin. The other side of the RX resistor should be connected to the center

core of the coaxial cable. The SHLDOUT pin should be connected to the metallic shield part

of the coaxial cable. A pull-up resistor (RSHIELD) should be added between SHLDOUT and

VDD as shown in Figure 20.

The example shown in Figure 20 is given for RX = 2 k, RSHIELD = 100 k, and VDD = 5 V

(a).

This setup has been successfully implemented with a coaxial cable of up to 4 m.

A longer coaxial cable could be used, but this would mean decreasing the RSHIELD resistor,

and consequently increasing current consumption.

Note:

A smaller RSHIELD ensures better shielding but increases current consumption (see

Figure 20).

VDD must range from 4.5 to 5.5 V to use the shield function.Please refer to Table 12: Operating characteristics

for the correct power supply operating voltage when using the shield function.

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