Application Note

Implementation

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

Implementation

ADC Operation

An MCU ADC typically has 8-bit

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.

ADC

INPUT

MCU

PIN

Freescale Semiconductor, Inc.

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