SD Card Mystery

[Kabron287]

Anybody could explain me, how this code from Datalogger example could work:

const int _CS = 22;
//#include <SPI.h>
#include <SD.h>
void setup() {
// Open serial communications and wait for port to open:
Serial.begin(115200);
while (!Serial);
Serial.print("Initializing SD card...");
// Ensure the SPI pinout the SD card is connected to is configured properly
//SPI1.setRX(_MISO);
//SPI1.setTX(_MOSI);
//SPI1.setSCK(_SCK);
// see if the card is present and can be initialized:
if (!SD.begin(_CS)) {
Serial.println("Card failed, or not present");
// don't do anything more:
return;
}
Serial.println("card initialized.");

Yes, it successfuly compiled and works onboard with the folliwing pins:
MISO = 16;
MOSI = 19;
SCK = 18;

[maxgerhardt]

So you have this code and it works but you don't know why?

Yes, it successfuly compiled and works onboard with the folliwing pins:
MISO = 16;
MOSI = 19;
SCK = 18;

Yeah that's great and matches the SPI pin map for a Pico.

arduino-pico/variants/rpipico/pins_arduino.h

Lines 17 to 20 in c4f3617

	// SPI
	#define PIN_SPI0_MISO (16u)
	#define PIN_SPI0_MOSI (19u)
	#define PIN_SPI0_SCK (18u)

Not sure what all the commented out code for SPI1 is about since, if you call SD.begin(_CS), it uses SPI (spi0). If you want to use SPI1 instead (with some different pin mappings), you also have to pass SPI1 as the SPI object in the begin call.

arduino-pico/libraries/SD/src/SD.h

Lines 34 to 41 in c4f3617

	boolean begin(uint8_t csPin, HardwareSPI &spi) {
	SDFS.setConfig(SDFSConfig(csPin, SPI_HALF_SPEED, spi));
	return (boolean)SDFS.begin();
	}
	boolean begin(uint8_t csPin, uint32_t cfg = SPI_HALF_SPEED, HardwareSPI &spi = SPI) {
	SDFS.setConfig(SDFSConfig(csPin, cfg, spi));
	return (boolean)SDFS.begin();
	}

[Kabron287]

BUT I did not defined SPI pins at all.

[maxgerhardt]

When you do not call SPI.setRX() etc., it will use the default pins as defined in the pins_arduino.h file of the selected board variant. And those very clearly say for SPI (0) that MISO = 16, MOSI = 19, SCK = 18. You can see these macros being used to initialize the SPI and SPI1 objects in

arduino-pico/libraries/SPI/src/SPI.cpp

Lines 366 to 372 in c4f3617

	#ifdef PIN_SPI0_MISO
	SPIClassRP2040 SPI(__SPI0_DEVICE, PIN_SPI0_MISO, PIN_SPI0_SS, PIN_SPI0_SCK, PIN_SPI0_MOSI);
	#endif
	#ifdef PIN_SPI1_MISO
	SPIClassRP2040 SPI1(__SPI1_DEVICE, PIN_SPI1_MISO, PIN_SPI1_SS, PIN_SPI1_SCK, PIN_SPI1_MOSI);
	#endif