AN3404

ProxSense and low power modes

Doc ID 018847 Rev 3

19/30

6

ProxSense and low power modes

6.1

Possible CPU low power modes combined with ProxSense

Once a ProxSense acquisition is launched, the CPU and its clock are not needed to perform

the acquisition. Wait modes, entered by a WFE or a WFI instruction, can be used as well as

Active-halt mode by executing a HALT instruction. If WFE is used, either a PXS interrupt or

the PXS event must be enabled but, if WFI or HALT is used then a PXS interrupt must be

enabled. In most cases, the PXS end of completion interrupt is used.

In Active-halt, it is possible to combine the auto-wakeup with the ProxSense. The acquisition

can be time limited by configuring the maximum counter enable register (PXS_MAXENR)

and maximum counter value register (PXS_MAXR).

Activation level (AL bit set in CFG_GCR register) can be used in the case of successive

acquisitions. The AL bit should be reset once the last acquisition has been performed in

order to return to the main processing and take the appropriate action according to the

acquisition results.

6.2

Main factor of the ProxSense acquisition consumption

The best way to reduce consumption of the ProxSense acquisition itself is to decrease the

acquisition time. This can be done by setting the ProxSense in order to get a lower count

when the key is not touched. In the STM8TL5x STMTouch Library, this is done by reducing

the KEY_TARGET_REFERENCE parameter in the touch sensing configuration file. This

must be done carefully as reducing this count also reduces the sensitivity of the touchkey. A

trade-off must be found between the consumption and the sensitivity.

Table 3.

ProxSense compatibility with low power modes

Low power mode

Instruction

Exit by PXS

CPU

HSI

HSI_PXS

1.

‘SYNCHRO’ indicates compatibility with the synchronization feature.

Wait

WFE

Yes

Off

Off

On

Yes

WFI

Yes

Off

Off

On

Yes

Active-halt

HALT

Yes

Off

Off

On

2.

Once in Active-halt, the synchronization edge cannot be detected by the ProxSense peripheral but, Active-

halt mode can be entered once the acquisition is started. To synchronize with an external signal, the

acquisition must be launched from an external interrupt routine.

Halt

HALT

No

Off

Off

Off

No