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AD7148

Rev. 0 | Page 17 of 56

NONCONTACT PROXIMITY DETECTION

The AD7148 internal signal processing continuously monitors

all capacitance sensors for noncontact proximity detection. This

feature provides the ability to detect when a user is approaching

a sensor, at which time all internal calibration is immediately

disabled while the AD7148 is automatically configured to detect

a valid contact.

The proximity control register bits are described in Table 12. The

FP_PROXIMITY_CNT and LP_PROXIMITY_CNT register bits

(Address 0x002[11:4]) control the length of the calibration disable

period after the user leaves the sensor and proximity is no longer

active in full and low power modes.

The calibration is disabled during this time and is enabled

again at the end of this period, provided that the user is no longer

approaching, or in contact with, the sensor. Figure 26 and Figure

show examples of how these registers are used to set the full

and

low power mode calibration disable periods.

The calibration disable period in full power mode is equal to

FP_PROXIMITY_CNT × 16 × time taken for one conversion

sequence in full power mode.

The calibration disable period in low power mode is equal to

LP_PROXIMITY_CNT × 4 × time taken for one conversion

sequence in low power mode.

Table 12. Proximity Control Registers (See Figure 30)

Bits

Length

Description

FP_PROXIMITY_CNT

4 bits

0x002[7:4]

Calibration disable time in full power mode.

LP_PROXIMITY_CNT

4 bits

0x002[11:8]

Calibration disable time in low power mode.

FP_PROXIMITY_RECAL

10 bits

0x004[9:0]

Full power mode proximity recalibration time.

LP_PROXIMITY_RECAL

6 bits

0x004[15:10]

Low power mode proximity recalibration time.

PROXIMITY_RECAL_LVL

8 bits

0x003[7:0]

Proximity recalibration level. This value, multiplied by 16, controls the

sensitivity of Comparator 2 in Figure 30.

PROXIMITY_DETECTION_RATE

6 bits

0x003[13:8]

Proximity detection rate. This value, multiplied by 16, controls the

sensitivity of Comparator 1 in Figure 30.

PROXIMITY DETECTION

(INTERNAL)

CALIBRATION

(INTERNAL)

12 3 4 5

6 78 9 10 11 12 13 14 15 16

CDC CONVERSION SEQUENCE

(INTERNAL)

CALIBRATION ENABLED

CALIBRATION DISABLED

tCONV_FP

USER LEAVES SENSOR

AREA HERE

USER APPROACHES

SENSOR HERE

tCALDIS

Figure 26. Full Power Mode Proximity Detection Example with FP_PROXIMITY_CNT = 1

NOTES

1. SEQUENCE CONVERSION TIME

tCONV_LP = tCONV_FP + LP_CONV_DELAY

2. PROXIMITY IS SET WHEN USER APPROACHES THE SENSOR AT WHICH TIME THE INTERNAL CALIBRATION IS DISABLED.

tCALDIS = (tCONV_LP × LP_PROXIMITY_CNT × 4)

CALIBRATION ENABLED

CALIBRATION DISABLED

PROXIMITY DETECTION

(INTERNAL)

CALIBRATION

(INTERNAL)

tCALDIS

tCONV_LP

12 34 5 6 7 8 9 10 11 1213 14 15 16

CDC CONVERSION SEQUENCE

(INTERNAL)

USER LEAVES

SENSOR AREA HERE

USER APPROACHES

SENSOR HERE

1718 19 20 21 2223 24

Figure 27. Low Power Mode Proximity Detection with LP_PROXIMITY_CNT = 4

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