Intermediary commit. Nothing's working, everything is broken. Let's start over now...

include/config.h

@@ -1,9 +1,32 @@
 #pragma once
 #include <Arduino.h>
 
-#define PIN_SD_CS            D1
-#define PIN_VS1053_CS        D2
-#define PIN_RC522_CS         D3
+// Pins according to https://www.instructables.com/id/NodeMCU-ESP8266-Details-and-Pinout/
+// D0 = 16
+// D1 = 5
+// D2 = 4
+// D3 = 0
+// D4 = 2
+// D5 = 14
+// D6 = 12
+// D7 = 13
+// D8 = 15
+// A0 = 17
+// Other usable pins:
+// 6, 7, 8, 11 not readable
+// 1, 3, 6, 7, 8, 11 nor writable -> 1 (TX) and 3 (RX) can be used read-only
+// 9 -> S2
+// 10 -> S3
+#define RX 3
+#define TX 1
+
+
+#define PIN_SD_CS            D4
+#define PIN_VS1053_XCS        D8
+#define PIN_VS1053_XRESET    D3
+#define PIN_VS1053_XDCS    D1
+#define PIN_VS1053_DREQ    D2
+#define PIN_RC522_CS         D0
 
 #define PIN_BTN_VOL_UP       D4
 #define PIN_BTN_VOL_DOWN     D5

include/controller.h

@@ -1,13 +1,15 @@
 #pragma once
 
 #include "player.h"
+#include "sd_card.h"
 
 class Controller {
 private:
 	Player* _player;
+	SDCard* _sd_card;
 
 public:
-	Controller(Player* p) : _player(p) {}
+	Controller(Player* p, SDCard* c) : _player(p), _sd_card(c) {}
 	void vol_up();
 	void vol_down();
 	void track_next();

include/player.h

@@ -1,7 +1,27 @@
 #pragma once
+#include "sd_card.h"
+#include "config.h"
+
+#define SCI_MODE 0x00
+#define SCI_STATUS 0x01
+#define SCI_CLOCKF 0x03
+
+#define CMD_WRITE 0x02
+#define CMD_READ 0x03
+
+#define XRESET PIN_VS1053_XRESET
+#define DREQ PIN_VS1053_DREQ
 
 class Player {
+private:
+	SDCard* _sd_card;
+	void _reset();
+	void _init();
+	void _wait();
+	uint16_t _read_register(uint8_t address, uint32_t spi_speed, uint16_t t);//=SPI_CLOCK_DIV4);
+	void _write_register(uint8_t address, uint16_t value, uint32_t spi_speed);//=SPI_CLOCK_DIV2);
 public:
+	Player(SDCard* c);
 	void vol_up();
 	void vol_down();
 	void track_next();

include/sd_card.h

@@ -0,0 +1,8 @@
+#pragma once
+#include <SD.h>
+#include "config.h"
+
+class SDCard {
+public:
+	SDCard();
+};

include/spi_master.h

@@ -0,0 +1,27 @@
+#pragma once
+
+class SPIMaster {
+public:
+	static void init() {
+		SPI.setHwCs(false);
+		pinMode(PIN_SD_CS, OUTPUT);
+		pinMode(PIN_VS1053_XCS, OUTPUT);
+		pinMode(PIN_VS1053_XDCS, OUTPUT);
+	}
+	static void enable(uint8_t pin) {
+		digitalWrite(PIN_SD_CS, pin==PIN_SD_CS ? LOW : HIGH);
+		digitalWrite(PIN_VS1053_XCS, pin==PIN_VS1053_XCS ? LOW : HIGH);
+		digitalWrite(PIN_VS1053_XDCS, pin==PIN_VS1053_XDCS ? LOW : HIGH);
+	}
+
+	static void printStatus() {
+		Serial.printf("CS state: SD:%d, VS1053_XCS:%d, VS1053_XDCS:%d\n",
+			digitalRead(PIN_SD_CS),
+			digitalRead(PIN_VS1053_XCS),
+			digitalRead(PIN_VS1053_XDCS));
+	}
+
+	static void disable() {
+		enable(142);
+	}
+};