MPR121 datasheet(16/57 Pages) FREESCALE

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MPR121

Sensors

Freescale Semiconductor

ELECTRODE CHARGE TIME

Figure 11. Electric Charge Time Register

AUTO-CONFIGURATION

One of the new features added in the MPR121 that was not included in the MPR03X is the ability to automatically configure

the Charge Current the Charge Time. This eliminates much of the guess involved with touch sensors and allows the same set-

tings to properly configure the device for a wide range of application and electrodes. As show earlier in this document, the sen-

sitivity of the sensor is maximized by having the baseline be as high as possible for a specific baseline capacitance. The

restriction on the high side is that a system should not charge above VDD - 0.7 V due to this being a non-linear region. Thus the

target voltage used is approximately VDD - 0.7 V.

This implies that the automatic configuration system should target approximately 156 when VDD is 1.8 V and 196 when VDDis

3.0 V. The following three registers should be set based on the VDD in the system. If the voltage is unregulated, set the values

assuming the lowest voltage necessary for the battery. If the final voltage supply in the system is not known, just use the 1.8 V

values as they represent the worst case. This lower setting will not dramatically affect the performance, thus the 1.8 V could be

considered default and be used in all cases where fine tuning is not required.

543210

CDT#

Reset:

000000

= Unimplemented

Table 12. Electrode Charge Time Register Field Descriptions

Field

Description

6:4

CDT#

Electrode # Charge Discharge Time – The Charge Discharge Time field selects the

amount of time an electrode charges and discharges.

000 Encoding 0 – Global value is used.

001 Encoding 1 – Time is set to 0.5

μs

010 Encoding 2 – Time is set to 1

μs

11 Encoding 7 – Time is set to 32

μs.

2:0

CDT#

Electrode # Charge Discharge Time – The Charge Discharge Time field selects the

amount of time an electrode charges and discharges.

000 Encoding 0 – Global value is used.

001 Encoding 1 – Time is set to 0.5

μs

010 Encoding 2 – Time is set to 1

μs

11 Encoding 7 – Time is set to 32

μs.

Table 13.

Voltage (VDD)

VDD - 0.7 V

ADC

Baseline

1.8 V

1.1 V

625

156

VDD

2.3 V

785

196

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