Fixed Issue #1, see explanations

In the “full main.cpp” I gave you, dumpSdPins() did this:

pinMode(SCK,  INPUT_PULLUP);
pinMode(MISO, INPUT_PULLUP);
pinMode(MOSI, INPUT_PULLUP);

…and you were calling dumpSdPins("after-idle-clocks") inside tryMountWithBus(), after bus.begin() and the 0xFF idle clocks, but before SD.begin().

That means: right before SD.begin(), you were accidentally turning the SPI pins back into inputs. The card then can’t respond, so you get endless:

sdCommand(): Card Failed! cmd: 0x00

f_mount failed: (3) The physical drive cannot work

That matches your new log perfectly.

exercises/05_SD_Card_Watcher/src/main.cpp

@@ -1,4 +1,4 @@
-// 20260213 ChatGPT
+// 20260214 ChatGPT
 // $Id$
 // $HeadURL$
 
@@ -7,8 +7,19 @@
 #include <FS.h>
 #include <SD.h>
 #include <SPI.h>
+#include "driver/gpio.h"          // gpio_get_level()
 #include "tbeam_supreme_adapter.h"
 
+// -------------------------
+// Configuration toggles
+// -------------------------
+#define ENABLE_SD_RAIL_CYCLE    1   // Power-cycle AXP2101 BLDO1 (SD rail) at boot.
+#define ENABLE_PIN_DUMPS        1   // Log SPI pin logic levels at key points (NON-INTRUSIVE).
+#define STARTUP_SERIAL_DELAY_MS 5000
+
+// -------------------------
+// Globals
+// -------------------------
 static SPIClass sdSpiH(HSPI);
 static SPIClass sdSpiF(FSPI);
 static SPIClass* g_sdSpi = nullptr;
@@ -16,10 +27,10 @@ static const char* g_sdBusName = "none";
 static uint32_t g_sdFreq = 0;
 static XPowersLibInterface* g_pmu = nullptr;
 
-static const char* kRootTestFile = "/Exercise_05_test.txt";
-static const char* kNestedDir = "/test/testsub1/testsubsub1";
+static const char* kRootTestFile   = "/Exercise_05_test.txt";
+static const char* kNestedDir      = "/test/testsub1/testsubsub1";
 static const char* kNestedTestFile = "/test/testsub1/testsubsub1/Exercise_05_test.txt";
-static const char* kPayload = "This is a test";
+static const char* kPayload        = "This is a test";
 
 enum class WatchState : uint8_t {
   UNKNOWN = 0,
@@ -33,37 +44,127 @@ static uint8_t g_absentVotes = 0;
 static uint32_t g_lastPollMs = 0;
 static uint32_t g_lastFullScanMs = 0;
 static uint32_t g_lastPeriodicActionMs = 0;
-static const uint32_t kPollIntervalAbsentMs = 1000;
-static const uint32_t kPollIntervalMountedMs = 2000;
-static const uint32_t kFullScanIntervalMs = 10000;
-static const uint32_t kPeriodicActionMs = 15000;
-static const uint8_t kVotesToPresent = 2;
-//static const uint8_t kVotesToAbsent = 4;
-static const uint8_t kVotesToAbsent = 5; // More votes needed to declare absent to prevent false removes on transient errors.
 
-//static const uint32_t kStartupWarmupMs = 800;
-static const uint32_t kStartupWarmupMs = 1500; // Longer warmup to allow PMU and card stabilization after power-on.
+static const uint32_t kPollIntervalAbsentMs  = 1000;
+static const uint32_t kPollIntervalMountedMs = 2000;
+static const uint32_t kFullScanIntervalMs    = 10000;
+static const uint32_t kPeriodicActionMs      = 15000;
+
+static const uint8_t  kVotesToPresent = 2;
+static const uint8_t  kVotesToAbsent  = 5;     // More votes needed to declare absent to prevent false removes.
+static const uint32_t kStartupWarmupMs = 1500; // Allow PMU and SD rail to stabilize.
+
 static uint32_t g_logSeq = 0;
 
