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AD7142
Rev. A | Page 20 of 72
05
702
-020
NOTES
1. SEQUENCE CONVERSION TIME
tCONV_LP = tCONV_FP + LP_CONV_DELAY
2.
tCALDIS = tCONV_LP × LP_PROXIMITY_CNT × 4
3.
tRECAL_TIMEOUT = tCONV_FP × LP_PROXIMITY_RECAL
4.
tRECAL = 2 × tCONV_LP
CALIBRATION ENABLED
tRECAL_TIMEOUT
16
30
70
tCONV_LP
RECALIBRATION TIME-OUT
PROXIMITY DETECTION
(INTERNAL)
CALIBRATION
(INTERNAL)
CDC CONVERSION SEQUENCE
(INTERNAL)
RECALIBRATION
(INTERNAL)
05
702
-020
USER APPROACHES
SENSOR HERE
USER LEAVES SENSOR
AREA HERE
MEASURED CDC VALUE > STORED AMBIENT
BY PROXIMITY_RECAL _LVL
tRECAL
tCALDIS
CALIBRATION DISABLED
Figure 31. Low Power Mode Proximity Detection with Forced Recalibration Example with LP_PROXIMITY_CNT = 4 and LP_PROXIMITY_RECAL = 40
FF_SKIP_CNT
The proximity detection fast FIFO is used by the on-chip logic to determine if proximity is detected. The fast FIFO expects to receive
samples from the converter at a set rate. FF_SKIP_CNT is used to normalize the frequency of the samples going into the FIFO, regardless
of how many conversion stages are in a sequence. In Register 0x02, Bits[3:0] are the fast filter skip control, FF_SKIP_CNT. This value
determines which CDC samples are not used (skipped) in the proximity detection fast FIFO.
Determining the FF_SKIP_CNT value is required only once during the initial setup of the capacitance sensor interface
. Table 13 shows
how FF_SKIP_CNT controls the update rate to the fast FIFO. Recommended value for this setting when using all 12 conversion stages on
the AD7142 is FF_SKIP_CNT = 0000 = no samples skipped.
Table 13. FF_SKIP_CNT Settings
FAST FIFO Update Rate
FF_SKIP_CNT
DECIMATION = 128
DECIMATION = 256
0
1.536 × (SEQUENCE_STAGE_NUM + 1) ms
3.072 × (SEQUENCE_STAGE_NUM + 1) ms
1
3.072 × (SEQUENCE_STAGE_NUM + 1) ms
6.144 × (SEQUENCE_STAGE_NUM + 1) ms
2
4.608 × (SEQUENCE_STAGE_NUM + 1) ms
9.216 × (SEQUENCE_STAGE_NUM + 1) ms
3
6.144 × (SEQUENCE_STAGE_NUM + 1) ms
12.288 × (SEQUENCE_STAGE_NUM + 1) ms
4
7.68 × (SEQUENCE_STAGE_NUM + 1) ms
15.36 × (SEQUENCE_STAGE_NUM + 1) ms
5
9.216 × (SEQUENCE_STAGE_NUM + 1) ms
18.432 × (SEQUENCE_STAGE_NUM + 1) ms
6
10.752 × (SEQUENCE_STAGE_NUM + 1) ms
21.504 × (SEQUENCE_STAGE_NUM + 1) ms
7
12.288 × (SEQUENCE_STAGE_NUM + 1) ms
24.576 × (SEQUENCE_STAGE_NUM + 1) ms
8
13.824 × (SEQUENCE_STAGE_NUM + 1) ms
27.648 × (SEQUENCE_STAGE_NUM + 1) ms
9
15.36 × (SEQUENCE_STAGE_NUM + 1) ms
30.72 × (SEQUENCE_STAGE_NUM + 1) ms
10
16.896 × (SEQUENCE_STAGE_NUM + 1) ms
33.792 × (SEQUENCE_STAGE_NUM + 1) ms
11
18.432 × (SEQUENCE_STAGE_NUM + 1) ms
36.864 × (SEQUENCE_STAGE_NUM + 1) ms
12
19.968 × (SEQUENCE_STAGE_NUM + 1) ms
39.936 × (SEQUENCE_STAGE_NUM + 1) ms
13
21.504 × (SEQUENCE_STAGE_NUM + 1) ms
43.008 × (SEQUENCE_STAGE_NUM + 1) ms
14
23.04 × (SEQUENCE_STAGE_NUM + 1) ms
46.08 × (SEQUENCE_STAGE_NUM + 1) ms
15
24.576 × (SEQUENCE_STAGE_NUM + 1) ms
49.152 × (SEQUENCE_STAGE_NUM + 1) ms