ProxSense and low power modes

AN3404

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Doc ID 018847 Rev 3

6.3

Low power features in the ProxSense peripheral

Power consumption can be reduced in different ways between ProxSense acquisitions. The

ProxSense can be switched off and its clock gated by resetting the PCKENR17 bit in the

CLK_PCKENR1. Consequently, the ProxSense has the following special low-power

features.

6.3.1

LOW_POWER bit

The most efficient way to reduce power consumption between two acquisitions is to set the

LOW_POWER bit in the PXS control register 1 (PXS_CR1). In this case, the ProxSense

clock (HSI_PXS) and the ProxSense regulator are switched off as soon as an acquisition is

completed and the results are still available in the PXS counter register of receiver channel n

(PXS_RXnCNTR).

By switching off the ProxSense regulator, a stabilization delay is introduced at the beginning

of each new acquisition. This delay time depends on the number of enabled PXS_RX pins.

For a few receivers, a 1.7 ms delay must be selected.

For successive acquisitions, it is recommended to reset the LOW_POWER bit and to set the

ProxSense interrupt to a high priority. This saves the stabilization time on the one hand and

greatly reduces the time between the acquisitions while the ProxSense clock and regulator

are set. The LOW_POWER bit must be set again once the last acquisition has been

launched. In this way, the regulator and the clock are switched off as soon as the last

acquisition is completed.

6.3.2

Stabilization time

The stabilization time is needed to ensure the regulator is fully operational at start-up. This

delay can be set at different values in the PXS control register 3 (PXS_CR3). The values

are: 1.7 ms (default), 500 µs or 120 µs.

The more the regulator is loaded, the faster the stabilization. In other words, launching an

acquisition with many receivers enabled speeds up the regulator stabilization. But, it is the

impedance of the receivers that is more important than their number. The best way to check

that the stabilization time is long enough, is to perform two successive acquisitions without

switching off the regulator between them, in ensuring that the ProxSense is off before the

first acquisition. If the results of the two acquisitions are similar, this means the regulator is

correctly stabilized before the first acquisition.

In case of successive acquisitions, in order to select the lowest stabilization time, the

acquisition with the maximum number of receivers must be launched first.

6.3.3

Bias parameter

This is the nominal bias current for the OpAmp which maintains the PXS_RX

n pins at the

final (discharged) voltage while the transmit lines are transitioning (back) low. This function

is valid only for projected capacitance measurements. A higher bias is needed when the

projected capacitance (the capacitance between each Rx line and each Tx line) is high.

This parameter is set in PXS_CR3. The lower its value, the less the consumption.