diff --git a/exercises/11_Set_RTC2GPS/README.md b/exercises/11_Set_RTC2GPS/README.md
new file mode 100644
index 0000000..462c2d2
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/README.md
@@ -0,0 +1,33 @@
+## Exercise 11: Set RTC to GPS (1PPS Discipline)
+
+This exercise extends Exercise 9 behavior (GPS + SD + OLED) and disciplines the onboard RTC from GPS UTC using the GPS `1PPS` (pulse-per-second) timing signal.
+
+Implemented behavior:
+
+1. Boots PMU, OLED, SD watcher, and GPS UART using the same T-Beam Supreme pin mapping from prior exercises.
+2. Parses NMEA (`RMC`, `GGA`, `GSV`) to track UTC validity and satellite counts.
+3. Every 1 minute, attempts to set RTC from GPS:
+   - Uses latest valid GPS UTC.
+   - Waits for next `1PPS` rising edge.
+   - Sets RTC to GPS time aligned to that edge (UTC + 1 second).
+4. Appends event records to SD file:
+   - Path: `/gps/discipline_rtc.log`
+   - Append-only writes (`FILE_APPEND`)
+   - Format:
+     - `YYYYMMDD_HH24MISS_z\t set RTC to GPS using 1PPS pulse-per-second discipline\trtc-gps drift=+/-Ns`
+5. OLED success message shows RTC disciplined confirmation and timestamp.
+6. If GPS time cannot be determined (or 1PPS edge is not seen in timeout), OLED shows failure status and the loop delays 30 seconds before retry.
+
+## Build
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a
+```
+
+## Upload
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a -t upload --upload-port /dev/ttyACM0
+```
diff --git a/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.cpp b/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.cpp
new file mode 100644
index 0000000..1e8791c
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.cpp
@@ -0,0 +1,360 @@
+#include "StartupSdManager.h"
+
+#include <stdarg.h>
+#include "driver/gpio.h"
+
+StartupSdManager::StartupSdManager(Print& serial) : serial_(serial) {}
+
+bool StartupSdManager::begin(const SdWatcherConfig& cfg, SdStatusCallback callback) {
+  cfg_ = cfg;
+  callback_ = callback;
+
+  forceSpiDeselected();
+  dumpSdPins("very-early");
+
+  if (!initPmuForSdPower()) {
+    return false;
+  }
+
+  cycleSdRail();
+  delay(cfg_.startupWarmupMs);
+
+  bool warmMounted = false;
+  for (uint8_t i = 0; i < 3; ++i) {
+    if (mountPreferred(false)) {
+      warmMounted = true;
+      break;
+    }
+    delay(200);
+  }
+
+  // Some cards need a longer power/settle window after cold boot.
+  // Before declaring ABSENT, retry with extended settle and a full scan.
+  if (!warmMounted) {
+    logf("Watcher: startup preferred mount failed, retrying with extended settle");
+    cycleSdRail(400, 1200);
+    delay(cfg_.startupWarmupMs + 1500);
+    warmMounted = mountCardFullScan();
+  }
+
+  if (warmMounted) {
+    setStateMounted();
+  } else {
+    setStateAbsent();
+  }
+  return true;
+}
+
+void StartupSdManager::update() {
+  const uint32_t now = millis();
+  const uint32_t pollInterval =
+      (watchState_ == SdWatchState::MOUNTED) ? cfg_.pollIntervalMountedMs : cfg_.pollIntervalAbsentMs;
+
+  if ((uint32_t)(now - lastPollMs_) < pollInterval) {
+    return;
+  }
+  lastPollMs_ = now;
+
+  if (watchState_ == SdWatchState::MOUNTED) {
+    if (verifyMountedCard()) {
+      presentVotes_ = 0;
+      absentVotes_ = 0;
+      return;
+    }
+
