SpiLoopback.ino

SPI loopback test: sends a counter with SPI.transfer() every 500 ms and prints the byte that comes back. With MOSI wired to MISO, every byte sent should come straight back.

Uses PhyduinoSPI::beginTransaction() with 1 MHz, MSBFIRST, SPI_MODE0.

Wiring

D11 (MOSI) to D12 (MISO).

Output

Serial Monitor at 115200 baud, for example:

sent 7 received 7

With D11 and D12 not connected, the received value is usually 0 or 255.

// SPI loopback test: connect D11 (MOSI) to D12 (MISO).
// Every byte sent should come straight back.
#include <SPI.h>
 
void setup() {
    Serial.begin(115200);
    SPI.begin();
}
 
void loop() {
    static uint8_t value = 0;
    SPI.beginTransaction(SPISettings(1000000, MSBFIRST, SPI_MODE0));
    const uint8_t received = SPI.transfer(value);
    SPI.endTransaction();
 
    Serial.print("sent ");
    Serial.print(value);
    Serial.print(" received ");
    Serial.println(received);
    value++;
    delay(500);
}