+// -------------------------
+// Logging helpers
+// -------------------------
 static void logf(const char* fmt, ...) {
   char msg[192];
   va_list args;
   va_start(args, fmt);
   vsnprintf(msg, sizeof(msg), fmt, args);
   va_end(args);
-  Serial.printf("[%10lu][%06lu] %s\r\n", (unsigned long)millis(), (unsigned long)g_logSeq++, msg);
+  Serial.printf("[%10lu][%06lu] %s\r\n",
+                (unsigned long)millis(),
+                (unsigned long)g_logSeq++,
+                msg);
+}
+
+static void dumpPmu(const char* tag, XPowersLibInterface* pmu) {
+  if (!pmu) {
+    logf("PMU(%s): pmu=null", tag);
+    return;
+  }
+
+  bool bldo1 = pmu->isPowerChannelEnable(XPOWERS_BLDO1);
+  int vbus = pmu->getVbusVoltage();
+  int batt = pmu->getBattVoltage();
+
+  logf("PMU(%s): BLDO1(SD)=%s  VBUS=%dmV  VBAT=%dmV",
+       tag, bldo1 ? "ON" : "OFF", vbus, batt);
+}
+
+// IMPORTANT: this function MUST NOT modify pin modes (regression cause).
+static void dumpSdPins(const char* tag) {
+#if ENABLE_PIN_DUMPS
+  const gpio_num_t CS   = (gpio_num_t)tbeam_supreme::sdCs();
+  const gpio_num_t SCK  = (gpio_num_t)tbeam_supreme::sdSck();
+  const gpio_num_t MISO = (gpio_num_t)tbeam_supreme::sdMiso();
+  const gpio_num_t MOSI = (gpio_num_t)tbeam_supreme::sdMosi();
+
+  int cs   = gpio_get_level(CS);
+  int sck  = gpio_get_level(SCK);
+  int miso = gpio_get_level(MISO);
+  int mosi = gpio_get_level(MOSI);
+
+  logf("PINS(%s): CS=%d SCK=%d MISO=%d MOSI=%d", tag, cs, sck, miso, mosi);
+#else
+  (void)tag;
+#endif
+}
+
+// -------------------------
+// Power + bus conditioning
+// -------------------------
+static void forceSpiDeselected() {
+  pinMode(tbeam_supreme::sdCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+
+  pinMode(tbeam_supreme::imuCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::imuCs(), HIGH);
 }
 
 static bool initPmuForSdPower() {
-  return tbeam_supreme::initPmuForPeripherals(g_pmu, &Serial);
+  bool ok = tbeam_supreme::initPmuForPeripherals(g_pmu, &Serial);
+  if (!ok) {
+    logf("ERROR: PMU init failed");
+    return false;
+  }
+
+  logf("PMU adapter: AXP2101 ready, BLDO1(SD)=%s",
+       g_pmu && g_pmu->isPowerChannelEnable(XPOWERS_BLDO1) ? "ON" : "OFF");
+  return true;
 }
 
+static void cycleSdRail(XPowersLibInterface* pmu,
+                        uint32_t off_ms = 250,
+                        uint32_t on_settle_ms = 600) {
+#if ENABLE_SD_RAIL_CYCLE
+  if (!pmu) {
+    logf("SD rail cycle skipped: pmu=null");
+    return;
+  }
+
+  dumpPmu("pre-sd-cycle", pmu);
+
+  // Ensure the card is NOT selected while power is unstable.
+  forceSpiDeselected();
+  dumpSdPins("pre-sd-cycle");
+
+  pmu->disablePowerOutput(XPOWERS_BLDO1);
+  delay(off_ms);
+
+  pmu->setPowerChannelVoltage(XPOWERS_BLDO1, 3300);
+  pmu->enablePowerOutput(XPOWERS_BLDO1);
+  delay(on_settle_ms);
+
+  dumpPmu("post-sd-cycle", pmu);
+  dumpSdPins("post-sd-cycle");
+#else
+  (void)pmu; (void)off_ms; (void)on_settle_ms;
+#endif
+}
+
+// -------------------------
+// SD helpers
+// -------------------------
 static const char* cardTypeToString(uint8_t type) {
   switch (type) {
-    case CARD_MMC: return "MMC";
-    case CARD_SD: return "SDSC";
+    case CARD_MMC:  return "MMC";
+    case CARD_SD:   return "SDSC";
     case CARD_SDHC: return "SDHC/SDXC";
-    default: return "UNKNOWN";
+    default:        return "UNKNOWN";
   }
 }
 