+    if (mountPreferred(false) && verifyMountedCard()) {
+      presentVotes_ = 0;
+      absentVotes_ = 0;
+      return;
+    }
+
+    absentVotes_++;
+    presentVotes_ = 0;
+    if (absentVotes_ >= cfg_.votesToAbsent) {
+      setStateAbsent();
+      absentVotes_ = 0;
+    }
+    return;
+  }
+
+  bool mounted = mountPreferred(false);
+  if (!mounted && (uint32_t)(now - lastFullScanMs_) >= cfg_.fullScanIntervalMs) {
+    lastFullScanMs_ = now;
+    if (cfg_.recoveryRailCycleOnFullScan) {
+      logf("Watcher: recovery rail cycle before full scan");
+      cycleSdRail(cfg_.recoveryRailOffMs, cfg_.recoveryRailOnSettleMs);
+      delay(150);
+    }
+    logf("Watcher: preferred probe failed, running full scan");
+    mounted = mountCardFullScan();
+  }
+
+  if (mounted) {
+    presentVotes_++;
+    absentVotes_ = 0;
+    if (presentVotes_ >= cfg_.votesToPresent) {
+      setStateMounted();
+      presentVotes_ = 0;
+    }
+  } else {
+    absentVotes_++;
+    presentVotes_ = 0;
+    if (absentVotes_ >= cfg_.votesToAbsent) {
+      setStateAbsent();
+      absentVotes_ = 0;
+    }
+  }
+}
+
+bool StartupSdManager::consumeMountedEvent() {
+  bool out = mountedEventPending_;
+  mountedEventPending_ = false;
+  return out;
+}
+
+bool StartupSdManager::consumeRemovedEvent() {
+  bool out = removedEventPending_;
+  removedEventPending_ = false;
+  return out;
+}
+
+void StartupSdManager::logf(const char* fmt, ...) {
+  char msg[196];
+  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)logSeq_++,
+                 msg);
+}
+
+void StartupSdManager::notify(SdEvent event, const char* message) {
+  if (callback_ != nullptr) {
+    callback_(event, message);
+  }
+}
+
+void StartupSdManager::forceSpiDeselected() {
+  pinMode(tbeam_supreme::sdCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+  pinMode(tbeam_supreme::imuCs(), OUTPUT);
+  digitalWrite(tbeam_supreme::imuCs(), HIGH);
+}
+
+void StartupSdManager::dumpSdPins(const char* tag) {
+  if (!cfg_.enablePinDumps) {
+    (void)tag;
+    return;
+  }
+
+  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();
+  logf("PINS(%s): CS=%d SCK=%d MISO=%d MOSI=%d",
+       tag, gpio_get_level(cs), gpio_get_level(sck), gpio_get_level(miso), gpio_get_level(mosi));
+}
+
+bool StartupSdManager::initPmuForSdPower() {
+  if (!tbeam_supreme::initPmuForPeripherals(pmu_, &serial_)) {
+    logf("ERROR: PMU init failed");
+    return false;
+  }
+  return true;
+}
+
+void StartupSdManager::cycleSdRail(uint32_t offMs, uint32_t onSettleMs) {
+  if (!cfg_.enableSdRailCycle) {
+    return;
+  }
+  if (!pmu_) {
+    logf("SD rail cycle skipped: pmu=null");
+    return;
+  }
+
+  forceSpiDeselected();
+  pmu_->disablePowerOutput(XPOWERS_BLDO1);
+  delay(offMs);
+  pmu_->setPowerChannelVoltage(XPOWERS_BLDO1, 3300);
+  pmu_->enablePowerOutput(XPOWERS_BLDO1);
+  delay(onSettleMs);
+}
+
+bool StartupSdManager::tryMountWithBus(SPIClass& bus, const char* busName, uint32_t hz, bool verbose) {
+  SD.end();
+  bus.end();
+  delay(10);
+  forceSpiDeselected();
+
+  bus.begin(tbeam_supreme::sdSck(), tbeam_supreme::sdMiso(), tbeam_supreme::sdMosi(), tbeam_supreme::sdCs());
+  digitalWrite(tbeam_supreme::sdCs(), HIGH);
+  delay(2);
+  for (int i = 0; i < 10; i++) {
+    bus.transfer(0xFF);
+  }
+  delay(2);
+
