• MCU: Microchip ATMEGA328P-MU
• Framework: Arduino AVR core
• Build system: PlatformIO or Arduino CLI
• Clock: External 16 MHz crystal on XTAL1/XTAL2
• Logic supply: VCC rail, intended 5 V operation for 16 MHz use

Ini
[env:atmega328p]
platform = atmelavr
board = ATmega328P
framework = arduino
board_build.f_cpu = 16000000L
upload_protocol = custom
monitor_speed = 115200

; Example avrdude upload command for USBasp. Adjust programmer and port for your tool.
upload_command = avrdude -p m328p -c usbasp -U flash:w:$SOURCE:i

Cpp
#include <Arduino.h>

// Nets from schematic
constexpr uint8_t PIN_UART_RXD = 0;   // PD0 / Arduino D0 / J2 pin 03
constexpr uint8_t PIN_UART_TXD = 1;   // PD1 / Arduino D1 / J2 pin 04
constexpr uint8_t PIN_GPIO_PB0 = 8;   // PB0 / Arduino D8 / J2 pin 06
constexpr uint8_t PIN_RS485_DE = 9;    // PB1 / Arduino D9 / U7 DE and ~RE
constexpr uint8_t PIN_CAN_CS   = 10;   // PB2 / Arduino D10 / U5 ~CS
constexpr uint8_t PIN_SPI_MOSI = 11;  // PB3 / Arduino D11 / ISP J1 pin 4
constexpr uint8_t PIN_SPI_MISO = 12;  // PB4 / Arduino D12 / ISP J1 pin 1
constexpr uint8_t PIN_SPI_SCK  = 13;  // PB5 / Arduino D13 / ISP J1 pin 3
constexpr uint8_t PIN_CAN_INT  = 2;    // PD2 / Arduino D2 / U5 ~INT
constexpr uint8_t PIN_ETH_INT  = 3;    // PD3 / Arduino D3 / J9 INTn via U9
constexpr uint8_t PIN_RS485_RX = 4;    // PD4 / Arduino D4 / U7 RO
constexpr uint8_t PIN_RS485_TX = 5;    // PD5 / Arduino D5 / U7 DI
constexpr uint8_t PIN_ETH_RST  = 6;    // PD6 / Arduino D6 / J9 RSTn via U9
constexpr uint8_t PIN_ETH_CS   = 7;    // PD7 / Arduino D7 / J9 SCNn via U9

constexpr unsigned long BLINK_INTERVAL_MS = 500;
unsigned long lastToggleMs = 0;
bool gpioState = false;

void setup() {
  // UART on PD0/PD1 via J2 pins 03/04
  Serial.begin(115200);
  Serial.println(F("ATmega328P minimal board firmware starting"));

  // Spare GPIO on PB0 / Arduino D8
  pinMode(PIN_GPIO_PB0, OUTPUT);
  digitalWrite(PIN_GPIO_PB0, LOW);

  pinMode(PIN_CAN_CS, OUTPUT);
  digitalWrite(PIN_CAN_CS, HIGH);
  pinMode(PIN_CAN_INT, INPUT_PULLUP);

  pinMode(PIN_RS485_DE, OUTPUT);
  digitalWrite(PIN_RS485_DE, LOW); // receive mode by default

  pinMode(PIN_ETH_CS, OUTPUT);
  digitalWrite(PIN_ETH_CS, HIGH);
  pinMode(PIN_ETH_RST, OUTPUT);
  digitalWrite(PIN_ETH_RST, LOW);
  delay(1); // WIZ850io requires at least 500 us reset low
  digitalWrite(PIN_ETH_RST, HIGH);
  delay(50); // wait before first SPI access
  pinMode(PIN_ETH_INT, INPUT_PULLUP);

  // Keep SPI pins in safe idle state unless used by firmware.
  pinMode(PIN_SPI_MOSI, INPUT_PULLUP);
  pinMode(PIN_SPI_MISO, INPUT_PULLUP);
  pinMode(PIN_SPI_SCK, INPUT_PULLUP);

  // AREF is decoupled in hardware. Use VCC as default ADC reference unless external reference is connected intentionally.
  analogReference(DEFAULT);

  Serial.println(F("Init complete. PB0/D8 will toggle every 500 ms."));
}

void loop() {
  const unsigned long now = millis();

  if (now - lastToggleMs >= BLINK_INTERVAL_MS) {
    lastToggleMs = now;
    gpioState = !gpioState;
    digitalWrite(PIN_GPIO_PB0, gpioState ? HIGH : LOW);
    Serial.print(F("PB0/D8 state: "));
    Serial.println(gpioState ? F("HIGH") : F("LOW"));
  }

  // UART echo test through J2 pins 03/04.
  if (Serial.available() > 0) {
    const int c = Serial.read();
    Serial.write(c);
  }
}

1. Connect a 5 V supply to VCC and GND.
2. Connect an AVR ISP programmer to J1:
   • J1-1: SPI_MISO
   • J1-2: VCC
   • J1-3: SPI_SCK
   • J1-4: SPI_MOSI
   • J1-5: RESET
   • J1-6: GND
3. Build:

Bash
pio run

4. Flash with the configured programmer:

Bash
pio run -t upload

5. Monitor UART through J2:

Bash
pio device monitor -b 115200