atmega-fast-gpio.md

Fast GPIO on ATMEGA

The classic Arduino use an MCU of the ATMEGA series.

Introduction

The typical way to set a GPIO pin in Arduino is to use the Arduino API:

#define PIN_TEST2 23
digitalWrite( PIN_TEST2, LOW );

The Arduino API might be simple, it is not fast.

Faster

A faster way is to write directly to the special function register ("SFR") that controls the pin state.

The ATMEGAs have 8 GPIO pins grouped in a so-called port. The ports get letters (A, B, ...). For each port x there are two registers. The 8 bits in these registers map to the 8 gpio pins. For example, the ATMEGA256 has GPIO pins 22,23,24,25,26,27,28,29 mapped to port A.

• DDRx data direction register x
  When a bit is 0 that GPIO is an input, when the bit is 1, the GPIO is an output.

• PORTx is the value at the pins of port x
  When a bit is 0 the pin is low, when a bit is 1 the pin is high.

Example

Output example of 8 pins in one go.

  // Port A: 22,23,24,25,26,27,28,29        DATA (7=MD7..0=MD0)
  // DATA pins: instruction NOP
  DDRA  = 0xFF; // all pins output
  PORTA = 0xEA; // NOP

Input example of a single pin

  // Port J: 15                             POW (0=MVCC)
  // POW pin: output and on
  DDRJ = 0x01;
  PORTJ = 0x00; // low active

These example are taken from an ATMEGA that feeds a 6502.

Nano ports

Here is a map of the ports of the Arduino Nano (rest of the document focuses on ATMEGA 256).

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