+  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)");
+    }
+    return false;
+  }
+
+  if (SD.cardType() == CARD_NONE) {
+    SD.end();
+    return false;
+  }
+
+  sdSpi_ = &bus;
+  sdBusName_ = busName;
+  sdFreq_ = hz;
+  return true;
+}
+
+bool StartupSdManager::mountPreferred(bool verbose) {
+  return tryMountWithBus(sdSpiH_, "HSPI", 400000, verbose);
+}
+
+bool StartupSdManager::mountCardFullScan() {
+  const uint32_t freqs[] = {400000, 1000000, 4000000, 10000000};
+
+  for (uint8_t i = 0; i < (sizeof(freqs) / sizeof(freqs[0])); ++i) {
+    if (tryMountWithBus(sdSpiH_, "HSPI", freqs[i], true)) {
+      logf("SD: card detected and mounted");
+      return true;
+    }
+  }
+  for (uint8_t i = 0; i < (sizeof(freqs) / sizeof(freqs[0])); ++i) {
+    if (tryMountWithBus(sdSpiF_, "FSPI", freqs[i], true)) {
+      logf("SD: card detected and mounted");
+      return true;
+    }
+  }
+
+  logf("SD: begin() failed on all bus/frequency attempts");
+  return false;
+}
+
+bool StartupSdManager::verifyMountedCard() {
+  File root = SD.open("/", FILE_READ);
+  if (!root) {
+    return false;
+  }
+  root.close();
+  return true;
+}
+
+const char* StartupSdManager::cardTypeToString(uint8_t type) {
+  switch (type) {
+    case CARD_MMC:
+      return "MMC";
+    case CARD_SD:
+      return "SDSC";
+    case CARD_SDHC:
+      return "SDHC/SDXC";
+    default:
+      return "UNKNOWN";
+  }
+}
+
+void StartupSdManager::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);
+
+  logf("SD type: %s", cardTypeToString(cardType));
+  logf("SD size: %llu MB", cardSizeMB);
+  logf("FS total: %llu MB", totalMB);
+  logf("FS used : %llu MB", usedMB);
+  logf("SPI bus: %s @ %lu Hz", sdBusName_, (unsigned long)sdFreq_);
+}
+
+bool StartupSdManager::ensureDirRecursive(const char* path) {
+  String full(path);
+  if (!full.startsWith("/")) {
+    full = "/" + full;
+  }
+
+  int start = 1;
+  while (start > 0 && start < (int)full.length()) {
+    int slash = full.indexOf('/', start);
+    String partial = (slash < 0) ? full : full.substring(0, slash);
+    if (!SD.exists(partial.c_str()) && !SD.mkdir(partial.c_str())) {
+      logf("ERROR: mkdir failed for %s", partial.c_str());
+      return false;
+    }
+    if (slash < 0) {
+      break;
+    }
+    start = slash + 1;
+  }
+
+  return true;
+}
+
+bool StartupSdManager::rewriteFile(const char* path, const char* payload) {
+  if (SD.exists(path) && !SD.remove(path)) {
+    logf("ERROR: failed to erase %s", path);
+    return false;
+  }
+
+  File f = SD.open(path, FILE_WRITE);
+  if (!f) {
+    logf("ERROR: failed to create %s", path);
+    return false;
+  }
+
+  size_t wrote = f.println(payload);
+  f.close();
+  if (wrote == 0) {
+    logf("ERROR: write failed for %s", path);
+    return false;
+  }
+  return true;
+}
+
+void StartupSdManager::permissionsDemo(const char* path) {
+  logf("Permissions demo: FAT has no Unix chmod/chown, use open mode only.");
+  File r = SD.open(path, FILE_READ);
+  if (!r) {
+    logf("Could not open %s as FILE_READ", path);
+    return;
+  }
+  size_t writeInReadMode = r.print("attempt write while opened read-only");
+  if (writeInReadMode == 0) {
+    logf("As expected, FILE_READ write was blocked.");
+  } else {
+    logf("NOTE: FILE_READ write returned %u (unexpected)", (unsigned)writeInReadMode);
