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AN1775 Datasheet(PDF) 3 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

AN1775 Datasheet(HTML) 3 Page - Freescale Semiconductor, Inc

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Application Note
AN1775 Rev. 1
ADC Alternative
In many cases, input pins are at a premium. One can't always freely
assign input pins to the function of user input. A more efficient use of
microcontroller resources can be devised. One common feature of many
Freescale MCUs is the analog-to-digital converter, or ADC.
The ADC of a Freesca;e MCU usually features four to eight channels of
analog input, which is compared with a reference voltage and converted
to an 8-bit digital value. When a resistor ladder is connected to an analog
input through switches in each segment, the conversion result can be
used to arbitrate an input. This allows many keys to be interfaced with
one input pin, with only a little more software overhead. Figure 2 shows
such an implementation.
Figure 2. Resistor Ladder Keys Using an Analog Input Pin
ADC Operation
An MCU ADC typically has 8-bit
precision. This means there are 28, or
256, distinguishable A/D inputs, including 0. The analog inputs are
converted to a binary number, which represents the magnitude of the
input voltage in relation to a reference voltage.
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com

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