MPR121 datasheet(38/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

For the system to recognize a touch the delta must be greater than the Touch Threshold.

Delta = Baseline - Data

Trigger Touch

→ Delta > Touch Threshold

A release is triggered when the Delta falls below the Release Threshold. This can happen for both changes to the Baseline

and actual Data changes. To understand how the Baseline can change, refer to AN3891.

Trigger Release

→ Delta < Touch Threshold

DEBOUNCE TOUCH AND RELEASE REGISTER

The last register available in this set is the Debounce register. The Debounce register maintains the accuracy of touch and

releases by further improving the performance. The debounce allows two different settings to prevent bounce in the end system.

If the value is set to 0x22, the requirement would be three sequential changes in status before the change would be recognized.

Figure 45. Debounce Touch and Release Register

CONCLUSION

The use of each of the features together can have a great effect of the jitter and false couch rejection. Jitter is prevented by

utilizing the two threshold settings. Thus the provided hysteresis prevent Jitter on the data from going through the to the output

Depending on environmental conditions, the Debounce can be used to eliminate the remainder of dramatic change of the signal

that aren’t really touches.

Additional filtering can be done before the data gets to the touch detection system. Refer to Freescale Application Note

AN3890.

543210

Reset:

000000

= Unimplemented

Table 33. Debounce Touch and Release Register Field Descriptions

Field

Description

6:4

Debounce Release – The Debounce Release determines the number of sequential

release detections before an interrupt is triggered and a release is reported.

000 Encoding 0 - Consecutive releases detection before Status change is 1

001 Encoding 1 - Consecutive releases detection before Status change is 2

007 Encoding 7 - Consecutive releases detection before Status change is 8

2:0

Debounce Touch – The Debounce Touch determines the number of sequential touch

detections before an interrupt is triggered and a touch is reported.

000 Encoding 0 - Consecutive touch detection before Status change is 1

001 Encoding 1 - Consecutive touch detection before Status change is 2

007 Encoding 7 - Consecutive touch detection before Status change is 8

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