Tuning of the thresholds

AN4316

12/26

DocID024847 Rev 2

1.

Connect the final hardware to a PC through the ST-Link and power the application

2.

Download the firmware which will be used in the final application with the final

parameters of the STMTouch driver.The default detection thresholds can be set to a

low value but keeping it higher than the noise Level.

3.

Launch STM Studio and configure it as explained in Section 2: STM Studio overview on

page 5.

4.

Use the standard finger as described in Section 4.1: Use of a standard test finger on

page 11.

5.

Touch a touchkey and move the finger in order to find the maximum delta and write

down this value, then repeat it for each touchkey. If around the maximum delta there is

still significant jittering on the measure, then compute the average and use it as

baseline.

These values will be the baseline of all the thresholds. If a significant variation exists

between the baseline of the application touchkeys, it is recommended to set a specific

threshold for each touchkey.

The detection threshold, which must be exceeded in order to report the touchkey as

detected, must be set between 55 % and 65 % of the baseline. The end-of-detection

threshold, below which the key is not detected anymore, must be set between 35 % and

45 % of the baseline.

Figure 5. Threshold position

An example of threshold firmware adjustment, with a baseline measured at 80 and a

threshold adjusted between ~65 % and ~55 % of the baseline is shown below:

MyTKeys[0].p_Param->DetectInTh = 50; // Key 1 detection threshold

MyTKeys[0].p_Param->DetectOutTh = 30; // Key 1 end of detection threshold

The calibration threshold (TSLPRM_TKEY_CALIB_TH) can be common to all keys and set

to 60 % of the maximum baseline.