Nuts and Volts - April 2015

therefore referenced by its port and pin number, prefixed with a “P” (e.g., PB1, PB0, PD7). Figure 2 is from the ATmega328P datasheet, and shows how the individual pins are named — you’ll notice that they don’t flow logically from the first to the 28th pin, not all ports have eight pins, and there’s no PORTA.

When I first looked at the pinout diagram in the datasheet, I nearly flipped! There was so much complexity and very little to explain what all the acronyms in parentheses were. Don’t panic! We’ll tackle these when we need them over the course of this series.

Registers

Okay, so we know that each of the I/O pins are numbered differently to the Arduino IDE and how they’re numbered. How do we read from or write to them? Most microcontrollers use registers to do this. A register (or more correctly, a hardware register) is a bit like a predefined variable — you can read the value stored in the register and change it (if it’s not a read-only register).

In the background, each hardware register is linked to specific hardware-related functionality, and writing to them causes the hardware to behave in certain ways. Registers have specific memory addresses, but we usually refer to them using the “friendly” names that are defined in the datasheets and header files.

The simplest hardware registers are simply called “PORT” registers. There is one of these registers for each of the ports on the MCU. For the ATmega328P, there are PORTB, PORTC, and PORTD registers. By writing to these, you cause the individual pins to go either high or low in the same way as digital Write() did in the Arduino IDE.

But which pin? A port contains up to eight pins, so how does the PORT register allow us to access a specific pin? Simple! Or, so I was told when I first tackled this. However, it took me a while.

As mentioned, each port has up to eight pins. A byte has eight bits. A register is usually one byte in size. If you make the connection, you’ll realize that each bit in the eight-bit register is linked to a pin on that port. Figure 3 shows an example of how this would look for PORTB, with PB0 and PB4 set high.

In order to make pin PB2 go high, you need to set bit 2 of the PORTB register to a 1. To make PB4 go low, you need to set bit 4 of the PORTB register to 0. Behind the scenes, this is exactly what digital Write() was doing for you.

One final piece of theory before we get stuck into a first project: bitwise operations. We need to use bitwise operations in order to set the bits in the PORT to 1 or 0.

Bitwise Operations

Bitwise operations are used to manipulate individual bits, or groups of bits. We won’t go into all the uses for these operations and operators here, but will focus on what we need them for: to set specific bits in registers.

Let’s assume you have PORTB configured as in Figure 3; bits 0 and 4 are set to 1 and the rest to 0. In binary, this would be represented as 0b00010001 (where the prefix 0b indicates a binary number follows). Now, let’s set PB2 high (i.e., bit 2 will be set to 1).

One way to do this is by spelling it out and setting PORTB to 0b00010101. However, you can only do this if you keep track of the values of all the other bits in the register — i.e., you also need to know the values of the bits that you aren’t setting. This is not very practical in a dynamic embedded system. Bitwise operators allow us to change the value of just PB2, without affecting (or needing to know) the values of the other bits in the register. Bitwise operation is critical in working with microcontrollers.

You may have come across the bitwise operators (AND, NOT, OR, XOR), as well as the shift operators ( Left-Shift and Right-Shift). I want to get us to a working project as soon as possible, so I won’t delve into the detailed theory behind these operators. What I will do is quickly summarize what you need to know to get working.

Bitwise OR

The bitwise OR operator compares two binary values and returns a value that includes the 1s from both of them. In the example in Figure 4, bit 2 contains a 0 in the original register, and a 1 in the value it is being OR’ed with; the result is therefore a 1. Practically, if we want to set PB2 to a 1, we simply need to perform the OR operation: PORTB = PORTB | 0b00000100.

Bitwise AND

The bitwise AND operator compares two binary values, and returns a value that only includes the 1s where both of the values contain a 1. In the example in Figure 4, we want to check whether bits 2 and 0 are set (i.e., are 1).

By ANDing a binary value of 0b00000101 (where bits 2 and 0 are 1), we can see that only bit 0 was set in the original register. In practice, this would look like PORTB & 0b00000101