參數(shù)資料
型號： EVAL-AD7879-1EBZ
廠商： Analog Devices Inc
文件頁數(shù)： 10/41頁
文件大?。?/td> 0K
描述： BOARD EVALUATION FOR AD7879-1
標(biāo)準(zhǔn)包裝： 1
主要目的： 接口，觸摸屏控制器
嵌入式： 否
已用 IC / 零件： AD7879-1
主要屬性： 4 線電阻式觸控屏控制器，I²C 和 SPI 接口
次要屬性： 圖形用戶界面，USB 接口
已供物品： 板，線纜，CD，觸控屏
AD7879/AD7889
Rev. C | Page 17 of 40
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
positional measurements.
The AD7879/AD7889 contain a filtering block to process the
data and discard the spurious noise before sending the infor-
mation to the host. The purpose of this block is not only the
suppression of noise; the on-chip filtering also greatly 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 measurements
are arranged in a temporary array, where the first value is the
smallest measurement and the last value is the largest measure-
ment. Bit 6 and Bit 5 in Control Register 2 (MED1, MED0) set
the window of the median filter and, therefore, the number of
measurements taken.
Table 8. Median Filter Size
MED1
MED0
Number of Measurements
0
Median filter disabled
0
1
4
1
0
8
1
16
The averaging filter size determines the number of values to
average. Bit 8 and Bit 7 in Control Register 2 (AVG1, AVG0)
set the average to 2, 4, 8, or 16 samples. Only the final averaged
result is written into the result register.
Table 9. Averaging Filter Size
AVG1
AVG0
Filter Size
0
Average of 2 middle samples
0
1
Average of 4 middle samples
1
0
Average of 8 middle samples
1
Average of 16 samples
When both filter values are 00, only one measurement is
transferred to the register map.
The number specified with the MED1 and MED0 settings must
be greater than or equal to the number specified with the AVG1
and AVG0 settings. If both settings specify the same number,
the median filter is switched off.
Table 10. Median Averaging Filters (MAVF) Settings
Setting
Function
M = A
Median filter is disabled; output is the average of
A converted results
M > A
Output is the average of the middle A values from
the array of M measurements
M < A
Not possible because the median filter size is always
larger than the averaging window size
Example
In this example, MED1, MED0 = 11 and AVG1, AVG0 = 10;
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 example is 8. The output is
the average of the middle eight values of the 16 measurements
taken with the median filter.
AVERAGING
FILTER
MEDIAN
FILTER
12-BIT SAR
ADC
6
2
13
4
16
5
15
10
9
3
11
8
1
12
14
7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
M = 16
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
A = 8
CONVERTED
RESULTS
16 MEASUREMENTS
ARRANGED
AVERAGE OF
MIDDLE 8 VALUES
07667-
027
Figure 27. Median and Averaging Filter Example
It takes approximately 2 μs to sort the data in the rank filter
(tSORT in Figure 34); tSORT adds to the update rate of the AD7879.
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