The ATmega128 microcontroller has seven general-purpose input-output (I/O) ports

Question

The ATmega128 microcontroller has seven general-purpose input-output (I/O) ports: Port A through Port G. An I/O port is a collection of pins, and these pins can be individually configured to send (output) or receive (input) a single binary bit. Each port has three I/O registers, which are used to control the behavior of its pins: PORTx, DDRx, and PINx. (The “x” is just a generic notation; for example, Port A’s three I/O registers are PORTA, DDRA, and PINA.)

1. Suppose you want to configure Port B so that all 8 of its pins are configured as outputs. Which I/O register is used to make this configuration, and what 8-bit binary value must be written to configure all 8 pins as outputs?

2. Suppose all 8 of Port D’s pins have been configured as inputs. Which I/O register must be used to read the current state of Port D’s pins?

3. Does the function of a PORTx register differ depending on the setting of its corresponding DDRx register? If so, explain any differences.

Explanation / Answer

1) In order to configure PORT B so that all 8 of its pins are configured as outputs then we have use "DDRB" I/O register and the binary value required to configure is 11111111

2)In order to read the current state of Port D's pins ; then we have to read "PIND" I/O register

3)Yes, the function of PORTx register differ depending on the setting of it's corresponding DDRx register.
Explanation :
a) If DDRx is set as 1 (i.e making physical pins as output port) then the corresponding PORTX will store the logic values that currently being outputted on the physical pins of Portx

b)If DDRx is set as 0 ( i.e making physical pins as input port ) then the corresponding PORTX are used to write values to a port, you write the values to the PORT register of that port.

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