@@ -72,22 +173,26 @@ static bool tryMountWithBus(SPIClass& bus, const char* busName, uint32_t hz, boo
   bus.end();
   delay(10);
 
-  // Keep inactive devices deselected on shared bus lines.
-  pinMode(tbeam_supreme::sdCs(), OUTPUT);
-  digitalWrite(tbeam_supreme::sdCs(), HIGH);
-  pinMode(tbeam_supreme::imuCs(), OUTPUT);
-  digitalWrite(tbeam_supreme::imuCs(), HIGH);
+  forceSpiDeselected();
 
   bus.begin(tbeam_supreme::sdSck(), tbeam_supreme::sdMiso(), tbeam_supreme::sdMosi(), tbeam_supreme::sdCs());
+
+  // SD SPI "idle clocks" ritual: CS HIGH + >= 74 clocks with MOSI high (0xFF).
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
   delay(2);
+  for (int i = 0; i < 10; i++) {
+    bus.transfer(0xFF); // 80 clocks total
+  }
+  delay(2);
+
+  dumpSdPins("after-idle-clocks");
 
   if (verbose) {
     logf("SD: trying bus=%s freq=%lu Hz", busName, (unsigned long)hz);
   }
+
   if (!SD.begin(tbeam_supreme::sdCs(), bus, hz)) {
-    if (verbose) {
-      logf("SD: mount failed (possible non-FAT format, power, or bus issue)");
-    }
+    if (verbose) logf("SD: mount failed (possible non-FAT format, power, or bus issue)");
     return false;
   }
 
@@ -107,7 +212,7 @@ static bool mountPreferred(bool verbose) {
   return tryMountWithBus(sdSpiH, "HSPI", 400000, verbose);
 }
 
-static bool mountCard() {
+static bool mountCardFullScan() {
   const uint32_t freqs[] = {400000, 1000000, 4000000, 10000000};
 
   for (uint8_t i = 0; i < (sizeof(freqs) / sizeof(freqs[0])); ++i) {
@@ -131,8 +236,8 @@ static bool mountCard() {
 static void printCardInfo() {
   uint8_t cardType = SD.cardType();
   uint64_t cardSizeMB = SD.cardSize() / (1024ULL * 1024ULL);
-  uint64_t totalMB = SD.totalBytes() / (1024ULL * 1024ULL);
-  uint64_t usedMB = SD.usedBytes() / (1024ULL * 1024ULL);
+  uint64_t totalMB    = SD.totalBytes() / (1024ULL * 1024ULL);
+  uint64_t usedMB     = SD.usedBytes() / (1024ULL * 1024ULL);
 
   logf("SD type: %s", cardTypeToString(cardType));
   logf("SD size: %llu MB", cardSizeMB);
@@ -143,9 +248,7 @@ static void printCardInfo() {
 
 static bool ensureDirRecursive(const char* path) {
   String full(path);
-  if (!full.startsWith("/")) {
-    full = "/" + full;
-  }
+  if (!full.startsWith("/")) full = "/" + full;
 
   int start = 1;
   while (start > 0 && start < (int)full.length()) {
@@ -160,12 +263,9 @@ static bool ensureDirRecursive(const char* path) {
       }
     }
 
-    if (slash < 0) {
-      break;
-    }
+    if (slash < 0) break;
     start = slash + 1;
   }
-
   return true;
 }
 
@@ -219,9 +319,7 @@ static void permissionsDemo(const char* path) {
 
 static bool verifyMountedCard() {
   File root = SD.open("/", FILE_READ);
-  if (!root) {
-    return false;
-  }
+  if (!root) return false;
   root.close();
   return true;
 }
@@ -245,6 +343,9 @@ static void runCardWorkflow() {
   permissionsDemo(kRootTestFile);
 }
 