+  }
+  r.close();
+}
+
+void StartupSdManager::setStateMounted() {
+  if (watchState_ != SdWatchState::MOUNTED) {
+    logf("EVENT: card inserted/mounted");
+    mountedEventPending_ = true;
+    notify(SdEvent::CARD_MOUNTED, "SD card mounted");
+  }
+  watchState_ = SdWatchState::MOUNTED;
+}
+
+void StartupSdManager::setStateAbsent() {
+  if (watchState_ == SdWatchState::MOUNTED) {
+    logf("EVENT: card removed/unavailable");
+    removedEventPending_ = true;
+    notify(SdEvent::CARD_REMOVED, "SD card removed");
+  } else if (watchState_ != SdWatchState::ABSENT) {
+    logf("EVENT: no card detected");
+    notify(SdEvent::NO_CARD, "Missing SD card or invalid FAT16/FAT32 format");
+  }
+  SD.end();
+  watchState_ = SdWatchState::ABSENT;
+}
diff --git a/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.h b/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.h
new file mode 100644
index 0000000..be9ef27
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/lib/startup_sd/StartupSdManager.h
@@ -0,0 +1,90 @@
+#pragma once
+
+#include <Arduino.h>
+#include <SD.h>
+#include <SPI.h>
+#include <Wire.h>
+#include "tbeam_supreme_adapter.h"
+
+enum class SdWatchState : uint8_t {
+  UNKNOWN = 0,
+  ABSENT,
+  MOUNTED
+};
+
+enum class SdEvent : uint8_t {
+  NO_CARD,
+  CARD_MOUNTED,
+  CARD_REMOVED
+};
+
+using SdStatusCallback = void (*)(SdEvent event, const char* message);
+
+struct SdWatcherConfig {
+  bool enableSdRailCycle = true;
+  bool enablePinDumps = true;
+  bool recoveryRailCycleOnFullScan = true;
+  uint32_t recoveryRailOffMs = 250;
+  uint32_t recoveryRailOnSettleMs = 700;
+  uint32_t startupWarmupMs = 1500;
+  uint32_t pollIntervalAbsentMs = 1000;
+  uint32_t pollIntervalMountedMs = 2000;
+  uint32_t fullScanIntervalMs = 10000;
+  uint8_t votesToPresent = 2;
+  uint8_t votesToAbsent = 5;
+};
+
+class StartupSdManager {
+ public:
+  explicit StartupSdManager(Print& serial = Serial);
+
+  bool begin(const SdWatcherConfig& cfg, SdStatusCallback callback = nullptr);
+  void update();
+
+  bool isMounted() const { return watchState_ == SdWatchState::MOUNTED; }
+  SdWatchState state() const { return watchState_; }
+
+  bool consumeMountedEvent();
+  bool consumeRemovedEvent();
+
+  void printCardInfo();
+  bool ensureDirRecursive(const char* path);
+  bool rewriteFile(const char* path, const char* payload);
+  void permissionsDemo(const char* path);
+
+ private:
+  void logf(const char* fmt, ...);
+  void notify(SdEvent event, const char* message);
+  void forceSpiDeselected();
+  void dumpSdPins(const char* tag);
+  bool initPmuForSdPower();
+  void cycleSdRail(uint32_t offMs = 250, uint32_t onSettleMs = 600);
+  bool tryMountWithBus(SPIClass& bus, const char* busName, uint32_t hz, bool verbose);
+  bool mountPreferred(bool verbose);
+  bool mountCardFullScan();
+  bool verifyMountedCard();
+  const char* cardTypeToString(uint8_t type);
+  void setStateMounted();
+  void setStateAbsent();
+
+  Print& serial_;
+  SdWatcherConfig cfg_{};
+  SdStatusCallback callback_ = nullptr;
+
+  SPIClass sdSpiH_{HSPI};
+  SPIClass sdSpiF_{FSPI};
+  SPIClass* sdSpi_ = nullptr;
+  const char* sdBusName_ = "none";
+  uint32_t sdFreq_ = 0;
+  XPowersLibInterface* pmu_ = nullptr;
+
+  SdWatchState watchState_ = SdWatchState::UNKNOWN;
