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AN1775 Datasheet(PDF) 6 Page - Freescale Semiconductor, Inc

Part No. AN1775
Description  Expanding Digital Input with an A/D Converter
Download  12 Pages
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Maker  FREESCALE [Freescale Semiconductor, Inc]
Homepage  http://www.freescale.com
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AN1775 Datasheet(HTML) 6 Page - Freescale Semiconductor, Inc

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Application Note
AN1775 Rev. 1
Some notes on the spreadsheet:
•VIn is determined by decrementing the high voltage reference by
the voltage step for each segment in the ladder. (See Figure 4.)
• Rn is the resistor value of the current key segment needed to form
desired equivalent resistance Req.
• Req represents the equivalent resistance of ladder, including the
current resistor (Rn) and excluding the pullup resistor (R0).
• Vinmin and Vinmax are the minimum and maximum voltages that
can be arbitrated as a particular key. In this case, VIn +/–Vstep/2
was used.
• ADDRmin and ADDRmax are the ADC data register value range
used to represent a given keypress. This range can be narrowed
or widened to affect precision. In this case, the ranges were
maximized, so no conversion result is undefined.
• This particular spreadsheet did not use resistor precision, but this
could be considered to further pad the error.
Table 1. Spreadsheet Calculations
VDD =
5
Converter Resolution =
0.01961 (Vdd/255)
# Keys =
8
Voltage Steps =
0.625 (VDD/# Keys)
R0 =
10000
(Pullup)
(Hex)
(Hex)
Key pressed
VIn
Req
Rn
Vinmin
Vinmax ADDRmin
ADDRmax
No Key
5
0
10000
4.6875
5
EF
FF
1
0
0
0
0
0.3125
0
F
2
0.625 1429
1429
0.3125
0.9375
F
2F
3
1.25
3333
1904
0.9375
1.5625
2F
4F
4
1.875 6000
2667
1.5625
2.1875
4F
6F
5
2.5
10000
4000
2.1875
2.8125
6F
8F
6
3.125 16667
6667
2.8125
3.4375
8F
AF
7
3.75
30000
13333
3.4375
4.0625
AF
CF
8
4.375 70000
40000
4.0625
4.6875
CF
EF
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com


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