Objective
In this tutorial we are going to discuss the port configuration of AVR/Atmel controllers or in general Atmega family. In this tutorial we will be using Atmega32 as reference, same will be applicable to other Atmega series controllers.
At the end of this tutorial you will be familiar with the Atmega GPIO's and the associated registers for configuring and accessing the GPIO's.
GPIO Registers
The basic and important feature of the any controllers is the number of gpio's available connecting the peripherals. Atmega32 has 32-gpio's grouped into four 8-bit ports namely PORTA-PORTD as shown in the below image.
As shown in the above image many I/O pins have 2-3 functions. If a pin is used for other function then it may not be used as a gpio.
Though the gpio pins are grouped into 8-bit ports they can still be configured and accessed individually.
Each Port is associated with 3 registers for direction configuration(Input/Output), read and write operation.
| Register | Description |
|---|---|
| DDRx | Used to configure the respective PORT as output/input |
| PORTx | Used to write the data to the Port pins |
| PINx | Used to Read the data from the port pins |
DDRx: Data Direction Register
Before reading or writing the data from the ports, their direction needs to be set. Unless the PORT is configured as output, the data from the registers will not go to controller pins.
This register is used to configure the PORT pins as Input or Output. Writing all 1's to DDRx will make the corresponding PORTx pins as output. Almost all the LPC1768 pins are multiplexed to support more than 1 function. Every GPIO pin has a minimum of one function and max of four functions. The required function can be selected by configuring the PINSEL register. As there can be up to 4 functions associated with a GPIO pin, two bits for each pin are available to select the function. This implies that we need two PINSEL registers to configure a PORT pins. By this the first 16(P0.0-P0.16) pin functions of PORT0 can be selected by 32 bits of PINSELO register. The remaining 16 bits(P0.16-P0.32) are configured using 32bits of PINSEL1 register. As mentioned earlier every pin has max of four functions. Below table shows how to select the function for a particular pin using two bits of the PINSEL register.
| Value | Function | Enumeration |
|---|---|---|
| 00 | Primary (default) function, typically GPIO port | PINSEL_FUNC_0 |
| 01 | First alternate function | PINSEL_FUNC_1 |
| 10 | Second alternate function | PINSEL_FUNC_2 |
| 11 | Third alternate function | PINSEL_FUNC_3 |
FIODIR:Fast GPIO Direction Control Register.
This register individually controls the direction of each port pin.
| Values | Direction |
|---|---|
| 0 | Input |
| 1 | Output |
FIOSET:Fast Port Output Set Register.
This register controls the state of output pins. Writing 1s produces highs at the corresponding port pins. Writing 0s has no effect. Reading this register returns the current contents of the port output register not the physical port value.
| Values | FIOSET |
|---|---|
| 0 | No Effect |
| 1 | Sets High on Pin |
FIOCLR:Fast Port Output Clear Register.
This register controls the state of output pins. Writing 1s produces lows at the corresponding port pins. Writing 0s has no effect.
| Values | FIOCLR |
|---|---|
| 0 | No Effect |
| 1 | Sets Low on Pin |
FIOPIN:Fast Port Pin Value Register.
This register is used for both reading and writing data from/to the PORT.
Output: Writing to this register places corresponding values in all bits of the particular PORT pins.
Input: The current state of digital port pins can be read from this register, regardless of pin direction or alternate function selection (as long as pins are not configured as an input to ADC).
Note:It is recommended to configure the PORT direction and pin function before using it.
As all the Atmega Ports and SFR's(Special Function Registers) are defined in io.h, this has to be included at the beginning of the project/code.