RM0312 datasheet(251/275 Pages) STMICROELECTRONICS

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RM0312

ProxSense™ (PXS)

273

22.3

PXS pins

Note: m = 0 to 14 (transmit outputs) and n = 0 to 9 (receivers).

22.4

Functional description

22.4.1

General description

Figure 95 shows the block diagram of the ProxSense peripheral.

There are 10 receiver channels so that the capacitance on 10 electrodes can be measured

in parallel. Two pins are associated with each receiver channel; one is from the A group and

the other is from the B group, enabling the measurement of 20 electrodes.

Each receiver channel converts the capacitance it measures on an electrode into a 16-bit

value (PXS_RXnCNTR). It acquires this result by repeatedly transferring a charge from the

electrode to an internal capacitor, Cs. The resulting value is the number of charge transfers

which are necessary to raise the voltage on the Cs above a reference voltage. Thus, a

larger capacitance results in a smaller value.

In Projected Capacitance mode (selected using PXS_RXCRn), the capacitance between a

Tx electrode and the corresponding Rx electrode is measured. In this case, the acquisition

gives larger data values as a conductive object approaches the intersection of these two

electrodes. This is because the Tx electrode starts to couple more with the conductive

object and less with the Rx electrode.

Table 53. PXS pins

Name

Signal type

Description

PXS_RXnA/

PXS_RXnB

Analog input/output

20 receivers, each associated with one of the 10 internal

CS capacitors into which the electrode charge is

transferred (RXAn and RXBn share one Cs).

PXS_TXm

Digital output

15 transmit outputs, each of which can drive an electrode

that transfers a charge (through a projected capacitor) into

a second electrode connected to a Rx pin.

PXS_RFIN

Analog input

Antenna input to detect radio frequency (RF) noise.

PXS_TRIG

Digital input

External trigger for the acquisition.

PXS_VREG

External decoupling

capacitor

A capacitance must be connected to stabilize the dedicated

ProxSense regulator.

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