AD7142 datasheet(20/68 Pages) AD

20 / 68 page

AD7142

Rev. 0 | Page 20 of 68

FIFO CONTROL

As shown in Figure 30, there are a number of FIFOs

implemented on the AD7142. These FIFOs are located in

Bank 3 of the on-chip memory. The FIFOs are used by the on-

chip logic to run the environmental calibration, adaptive

threshold, and proximity algorithms.

AVG_FP_SKIP and AVG_LP_SKIP

In Register 0x001, Bits[13:12]are the slow FIFO skip control for

full power mode, AVG_FP_SKIP. Bits[15:14] in the same

AVG_LP_SKIP. These values determine which CDC samples

are not used (skipped) in the slow FIFO. Changing theses values

slows down or speeds up the rate at which the ambient

capacitance value tracks the measured capacitance value read by

the converter.

The slow FIFO is used by the on-chip logic to track the ambient

capacitance value. The slow FIFO expects to receive samples

from the converter at a rate of 33 ms to 40 ms. AVG_FP_SKIP

and AVG_LP_SKIP are used to normalize the frequency of the

samples going into the FIFO, regardless of how many

conversion stages are in a sequence.

Determining the AVG_FP_SKIP and AVG_LP_SKIP value is

only required once during the initial setup of the capacitance

sensor interface. Recommended values for these settings when

using all 12 conversion stages on the AD7142 are:

AVG_FP_SKIP = 11 = skip 31 samples

AVG_LP_SKIP = 11 = skip 3 samples

FF_SKIP_CNT

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.

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.

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

FF_SKIP_CNT

FAST FIFO Update Rate

DECIMATION = 128

DECIMATION = 256

1.536 × (SEQUENCE_STAGE_NUM + 1) ms

3.072 × (SEQUENCE_STAGE_NUM + 1) ms

6.144 × (SEQUENCE_STAGE_NUM + 1) ms

4.608 × (SEQUENCE_STAGE_NUM + 1) ms

9.216 × (SEQUENCE_STAGE_NUM + 1) ms

6.144 × (SEQUENCE_STAGE_NUM + 1) ms

12.288 × (SEQUENCE_STAGE_NUM + 1) ms

7.68 × (SEQUENCE_STAGE_NUM + 1) ms

15.36 × (SEQUENCE_STAGE_NUM + 1) ms

9.216 × (SEQUENCE_STAGE_NUM + 1) ms

18.432 × (SEQUENCE_STAGE_NUM + 1) ms

10.752 × (SEQUENCE_STAGE_NUM + 1) ms

21.504 × (SEQUENCE_STAGE_NUM + 1) ms

12.288 × (SEQUENCE_STAGE_NUM + 1) ms

24.576 × (SEQUENCE_STAGE_NUM + 1) ms

13.824 × (SEQUENCE_STAGE_NUM + 1) ms

27.648 × (SEQUENCE_STAGE_NUM + 1) ms

15.36 × (SEQUENCE_STAGE_NUM + 1) ms

30.72 × (SEQUENCE_STAGE_NUM + 1) ms

16.896 × (SEQUENCE_STAGE_NUM + 1) ms

33.792 × (SEQUENCE_STAGE_NUM + 1) ms

18.432 × (SEQUENCE_STAGE_NUM + 1) ms

36.864 × (SEQUENCE_STAGE_NUM + 1) ms

19.968 × (SEQUENCE_STAGE_NUM + 1) ms

39.936 × (SEQUENCE_STAGE_NUM + 1) ms

21.504 × (SEQUENCE_STAGE_NUM + 1) ms

43.008 × (SEQUENCE_STAGE_NUM + 1) ms

23.04 × (SEQUENCE_STAGE_NUM + 1) ms

46.08 × (SEQUENCE_STAGE_NUM + 1) ms

24.576 × (SEQUENCE_STAGE_NUM + 1) ms

49.152 × (SEQUENCE_STAGE_NUM + 1) ms

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