ad7879-1acbz-rl Analog Devices, Inc., ad7879-1acbz-rl Datasheet - Page 16

• Manufacturer Part Number: ad7879-1acbz-rl
• Description: Low Voltage Controller For Touch Screens
• Manufacturer: Analog Devices, Inc.

AD7879
MEDIAN AND AVERAGING FILTERS
As explained in the Touch Screen Principles section, touch
screens are composed of two resistive layers, normally placed
over an LCD screen. Because these layers are in close proximity
to the LCD screen, noise can be coupled from the screen onto
these resistive layers, causing errors in the touch screen posi-
tional measurements.
The AD7879 contains a filtering block to process the data and
discard the spurious noise before sending the information to
the host. The goal of this block is not just the suppression of
noise; the on-chip filtering also remarkably reduces the host
processing loading.
The processing function consists of two filters that are applied
to the converted results: the median filter and the averaging filter.
The median filter suppresses the isolated out-of-range noise and
sets the number of measurements to be taken. These measure-
ments are arranged in a temporary array, where the first value
is the smallest measurement and the last value is the largest
measurement. Bit 6 and Bit 5 in Control Register 2 (M1, M0)
set the window of the median filter, and therefore, the number
of measurements taken.
Table 6. Median Filter Size
M1
0
0
1
1
The averaging filter size determines the number of values to
average. Bit 8 and Bit 7 in Control Register 2 (A1, A0) allow the
average of 2, 4, 8, or 16 samples. Only the final averaged result is
written into the results register.
Table 7. Averaging filter Size
A1
0
0
1
1
0
1
0
1
0
1
0
1
M0
A0
Function
Median filter does not operate
4 measurements
8 measurements
16 measurements
Function
Average of 2 middle samples
Average of 4 middle samples
Average of 8 middle samples
Average of 16 samples
Rev. 0 | Page 16 of 36
When both filter values are 00, only one measurement is
transferred to the register map.
The number chosen with the M1 and M0 settings must be
equal to or larger than the number chosen with the A1 and
A0 settings. If both settings select the same number, the median
filter is switched off.
Table 8. Median Averaging Filters (MAVF) Settings
M = A
M < A
M > A
Example
M1, M0 = 11, A1, A0 = 10; in this example the median filter
has a window size of 16. This means that 16 measurements are
taken and arranged in descending order in a temporary array.
The averaging window size in this case is 8. The output is an
average of the middle 8 values of the 16 measurements taken
with the median filter.
CONVERTED
12-BIT SAR
RESULTS
ADC
13
16
15
10
11
12
14
6
2
4
5
9
3
8
1
7
Function
Median filter does not operate; output is the
average of A converted results
Not possible because the median filter size is
always bigger than the averaging window size
Output is the average of the middle A values
from the array of M measurements
Figure 27. Median and Averaging Filter Example
16 MEASUREMENTS
ARRANGED
MEDIAN
FILTER
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
M = 16
MIDDLE 8 VALUES
AVERAGE OF
AVERAGING
FILTER
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
A = 8