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Port registers

https://www.arduino.cc/en/Reference/PortManipulation

Port registers allow for lower-level and faster manipulation of the i/o pins of the microcontroller on an Arduino board. The chips used on the Arduino board have three ports:

• B (digital pin 8 to 13)
• C (analog input pins)
• D (digital pins 0 to 7)

Each port is controlled by three registers, which are also defined variables in the arduino language.

• The DDR register, determines whether the pin is an INPUT or OUTPUT.
• The PORT register controls whether the pin is HIGH or LOW
• The PIN register reads the state of INPUT pins set to input with pinMode().

DDR and PORT registers may be both written to, and read. PIN registers correspond to the state of inputs and may only be read.

PORTD

maps to Arduino digital pins 0 to 7

• DDRD - The Port D Data Direction Register - read/write
• PORTD - The Port D Data Register - read/write
• PIND - The Port D Input Pins Register - read only

PORTB

maps to Arduino digital pins 8 to 13. The two high bits (6 & 7) map to the crystal pins and are not usable

• DDRB - The Port B Data Direction Register - read/write
• PORTB - The Port B Data Register - read/write
• PINB - The Port B Input Pins Register - read only

PORTC

maps to Arduino analog pins 0 to 5. Pins 6 & 7 are only accessible on the Arduino Mini

• DDRC - The Port C Data Direction Register - read/write
• PORTC - The Port C Data Register - read/write
• PINC - The Port C Input Pins Register - read only

PORTC data register

bit	7	6	5	4	3	2	1	0
	(high)							(low)
	-	PORTC6	PORTC5	PORTC4	PORTC3	PORTC2	PORTC1	PORTC0
Read/Write	R	R/W	R/W	R/W	R/W	R/W	R/W	R/W
initial value	0	0	0	0	0	0	0	0

Port mapping

pin	Ax	PORTCx	PCx	ADCx
analog input 0	A0	PORTC0	PC0	ADC0
analog input 1	A1	PORTC1	PC1	ADC1
analog input 2	A2	PORTC2	PC2	ADC2
analog input 3	A3	PORTC3	PC3	ADC3
analog input 4	A4	PORTC4	PC4	ADC4
analog input 5	A5	PORTC5	PC5	ADC5
reset		PORTC6	PC6	RESE

Each bit of these registers corresponds to a single pin; e.g. the low bit of DDRC, PORTC, and PINC refers to pin PC0 (analog pin 0, A0). For a complete mapping of Arduino pin numbers to ports and bits, see the diagram for your chip: ATmega8, ATmega168. (Note that some bits of a port may be used for things other than i/o; be careful not to change the values of the register bits corresponding to them.)

Examples

Referring to the pin map above, the PortC registers control Arduino analog pins 0 to 5.

DDRC is the direction register for Port C (Arduino analog pins 0-5). The bits in this register control whether the pins in PORTC are configured as inputs or outputs so, for example:

DDRC = B00000000;  // sets Arduino analog pins 0 to 5 as inputs (0)
DDRC = B00111111;  // sets Arduino analog pins 0 to 5 as outputs (1)

PORTC is the register for the state of the outputs. For example:

PORTC = B00101000; // sets analog pins 5,3 HIGH

You will only see 5 volts on these pins however if the pins have been set as outputs using the DDRC register or with pinMode().

PINC is the input register variable It will read all of the analog input pins at the same time.

ADMUX - ADC multiplexer selection register

bit	7	6	5	4	3	2	1	0
	(high)							(low)
	REFS1	REFS0	ADLAR	-	MUX3	MUX2	MUX1	MUX0
Read/Write	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
initial value	0	0	0	0	0	0	0	0

Voltage reference selections for ADC

REFS1	REFS0	Voltage reference selection
0	0	AREF, internal Vref turned off
0	1	AVcc with external capacitor at AREF pin
1	0	Reserved
1	1	Internal 1.1V voltage reference with external capacitor at AREF pin

The ADLAR bit affects the presentation of the ADC conversion result in the ADC data register. Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the ADLAR bit will affect the ADC data register immediately, regardless of any ongoing conversions.

Input channel selection

MUX3..0	Single Ended Input
0000	ADC0
0001	ADC1
0010	ADC2
0011	ADC3
0100	ADC4
0101	ADC5
0110	ADC6
0111	ADC7
1000	ADC8
1001	(Reserved)
1010	(Reserved)
1011	(Reserved)
1100	(Reserved)
1101	(Reserved)
1110	1.1V (Vbg)
1111	0V (GND)