DocID17613 Rev 3

13/15

AN3236

Working around the limitations

14

5

Working around the limitations

The limitations can be reduced through careful management of the touchkeys and touchkey

matrices.

5.1

Touchkeys set definition

For extra touchkeys and touchkey matrices, it is important to define sets of touchkeys in

which the touchkeys cannot be touched at the same time. This allows multi-touching

capability across sets.

Sets can be catagorized as follows:

•

a touchkey matrix and a few independent touchkeys which allow simultaneous touches

on any of the independent touchkeys plus one touch on the touchkey matrix.

•

two touchkey matrices which allow simultaneous touches on each touchkey matrix.

5.2

Detection exclusion system (DxS) usage

For a touchkey matrix (without single electrode touchkeys), DxS allows only the first

detected touch inside a group to be reported. By defining a DxS group with channel rows

and another group with channel columns, only one touchkey can be activated at a time. For

application managing only single touch, this simplifies the processing at application level.

6

Conclusion

The described guidelines allow the touchkey capabilities of STMicroelectronics

microcontrollers to be improved without modifying the touch sensing firmware library. In

application software, the processing must provide a simple decoding step to determine

which touchkey is touched when a pair of channels is activated.