AVR IO port characteristics and applications

Analyze the IO pin Pxn. Only when DDRxn is 1, the controllable unidirectional switch will work, and the value of PORTxn can be sent to Pxn through the controllable unidirectional switch.

Conclusion: When DDRxn=1, it is in output state. The output value is equal to PORTxn. Therefore, DDRxn is the direction register. PORTxn is the data register.

Analyze the pull-up resistor. When the potential of E is 0, that is, when D is 1, the pull-up resistor is effective.

From the analysis of the AND gate input, the pull-up resistor is effective only when the following conditions are met at the same time

1. PUD is 0

2. DDxn is 0

3. PORTxn is 1

The conclusion is: the pull-up resistor is effective only when DDRxn = 0, that is, the pin is defined as input state, PORTxn = 1, and UPD is set to 0.

Analyze Pxn and SLEEP. Only when SLEEP = 0, the controllable switch 2 is turned on, SD1 does not work, the input of the Schmitt trigger is equal to Pxn, and the signal is sent to the synchronizer for reading.

Conclusion: Pxn can be read by the AVR regardless of whether it is in input or output state. Input can only be read when SLEEP=0.

Notes on the use of AVR IO ports:

If there are pins that are not used, it is recommended that these pins be given a certain level. The simplest way to ensure that unused pins have a certain level is to enable the internal pull-up resistor.

If you have just defined the input status of a pin and want to read it back immediately, you can insert a _nop() statement before reading it back.

When the system is reset, all DDR bits are 0 and all Port bits are 0, so the pull-up resistor will fail during reset.

How to use C language to manipulate AVR's IO ports (taking ICCAVR as an example):

Example 1: Define PB0 as output, and the output is high level

DDRB=BIT(0); //define PB0 as output

PORTB|=BIT(0); //PB0 outputs high level

Example 2: Define PB0 and PB1 as outputs, and both PB0 and PB1 are high level

DDRB|=BIT(0)|BIT(1); //Define PB0 and PB1 as output

PORTB|=BIT(0)|BIT(1); // PB0, PB1 output high level

Example 3: Flip the value of the PB0 data register, that is, if it is 1, it becomes 0, and if it is 0, it becomes 1

PORTB^=BIT(0); //Flip PB0 port

Example 4: Flip the values ​​of the PB0 and PB1 data registers, that is, if it is 1, it becomes 0, and if it is 0, it becomes 1

PORTB^=BIT(0)|BIT(1); // Flip PB0 and PB1 ports

Example 5: Define PB2 and PB3 as input without pull-up resistors

DDRB&=~(BIT(2)|BIT(3)); //Define PB2 and PB3 as input

PORTB&=~(BIT(2)|BIT(3)); // Set PORT to 0, no pull-up resistor

Example 6: Define PB2 and PB3 as inputs with pull-up resistors. That is, when these pins are not referenced, the default value is high level.

SFIOR&=~BIT(PUD); // Set the pull-up resistor control bit PUD of the SFIOR register to 0. This sentence may not appear in the entire code, or may only appear once, because it is a control bit that controls all pull-up resistors.

DDRB&=~(BIT(2)|BIT(3)); //Define PB2 and PB3 as input

PORTB|=BIT(2)|BIT(3); // Set PORT to 1 to meet another condition of the pull-up resistor

Example 7: The difference between DDRB=BIT(0)|BIT(1) and DDRB|=BIT(0)|BIT(1)

Assume that before executing the above two instructions, the status of DDRB is: 1000 0000

If DDRB=BIT(0)|BIT(1) is executed, the state of DDRB becomes: 0000 0011 
If DDRD|=BIT(0)|BIT(1) is executed, the state of DDRB becomes: 1000 0011

The first sentence will clear all previous states, and the second sentence will retain the previous states.

In practical applications, the latter sentence is more commonly used.

Example 8: Besides using BIT(3), is there any other way to set the third bit to 1?

DDRB|=BIT(3);

DDRB|=1<<3;

DDRB|=0x08;

DDRB|=0b00001000;