my changes, untested, before ChatGTP complete revision... for posterity

exercises/05_SD_Card_Watcher/src/main.cpp

@@ -54,6 +54,66 @@ static void logf(const char* fmt, ...) {
   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);
+
+  // These should work because your adapter calls:
+  // enableVbusVoltageMeasure() and enableBattVoltageMeasure()
+  int vbus = pmu->getVbusVoltage();
+  int batt = pmu->getBattVoltage();
+
+  logf("PMU(%s): BLDO1(SD)=%s  VBUS=%dmV  VBAT=%dmV",
+       tag, bldo1 ? "ON" : "OFF", vbus, batt);
+}
+
+static void cycleSdRail(XPowersLibInterface* pmu,
+                        uint32_t off_ms = 250,
+                        uint32_t on_settle_ms = 600) {
+  if (!pmu) return;
+
+  dumpPmu("pre-sd-cycle", pmu);
+
+  // Make sure CS is HIGH while we yank power.
+  pinMode(tbeam_supreme::sdCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+
+  pmu->disablePowerOutput(XPOWERS_BLDO1);
+  delay(off_ms);
+
+  pmu->setPowerChannelVoltage(XPOWERS_BLDO1, 3300);  // reaffirm
+  pmu->enablePowerOutput(XPOWERS_BLDO1);
+  delay(on_settle_ms);
+
+  dumpPmu("post-sd-cycle", pmu);
+}
+
+static void dumpSdPins(const char* tag) {
+  const int CS   = tbeam_supreme::sdCs();
+  const int SCK  = tbeam_supreme::sdSck();
+  const int MISO = tbeam_supreme::sdMiso();
+  const int MOSI = tbeam_supreme::sdMosi();
+
+  // Try to make floating lines visible.
+  pinMode(CS,   INPUT_PULLUP);
+  pinMode(SCK,  INPUT_PULLUP);
+  pinMode(MISO, INPUT_PULLUP);
+  pinMode(MOSI, INPUT_PULLUP);
+
+  delay(2);
+
+  logf("PINS(%s): CS=%d SCK=%d MISO=%d MOSI=%d",
+       tag,
+       digitalRead(CS),
+       digitalRead(SCK),
+       digitalRead(MISO),
+       digitalRead(MOSI));
+}
+
 static bool initPmuForSdPower() {
   return tbeam_supreme::initPmuForPeripherals(g_pmu, &Serial);
 }
@@ -79,6 +139,17 @@ static bool tryMountWithBus(SPIClass& bus, const char* busName, uint32_t hz, boo
   digitalWrite(tbeam_supreme::imuCs(), HIGH);
 
   bus.begin(tbeam_supreme::sdSck(), tbeam_supreme::sdMiso(), tbeam_supreme::sdMosi(), tbeam_supreme::sdCs());
+  // SD SPI reset ritual: CS high + >= 74 clocks with MOSI high.
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+  delay(2);
+  for (int i = 0; i < 10; i++) {
+    bus.transfer(0xFF); // 8 clocks each => 80 clocks
+  }
+  delay(2);
+
+  // Snapshot pins after clocks.
+  dumpSdPins("after-idle-clocks");
+
   delay(2);
 
   if (verbose) {
@@ -267,6 +338,16 @@ static void setStateAbsent() {
 void setup() {
   Serial.begin(115200);
   Serial.println("[WATCHER: startup]");
+  // Force all SPI devices deselected ASAP (before any delays).
+  pinMode(tbeam_supreme::sdCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+
+  pinMode(tbeam_supreme::imuCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::imuCs(), HIGH);
+  Serial.println("calling dumpSdPins early to check for floating pins before PMU config");
+  dumpSdPins("very-early");
+
+
   Serial.println("Sleeping for 5 seconds to allow Serial Monitor connection...");
   delay(5000); // Time to open Serial Monitor after reset
 
@@ -293,11 +374,13 @@ void setup() {
     }
     delay(200);
   }
+  dumpSdPins("pre-warmup-mount");
   if (warmMounted) {
     logf("Watcher: startup warmup mount succeeded");
     setStateMounted();
   } else {
     logf("Watcher: startup warmup did not mount card");
+    dumpSdPins("warmup-mount-failed");
     setStateAbsent();
   }
 }