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On the top left of the schematic 1 is the pinout of the board, and in the centre is the pinout of the AVR chip an ATmega or ATmegap 2. The schematic is confusing at first but gets simpler once you see that every label is unique, and anywhere a label appears multiple times means they are connected at those points within the board.
Alternatively, USB can power the board directly. The components thus use a 5 V logic level to communicate binary signals. A binary signal is a form of modulation the concept of encoding data within a signal, in this case electrical using voltage level to determine a high value or a low value. Whether high is considered a logical 1 active-high or a logical 0 active-low is dependent on the pin configuration. When a pin is configured to be active-low, it is usually labelled with a bar over it as in RESET or appended with a , but other conventions are also used.
We will look at how to use the UART in a later section. For most systems, you will need to install a separate instance of gcc and binutils built to target AVR. The toolchain modules are invoked using the prefix avr- ; all the same flags and features are available. The programmer requires a physical connection to the AVR, and uses an algorithm specified in the datasheet chapter 28 to write the passed program to the chip. The usual tool for interfacing with the programmer is avrdude 5.
The bootloader reads the binary via the UART and writes to the application memory space. Instead, we use Intel hex format binaries:. How do we know the baud rate should be ? Well, avrdude is communicating directly with the bootloader, so all we have to do is look at the bootloader code. The differences show up when we try to actually write a program for the AVR. An MCU does not have the same features as a microprocessor.
There is only one process, delineated by the main function, as process preemption requires a multitasking operating system generally facilitated by virtual memory, which MCUs do not have. As such, your program must do all hardware initialization explicitly. Fortunately, MCUs are simple enough that this is a reasonable task. We can use avr-libc 7 to provide basic macros and functions. The project attempts to mimic the C standard library, with the limitation that there is no operating system for hardware abstraction.
Here is the AVR pinout from the datasheet, which you can cross-reference with the Arduino schematic:. The idea is that each port has 8 pins except Port C, read Pin Descriptions in section 1. Within each port, each pin also has alternatively configurable functions, which correspond to the parenthesized labels. When the MCU begins program execution, no alternative pin functions are initialized.
The only exception is PC6 , which is programmed with a fuse bit to configure reset functionality. All other port pins are set as tri-stated inputs high-impedance, i. The program must then configure how each pin should be used. A simple program could be to set Port B as an output, and increment the value input on Port D:.
The DDRx registers determine whether each pin of the corresponding port is an output or an input. Setting the register to 0xff configures all Port B pins as outputs. The PINx registers are readable addresses for capturing inputs. We use an infinite loop so that we are continuously sampling the input at Port D. A UART is a simple device for serial communication. Serial means data is sent in sequence over the same wire, as opposed to having, say, 8 wires transmit a byte all at once called parallel communication.
Both the AVR and the FTR are full-duplex, meaning they can transmit and receive simultaneously, without having to take turns half-duplex. If you then read from the TTY e. Because UARTs are asynchronous, and do not have a clock line for synchronization, they must synchronize over the same lines used for data transmission.
This is only possible if the baud rate same as the bit rate when using binary modulation and frame format are agreed upon prior to communication. UARTs use a prescaler to divide the provided clock to a desired baud rate. With a given clock frequency, only a set number of baud rates can be configured. Not all standard baud rates are available for every frequency—due to the discrete number of prescaler values available—so the error to the nearest standard baud rate is listed.
The range of acceptable errors is tabulated in section The default is 8 data bits, no parity, and 1 stop bit—commonly used for serial communication with PCs. You can also affect the TTY programmatically, via the termios structure in libc.
The datasheet specifies prescaler options in section 4. As an example, we could write a simple program that accepts characters from the host, and sends each character back incremented by one:. Finally, we block on the values of RXC0 and UDRE0 to know when the receive and transmit buffers are ready for reading and writing, respectively.
Instead, we use Intel hex format binaries: