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Introduction to the Arduino Uno programming in Assembler
The following chapter assumes that you are familiar with basic assembler operations for AVR microcontrollers. Below, we explain the most important construction elements and assembler instructions for manipulating the Arduino Uno's (figure 1) GPIOs, based on the ATmega328P microcontroller.
GPIO and Ports
The Arduino Uno exposes a number of GPIOs that can serve as binary inputs and outputs, analogue inputs, and many of them provide advanced, hardware-accelerated functions, such as UART, SPI, I2C, PWM, and ADC. In fact, not all of the pins on the development board are such “general-purpose”: some of them provide specific features, while others do not: there is no internal multiplexer, so functions such as UART, I2C, SPI, PWM and ADC are bound to particular GPIOs and cannot be changed.
On the programming level, GPIO ports are grouped into 3 “ports” (figure 2), and it is how you can access them:
- PortB, with GPIOs from D8 to D13,
- PortC, with GPIOs from port A0 to A5,
- PortD, with GPIOs from D0 to D7.
A bit in the port corresponds to a single GPIO pin, e.g. bit 5 (6th, zero-ordered) of PortB corresponds to GPIO D13 and is connected to the built-in LED.
IO Registers
Each Port has assigned three 8-bit registers:
- DDRx (Data Direction Register): there are 3 of those registers, one per Port (B, C, D): DDRB, DDRC and DDRD. This registers configures GPIO as Input (0) or Output (1). Configuration is done “per bit”, so it is equivalent to controlling each GPIO individually.
- PORTx (Port Data Register): there are also 3 of those registers: PORTB, PORTC and PORTD. The operation depends on the value of the specific bit in the corresponding DDR register; either pin is configured as input or output:
- If a specific GPIO pin (represented as a bit in the related DDRx register) is set as output, then PORTx bit directly affects the GPIO output: 1 is HIGH (+5V), while 0 is LOW (0V).
- If a specific GPIO pin is set to input, PORTx value controls the internal pull-up resistor: 1 enables pull-up, 0 disables it.
- PINx (Pin Value Register) represents the current input state of the GPIO.
Instructions
There is a set of assembler instructions that operate on port registers, represented in table 1. Assembler-level operations are much faster than C++.
Table 1: Common GPIO-operating, bit level instructions
| Instruction | Description |
|---|---|
SBI | Set bit in register |
CBI | Clear bit in register |
SBIS | Skif if bit in register is set (1) |
SBIC | Skip if bit in register is clear (0) |
IN | Read hardware register to the general purpose register (R0-R31) |
OUT | Write general purpose register to the hardware register. |
A common scenario is to first set either the GPIO is input or output (using the correct DDRx register), then either set (SBI), reset (CBI), check (SBIS, SBIC), read the whole register (IN) or write the whole register (OUT).