+  uint8_t presentVotes_ = 0;
+  uint8_t absentVotes_ = 0;
+  uint32_t lastPollMs_ = 0;
+  uint32_t lastFullScanMs_ = 0;
+  uint32_t logSeq_ = 0;
+
+  bool mountedEventPending_ = false;
+  bool removedEventPending_ = false;
+};
diff --git a/exercises/11_Set_RTC2GPS/lib/startup_sd/library.json b/exercises/11_Set_RTC2GPS/lib/startup_sd/library.json
new file mode 100644
index 0000000..4978fdd
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/lib/startup_sd/library.json
@@ -0,0 +1,12 @@
+{
+  "name": "startup_sd",
+  "version": "0.1.0",
+  "dependencies": [
+    {
+      "name": "XPowersLib"
+    },
+    {
+      "name": "Wire"
+    }
+  ]
+}
diff --git a/exercises/11_Set_RTC2GPS/platformio.ini b/exercises/11_Set_RTC2GPS/platformio.ini
new file mode 100644
index 0000000..fff0d30
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/platformio.ini
@@ -0,0 +1,41 @@
+; 20260213 ChatGPT
+; $Id$
+; $HeadURL$
+
+[platformio]
+default_envs = node_a
+
+[env]
+platform = espressif32
+framework = arduino
+board = esp32-s3-devkitc-1
+monitor_speed = 115200
+lib_deps =
+  lewisxhe/XPowersLib@0.3.3
+  Wire
+  olikraus/U8g2@^2.36.4
+
+; SD pins based on T-Beam S3 core pin mapping
+build_flags =
+  -I ../../shared/boards
+  -I ../../external/microReticulum_Firmware
+  -D BOARD_MODEL=BOARD_TBEAM_S_V1
+  -D OLED_SDA=17
+  -D OLED_SCL=18
+  -D OLED_ADDR=0x3C
+  -D GPS_RX_PIN=9
+  -D GPS_TX_PIN=8
+  -D GPS_WAKEUP_PIN=7
+  -D GPS_1PPS_PIN=6
+  -D ARDUINO_USB_MODE=1
+  -D ARDUINO_USB_CDC_ON_BOOT=1
+
+[env:node_a]
+build_flags =
+  ${env.build_flags}
+  -D NODE_LABEL=\"A\"
+
+[env:node_b]
+build_flags =
+  ${env.build_flags}
+  -D NODE_LABEL=\"B\"
diff --git a/exercises/11_Set_RTC2GPS/src/main.cpp b/exercises/11_Set_RTC2GPS/src/main.cpp
new file mode 100644
index 0000000..d2a90cb
--- /dev/null
+++ b/exercises/11_Set_RTC2GPS/src/main.cpp
@@ -0,0 +1,636 @@
+// 20260217 ChatGPT
+// $Id$
+// $HeadURL$
+
+#include <Arduino.h>
+#include <Wire.h>
+#include <SD.h>
+#include <U8g2lib.h>
+
+#include "StartupSdManager.h"
+#include "tbeam_supreme_adapter.h"
+
+#ifndef OLED_SDA
+#define OLED_SDA 17
+#endif
+
+#ifndef OLED_SCL
+#define OLED_SCL 18
+#endif
+
+#ifndef OLED_ADDR
+#define OLED_ADDR 0x3C
+#endif
+
+#ifndef RTC_I2C_ADDR
+#define RTC_I2C_ADDR 0x51
+#endif
+
+#ifndef GPS_BAUD
+#define GPS_BAUD 9600
+#endif
+
+#ifndef FILE_APPEND
+#define FILE_APPEND FILE_WRITE
+#endif
+
+static const uint32_t kSerialDelayMs = 5000;
+static const uint32_t kLoopMsDiscipline = 60000;
+static const uint32_t kNoTimeDelayMs = 30000;
+static const uint32_t kGpsStartupProbeMs = 20000;
+static const uint32_t kPpsWaitTimeoutMs = 1500;
+
+static XPowersLibInterface* g_pmu = nullptr;
+static StartupSdManager g_sd(Serial);
+static U8G2_SH1106_128X64_NONAME_F_HW_I2C g_oled(U8G2_R0, /* reset=*/U8X8_PIN_NONE);
+static HardwareSerial g_gpsSerial(1);
+
+static uint32_t g_logSeq = 0;
+static uint32_t g_nextDisciplineMs = 0;
+static bool g_gpsPathReady = false;
+
+static char g_gpsLine[128];
+static size_t g_gpsLineLen = 0;
+
+static volatile uint32_t g_ppsEdgeCount = 0;
+
+struct DateTime {
+  uint16_t year;
+  uint8_t month;
+  uint8_t day;
+  uint8_t hour;
+  uint8_t minute;
+  uint8_t second;
+};
+
+struct GpsState {
+  bool sawAnySentence = false;
+  bool hasValidUtc = false;
+  uint8_t satsUsed = 0;