+// -------------------------
+// Watcher state transitions
+// -------------------------
 static void setStateMounted() {
   if (g_watchState != WatchState::MOUNTED) {
     logf("EVENT: card inserted/mounted");
@@ -264,13 +365,22 @@ static void setStateAbsent() {
   g_watchState = WatchState::ABSENT;
 }
 
+// -------------------------
+// Arduino entry points
+// -------------------------
 void setup() {
   Serial.begin(115200);
   Serial.println("[WATCHER: startup]");
-  Serial.println("Sleeping for 5 seconds to allow Serial Monitor connection...");
-  delay(5000); // Time to open Serial Monitor after reset
 
-  Serial.println("\r\n==================================================");
+  // De-select SPI devices immediately.
+  forceSpiDeselected();
+  dumpSdPins("very-early");
+
+  logf("Sleeping for %lu ms to allow Serial Monitor connection...", (unsigned long)STARTUP_SERIAL_DELAY_MS);
+  delay(STARTUP_SERIAL_DELAY_MS);
+
+  Serial.println();
+  Serial.println("==================================================");
   Serial.println("Exercise 05: SD Card Watcher");
   Serial.println("==================================================");
   Serial.printf("Pins: CS=%d SCK=%d MISO=%d MOSI=%d\r\n",
@@ -278,13 +388,20 @@ void setup() {
                 tbeam_supreme::sdMiso(), tbeam_supreme::sdMosi());
   Serial.printf("PMU I2C: SDA1=%d SCL1=%d\r\n",
                 tbeam_supreme::i2cSda(), tbeam_supreme::i2cScl());
-  Serial.println("Note: SD must be FAT16/FAT32 for Arduino SD library.\r\n");
+  Serial.println("Note: SD must be FAT16/FAT32 for Arduino SD library.");
+  Serial.println();
 
   initPmuForSdPower();
+  dumpPmu("post-pmu-init", g_pmu);
+
+  // Software equivalent of "remove/insert card".
+  cycleSdRail(g_pmu);
+
   logf("Watcher: waiting %lu ms for SD rail/card stabilization", (unsigned long)kStartupWarmupMs);
   delay(kStartupWarmupMs);
 
-  // Warm-up attempts before first status decision.
+  dumpSdPins("pre-warmup-mount");
+
   bool warmMounted = false;
   for (uint8_t i = 0; i < 3; ++i) {
     if (mountPreferred(false)) {
@@ -293,6 +410,7 @@ void setup() {
     }
     delay(200);
   }
+
   if (warmMounted) {
     logf("Watcher: startup warmup mount succeeded");
     setStateMounted();
@@ -304,9 +422,9 @@ void setup() {
 
 void loop() {
   const uint32_t now = millis();
-  const uint32_t pollInterval = (g_watchState == WatchState::MOUNTED)
-                                    ? kPollIntervalMountedMs
-                                    : kPollIntervalAbsentMs;
+  const uint32_t pollInterval =
+      (g_watchState == WatchState::MOUNTED) ? kPollIntervalMountedMs : kPollIntervalAbsentMs;
+
   if ((uint32_t)(now - g_lastPollMs) < pollInterval) {
     delay(10);
     return;
@@ -321,14 +439,14 @@ void loop() {
         g_lastPeriodicActionMs = now;
       }
       g_presentVotes = 0;
-      g_absentVotes = 0;
+      g_absentVotes  = 0;
       return;
     }
 
-    // One immediate remount attempt prevents false remove events on transient SPI errors.
+    // One immediate remount attempt prevents false removes on transient SPI errors.
     if (mountPreferred(false) && verifyMountedCard()) {
       g_presentVotes = 0;
-      g_absentVotes = 0;
+      g_absentVotes  = 0;
       return;
     }
 
@@ -341,11 +459,12 @@ void loop() {
     return;
   }
 
+  // ABSENT/UNKNOWN state
   bool mounted = mountPreferred(false);
   if (!mounted && (uint32_t)(now - g_lastFullScanMs) >= kFullScanIntervalMs) {
     g_lastFullScanMs = now;
     logf("Watcher: preferred probe failed, running full scan");
-    mounted = mountCard();
+    mounted = mountCardFullScan();
   }
 
   if (mounted) {