This section will summarize the key details from the GPIO lecture (Lecture #8). There is usually a lot of initial confusion over the naming of the different parts of the GPIO registers, which will be addressed here.

GPIO stands for General Purpose Input / Output and that is not the same as your General Purpose registers (GP registers which are R0 to R31). The key difference between the two is that the GP registers are within the CPU while the GPIO registers are outside of it. The GP registers are meant for holding data for arithmetic and logical operations while the GPIO registers can have a wide range of purposes such as holding the data from sensor readings or the data that needs to be outputted to generate a specific signal. Given the possibilities, it is important to understand how to configure the GPIO registers for the specific application needed.

As you may remember from our discussion of the basic memory models for computers, the input and output are treated as separate components. In order to make things more efficient and compact, they have been combined for nearly all modern microcontrollers. It is similar to comparing it to a 2-way street. Data can either flow into the microcontroller or out of it depending on what is needed. With this increased complexity, understanding the different terminology used to refer to each part is important.

There are several names that are used interchangeably when referring to the GPIO such as port, pin, and register. We will attempt to distinguish these from each other as best as possible.

First, we have to separate the hardware related terms from the software related terms. Each physical connection that can act as a general purpose input/output is referred to as a pin. They can be referred to individually by number or grouped together based on what they can be used for. These groupings are given unique names to differentiate them. This is where you have the Port B pins, Port C pins, Port D pins, and Port F pins. Each of the ports have a maximum of 8 pins each with some exceptions where certain pins are not available. The pins are usually referred to by their location within each port, which is why we have names such as PF5 and PF6 which stand for Port F pin 5 and Port F pin 6 respectively.

Now, we have the confusing part with the software terms. Because each pin can act as either an input or output, the direction and value has to be defined within the code. That is why there are three GPIO registers associated with each Port. These registers are the Data Direction Register (DDR), PORT register, and PIN register. While they share the same name, the PORT and PIN registers do not have the same meaning as the hardware related terms. Each of these GPIO registers serves a specific purpose.

• The DDR register is used to configure the direction of the pins for that Port. For example, if all of the pins for Port D need to be inputs, the value of the DDRD register would be 0b00000000. Each pin can be configured individually, so you can have combinations such as 0b10010010. A value of 1 indicates it will function as an output while a value of 0 is for inputs.
• The PIN register is used to indicate the current value on the pin. This is important for inputs as value here represents the input value that the microcontroller will see. You will typically only read(load) data from this register as writing to it (set or clear) will override any inputs.
• The PORT register serves different purposes depending on whether the pin is being used as an input or an output. It would be best to remember this register as the configuration for the GPIO pin. If the pin is to be used as an input, the value in this register determines if the input will have a pull-up resistor or not. A zero means no pull-up and a one means there is one. On the other hand, if the pin is to be used as an output, the value in this register determines the output value that is seen. If the output needs to be low, then a zero needs to be here. If the output needs to be high, then a one needs to be here.

Now that you know what each register is used for, you can put together that information to determine the appropriate value needed to configure the GPIO pins for the desired application. Make sure to utilize the CBR and SBR instructions as needed. In some cases, it may be better to use LDI.