+  uint8_t satsInView = 0;
+  DateTime utc{};
+  uint32_t lastUtcMs = 0;
+};
+
+static GpsState g_gps;
+
+static void logf(const char* fmt, ...) {
+  char msg[240];
+  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);
+}
+
+static void oledShowLines(const char* l1,
+                          const char* l2 = nullptr,
+                          const char* l3 = nullptr,
+                          const char* l4 = nullptr,
+                          const char* l5 = nullptr) {
+  g_oled.clearBuffer();
+  g_oled.setFont(u8g2_font_5x8_tf);
+  if (l1) g_oled.drawUTF8(0, 12, l1);
+  if (l2) g_oled.drawUTF8(0, 24, l2);
+  if (l3) g_oled.drawUTF8(0, 36, l3);
+  if (l4) g_oled.drawUTF8(0, 48, l4);
+  if (l5) g_oled.drawUTF8(0, 60, l5);
+  g_oled.sendBuffer();
+}
+
+static uint8_t toBcd(uint8_t v) {
+  return (uint8_t)(((v / 10U) << 4U) | (v % 10U));
+}
+
+static uint8_t fromBcd(uint8_t b) {
+  return (uint8_t)(((b >> 4U) * 10U) + (b & 0x0FU));
+}
+
+static bool rtcRead(DateTime& out, bool& lowVoltageFlag) {
+  Wire1.beginTransmission(RTC_I2C_ADDR);
+  Wire1.write(0x02);
+  if (Wire1.endTransmission(false) != 0) {
+    return false;
+  }
+
+  const uint8_t need = 7;
+  uint8_t got = Wire1.requestFrom((int)RTC_I2C_ADDR, (int)need);
+  if (got != need) {
+    return false;
+  }
+
+  uint8_t sec = Wire1.read();
+  uint8_t min = Wire1.read();
+  uint8_t hour = Wire1.read();
+  uint8_t day = Wire1.read();
+  (void)Wire1.read();
+  uint8_t month = Wire1.read();
+  uint8_t year = Wire1.read();
+
+  lowVoltageFlag = (sec & 0x80U) != 0;
+  out.second = fromBcd(sec & 0x7FU);
+  out.minute = fromBcd(min & 0x7FU);
+  out.hour = fromBcd(hour & 0x3FU);
+  out.day = fromBcd(day & 0x3FU);
+  out.month = fromBcd(month & 0x1FU);
+  uint8_t yy = fromBcd(year);
+  bool century = (month & 0x80U) != 0;
+  out.year = century ? (1900U + yy) : (2000U + yy);
+  return true;
+}
+
+static bool rtcWrite(const DateTime& dt) {
+  Wire1.beginTransmission(RTC_I2C_ADDR);
+  Wire1.write(0x02);
+  Wire1.write(toBcd(dt.second & 0x7FU));
+  Wire1.write(toBcd(dt.minute));
+  Wire1.write(toBcd(dt.hour));
+  Wire1.write(toBcd(dt.day));
+  Wire1.write(0x00);
+
+  uint8_t monthReg = toBcd(dt.month);
+  if (dt.year < 2000U) {
+    monthReg |= 0x80U;
+  }
+  Wire1.write(monthReg);
+  Wire1.write(toBcd((uint8_t)(dt.year % 100U)));
+
+  return Wire1.endTransmission() == 0;
+}
+
+static bool isLeapYear(uint16_t y) {
+  return ((y % 4U) == 0U && (y % 100U) != 0U) || ((y % 400U) == 0U);
+}
+
+static uint8_t daysInMonth(uint16_t year, uint8_t month) {
+  static const uint8_t kDays[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
+  if (month == 2) {
+    return (uint8_t)(isLeapYear(year) ? 29 : 28);
+  }
+  if (month >= 1 && month <= 12) {
+    return kDays[month - 1];
+  }
+  return 30;
+}
+
+static bool isValidDateTime(const DateTime& dt) {
+  if (dt.year < 2000U || dt.year > 2099U) return false;
+  if (dt.month < 1 || dt.month > 12) return false;
+  if (dt.day < 1 || dt.day > daysInMonth(dt.year, dt.month)) return false;
+  if (dt.hour > 23 || dt.minute > 59 || dt.second > 59) return false;
+  return true;
+}
+
+static int64_t daysFromCivil(int y, unsigned m, unsigned d) {
+  y -= (m <= 2);
+  const int era = (y >= 0 ? y : y - 399) / 400;
+  const unsigned yoe = (unsigned)(y - era * 400);
