SainSmart UNO R3 Starter Kit For Arduino
Chapter9 Potentiometer
What’s Analog Pins?
1.
A/D converter
The Atmega168 contains an onboard 6 channel analog-to-digital (A/D) converter. The converter
has 10 bit resolution, returning integers from 0 to 1023. While the main function of the analog
pins for most Arduino users is to read analog sensors, the analog pins also have all the
functionality of general purpose input/output (GPIO) pins (the same as digital pins 0 - 13).
Consequently, if a user needs more general purpose input output pins, and all the analog pins
are not in use, the analog pins may be used for GPIO.
2.
Pin mapping
The Arduino pin numbers corresponding to the analog pins are 14 through 19. Note that these
are Arduino pin numbers, and do not correspond to the physical pin numbers on the Atmega168
chip. The analog pins can be used identically to the digital pins, so for example, to set analog pin
0 to an output, and to set it HIGH, the code would look like this:
pinMode(14, OUTPUT);
digitalWrite(14, HIGH);
3.
Pullup resistors
The analog pins also have pullup resistors, which work identically to pullup resistors on the digital
pins. They are enabled by issuing a command such as
digitalWrite(14, HIGH); // set pullup on analog pin 0
while the pin is an input.
Be aware however that turning on a pullup will affect the value reported by analogRead() when
using some sensors if done inadvertently. Most users will want to use the pullup resistors only
when using an analog pin in its digital mode.
4.
Details and Caveats
The analogRead command will not work correctly if a pin has been previously set to an output,
so if this is the case, set it back to an input before using analogRead. Similarly if the pin has been
set to HIGH as an output, the pullup resistor will be on, after setting it back to an INPUT with
pinMode.
The Atmega168 datasheet also cautions against switching digital pins in close temporal proximity
to making A/D readings (analogRead) on other analog pins. This can cause electrical noise and
introduce jitter in the analog system. It may be desirable, after manipulating analog pins (in
digital mode), to add a short delay before using analogRead() to read other analog pins.
analogRead()
Description
Reads the value from the specified analog pin. The Arduino board contains a 6 channel (8
channels on the Mini and Nano, 16 on the Mega), 10-bit analog to digital converter. This means
that it will map input voltages between 0 and 5 volts into integer values between 0 and 1023.
This yields a resolution between readings of: 5 volts / 1024 units or, .0049 volts (4.9 mV) per unit.
The input range and resolution can be changed using analogReference().
