Atmega328p pin manipulation

A normal Atmega328p microcontroller which is commonly used in robotics has 32 pin and 14 of them are digital pin. Digital pin is the most common pin for microcontroller to communicate and receive signal from the circuit. Atmega328p usually run on 16MHz clock speed or 62.5 ns to complete a clock signal. Theorically, Atmega328p should only need 62.5 ns to flip the power state of a digital pin. However, this simple operation will take 3.40 microsecond (3400 ns) to complete if using digitalWrite() from Arduino library. This is 54.4 times longer than the theorical speed and the root cause is digitalWrite() performs a lot of state checking before doing the real state switching. This can protect the Atmega328p from nasty electronic stuff and beginner programmer. As an “experienced” programmer, we need to develop our own fast pin manipulation code.

Mechanism behind the pins

Based on the diagram above provided by AVR, each digital pins is consist of three register, DDRX, PORTX, PINX. Fast pin manipulation can be achieve by reading and writing to the registers. Before we start, we need to know their purpose.

DDRX register

DDRX register aka data direction register is register that control the direction of data flow of a digital pin.

If DDRX state is 0, then digital pin will be input mode.

If DDRx state is 1, the digital pin will be output mode.

register description

DDRB – The Port B Data Direction Register

Bit	7	6	5	4	3	2	1	0
Name	DB7	DB6	DB5	DB4	DB3	DB2	DB1	DB0
IO	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

DDRC – The Port C Data Direction Register

Bit	7	6	5	4	3	2	1	0
Name	-	DC6	DC5	DC4	DC3	DC2	DC1	DC0
IO	R	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

DDRD – The Port D Data Direction Register

Bit	7	6	5	4	3	2	1	0
Name	DD7	DD6	DD5	DD4	DD3	DD2	DD1	DD0
IO	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

PORTX register

PORTX register is the register that store data for pin.

In output mode, digital pin will output signal according to PORTX register.

In input mode, digital pin will set pull-up resistor according to PORTX

register description

PORTB – The B Data Register

Bit	7	6	5	4	3	2	1	0
Name	PB7	PB6	PB5	PB4	PB3	PB2	PB1	PB0
IO	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

PORTC – The C Data Register

Bit	7	6	5	4	3	2	1	0
Name	-	PC6	PC5	PC4	PC3	PC2	PC1	PC0
IO	R	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

PORTD – The D Data Register

Bit	7	6	5	4	3	2	1	0
Name	PD7	PD6	PD5	PD4	PD3	PD2	PD1	PD0
IO	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

PINX register

PINX register will receive and store the signal from pin in input mode.

register description

PINB – The B Pin Register

Bit	7	6	5	4	3	2	1	0
Name	PB7	PB6	PB5	PB4	PB3	PB2	PB1	PB0
IO	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

PINC – The C Pin Register

Bit	7	6	5	4	3	2	1	0
Name	-	PC6	PC5	PC4	PC3	PC2	PC1	PC0
IO	R	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

PIND – The D Pin Register

Bit	7	6	5	4	3	2	1	0
Name	PD7	PD6	PD5	PD4	PD3	PD2	PD1	PD0
IO	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Init	0	0	0	0	0	0	0	0

MCUCR register

MCUCR register as known as MCU control register is a special register for pull-up register control.

register description

MCUCR - The MCU control register

Bit	7	6	5	4	3	2	1	0
Name	-	BODS	BODSE	PUD	-	-	IVSEL	IVCE
IO	R	R	R	R/W	R	R	R/W	R/W
Init	0	0	0	0	0	0	0	0

Pin configuration

Different state of register will also change the behavior of digital pin.

DDxn	PORTxn	PUD (in MCUCR)	pull-up resister	mode	comment
0	0	X	Input	No	Tri-state (Hi-Z)
0	1	0	Input	Yes	Pxn will source current if ext. pulled low.
0	1	1	Input	No	Tri-state (Hi-Z)
1	0	X	Output	No	Output low (sink)
1	1	X	Output	No	Output high (source)

Coding part

The theory part is done and lets write some code.

Here’s the code that will turn on pin 6 and turn off pin 9

#include <avr/io.h>

int main(void) {
    // set pin 6 to output mode
    DDRD |= 0b01000000;

    // set pin 9 to output mode
    DDRB |= 0b00000010;

    // turn on pin 6
    PORTD |= 0b01000000;

    // turn off pin 9
    PORTB &= 0b11111101;

    while(1) {};
}