+  const unsigned doy = (153 * (m + (m > 2 ? (unsigned)-3 : 9)) + 2) / 5 + d - 1;
+  const unsigned doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
+  return era * 146097 + (int)doe - 719468;
+}
+
+static int64_t toEpochSeconds(const DateTime& dt) {
+  int64_t days = daysFromCivil((int)dt.year, dt.month, dt.day);
+  return days * 86400LL + (int64_t)dt.hour * 3600LL + (int64_t)dt.minute * 60LL + (int64_t)dt.second;
+}
+
+static bool fromEpochSeconds(int64_t sec, DateTime& out) {
+  if (sec < 0) {
+    return false;
+  }
+
+  int64_t days = sec / 86400LL;
+  int64_t rem = sec % 86400LL;
+  if (rem < 0) {
+    rem += 86400LL;
+    days -= 1;
+  }
+
+  out.hour = (uint8_t)(rem / 3600LL);
+  rem %= 3600LL;
+  out.minute = (uint8_t)(rem / 60LL);
+  out.second = (uint8_t)(rem % 60LL);
+
+  days += 719468;
+  const int era = (days >= 0 ? days : days - 146096) / 146097;
+  const unsigned doe = (unsigned)(days - era * 146097);
+  const unsigned yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;
+  int y = (int)yoe + era * 400;
+  const unsigned doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
+  const unsigned mp = (5 * doy + 2) / 153;
+  const unsigned d = doy - (153 * mp + 2) / 5 + 1;
+  const unsigned m = mp + (mp < 10 ? 3 : (unsigned)-9);
+  y += (m <= 2);
+
+  out.year = (uint16_t)y;
+  out.month = (uint8_t)m;
+  out.day = (uint8_t)d;
+  return isValidDateTime(out);
+}
+
+static void addOneSecond(DateTime& dt) {
+  int64_t t = toEpochSeconds(dt);
+  DateTime out{};
+  if (fromEpochSeconds(t + 1, out)) {
+    dt = out;
+  }
+}
+
+static void formatUtcCompact(const DateTime& dt, char* out, size_t outLen) {
+  snprintf(out,
+           outLen,
+           "%04u%02u%02u_%02u%02u%02u_z",
+           (unsigned)dt.year,
+           (unsigned)dt.month,
+           (unsigned)dt.day,
+           (unsigned)dt.hour,
+           (unsigned)dt.minute,
+           (unsigned)dt.second);
+}
+
+static void formatUtcHuman(const DateTime& dt, char* out, size_t outLen) {
+  snprintf(out,
+           outLen,
+           "%04u-%02u-%02u %02u:%02u:%02u UTC",
+           (unsigned)dt.year,
+           (unsigned)dt.month,
+           (unsigned)dt.day,
+           (unsigned)dt.hour,
+           (unsigned)dt.minute,
+           (unsigned)dt.second);
+}
+
+static bool parseUInt2(const char* s, uint8_t& out) {
+  if (!s || !isdigit((unsigned char)s[0]) || !isdigit((unsigned char)s[1])) {
+    return false;
+  }
+  out = (uint8_t)((s[0] - '0') * 10 + (s[1] - '0'));
+  return true;
+}
+
+static void parseGga(char* fields[], int count) {
+  if (count <= 7) {
+    return;
+  }
+  int sats = atoi(fields[7]);
+  if (sats >= 0 && sats <= 255) {
+    g_gps.satsUsed = (uint8_t)sats;
+  }
+}
+
+static void parseGsv(char* fields[], int count) {
+  if (count <= 3) {
+    return;
+  }
+  int sats = atoi(fields[3]);
+  if (sats >= 0 && sats <= 255) {
+    g_gps.satsInView = (uint8_t)sats;
+  }
+}
+
+static void parseRmc(char* fields[], int count) {
+  if (count <= 9) {
+    return;
+  }
+
+  const char* utc = fields[1];
+  const char* status = fields[2];
+  const char* date = fields[9];
+
+  if (!status || status[0] != 'A') {
+    return;
+  }
+
+  if (!utc || strlen(utc) < 6 || !date || strlen(date) < 6) {
+    return;
+  }
+
+  uint8_t hh = 0, mm = 0, ss = 0;
