MPR084

Sensors

24

Freescale Semiconductor

Preliminary

8Calibration

8.1

Introduction

The MPR084 is self-calibrating. This is done both at initial start-up of the device and during run time.

8.2

Initial Start-up Conditions

Initial calibration of the MPR084 occurs every time the device resets. The first key detection cycle is used as a baseline

capacitance value for all remaining calculations. Thus, a touch is detected by taking the difference between this baseline value

and the current capacitance on the electrode.

8.3

Auto-Calibration

The MPR084 has an auto-calibration feature. This is enabled through the Touch Pad Configuration Register (Section 3.4), by

setting the ACE bit high. Auto calibration is done by two mechanisms. The basic auto-calibration will recalculate the baseline

value after 6 sample periods. Thus the auto calibrate period can be calculate by multiplying the master clock period (in

milliseconds) and the touch acquisition sample period with a factor of 64.

Equation 6

If a touch is currently being detected the auto-calibration will not engage and calibration will be ignored. The device can also be

calibrated when a key is being touched, this is controlled by stuck key detection.

8.4

Stuck Key Detection

The Stuck Key Detection system allows the application to specify the maximum amount of time a touch should be detected before

it is calibrated into the baseline and the touch is ignored. This is controlled by setting the Stuck Key Timeout multiplication factor

(SKT). The timeout period can be calculated by multiplying the SKT, master clock period (in ms) and touch acquisition sample

period with a factor of 64.

Equation 7

When Stuck Key Detection is off a touched key will remain touched indefinitely and never be calibrated into the baseline value.

8.4.1

Stuck Key Timeout Register

Key Timeout Register is 0x12.

Figure 27. Stuck Key Timeout Register

7

6

543210

R

SKT

W

Reset:

0

0

000000

= Unimplemented

Table 16. Stuck Key Timeout Register Field Descriptions

Field

Description

7:0

SKT

Stuck Key Timeout – The Stuck Key Timeout field selects or reports the

multiplication factor that is used to determine how often electrodes are calibrated

while a touch is being detected.

00000000 Encoding 0 – Turns off Stuck Key Detection

00000001 Encoding 1 – Sets the SKT multiplication factor to 2

~

11111111 Encoding 255 – Sets the SKT multiplication factor to 256

AutoCalibrationPeriod ms

()

MCP

TASP

×

64

×

=

AutoCalibrationPeriod ms

()

MCPTASPSKT

×

×

64

×

=