+  uint8_t dd = 0, mo = 0, yy = 0;
+  if (!parseUInt2(utc + 0, hh) || !parseUInt2(utc + 2, mm) || !parseUInt2(utc + 4, ss)) {
+    return;
+  }
+  if (!parseUInt2(date + 0, dd) || !parseUInt2(date + 2, mo) || !parseUInt2(date + 4, yy)) {
+    return;
+  }
+
+  g_gps.utc.hour = hh;
+  g_gps.utc.minute = mm;
+  g_gps.utc.second = ss;
+  g_gps.utc.day = dd;
+  g_gps.utc.month = mo;
+  g_gps.utc.year = (uint16_t)(2000U + yy);
+  g_gps.hasValidUtc = true;
+  g_gps.lastUtcMs = millis();
+}
+
+static void processNmeaLine(char* line) {
+  if (!line || line[0] != '$') {
+    return;
+  }
+
+  g_gps.sawAnySentence = true;
+
+  char* star = strchr(line, '*');
+  if (star) {
+    *star = '\0';
+  }
+
+  char* fields[24] = {0};
+  int count = 0;
+  char* saveptr = nullptr;
+  char* tok = strtok_r(line, ",", &saveptr);
+  while (tok && count < 24) {
+    fields[count++] = tok;
+    tok = strtok_r(nullptr, ",", &saveptr);
+  }
+
+  if (count <= 0 || !fields[0]) {
+    return;
+  }
+
+  const char* header = fields[0];
+  size_t n = strlen(header);
+  if (n < 6) {
+    return;
+  }
+
+  const char* type = header + (n - 3);
+  if (strcmp(type, "GGA") == 0) {
+    parseGga(fields, count);
+  } else if (strcmp(type, "GSV") == 0) {
+    parseGsv(fields, count);
+  } else if (strcmp(type, "RMC") == 0) {
+    parseRmc(fields, count);
+  }
+}
+
+static void pollGpsSerial() {
+  while (g_gpsSerial.available() > 0) {
+    char c = (char)g_gpsSerial.read();
+    if (c == '\r') {
+      continue;
+    }
+    if (c == '\n') {
+      if (g_gpsLineLen > 0) {
+        g_gpsLine[g_gpsLineLen] = '\0';
+        processNmeaLine(g_gpsLine);
+        g_gpsLineLen = 0;
+      }
+      continue;
+    }
+
+    if (g_gpsLineLen + 1 < sizeof(g_gpsLine)) {
+      g_gpsLine[g_gpsLineLen++] = c;
+    } else {
+      g_gpsLineLen = 0;
+    }
+  }
+}
+
+static bool collectGpsTraffic(uint32_t windowMs) {
+  uint32_t start = millis();
+  bool sawBytes = false;
+  while ((uint32_t)(millis() - start) < windowMs) {
+    if (g_gpsSerial.available() > 0) {
+      sawBytes = true;
+    }
+    pollGpsSerial();
+    g_sd.update();
+    delay(2);
+  }
+  return sawBytes || g_gps.sawAnySentence;
+}
+
+static void initialGpsProbe() {
+  logf("GPS startup probe at %u baud", (unsigned)GPS_BAUD);
+  (void)collectGpsTraffic(kGpsStartupProbeMs);
+  logf("GPS probe complete: nmea=%s sats_used=%u sats_view=%u utc=%s",
+       g_gps.sawAnySentence ? "yes" : "no",
+       (unsigned)g_gps.satsUsed,
+       (unsigned)g_gps.satsInView,
+       g_gps.hasValidUtc ? "yes" : "no");
+}
+
+static IRAM_ATTR void onPpsEdge() {
+  g_ppsEdgeCount++;
+}
+
+static uint8_t bestSatelliteCount() {
+  return (g_gps.satsUsed > g_gps.satsInView) ? g_gps.satsUsed : g_gps.satsInView;
+}
+
+static bool ensureGpsLogPathReady() {
+  if (!g_sd.isMounted()) {
+    g_gpsPathReady = false;
+    return false;
+  }
+
+  if (g_gpsPathReady) {
+    return true;
+  }
+
+  if (!g_sd.ensureDirRecursive("/gps")) {
+    logf("Could not create /gps directory");
+    return false;
+  }
+
+  // Touch the log file so a clean SD card is prepared before first discipline event.
+  File f = SD.open("/gps/discipline_rtc.log", FILE_APPEND);
+  if (!f) {
+    logf("Could not open /gps/discipline_rtc.log for append");
+    return false;
+  }
+  f.close();
+
+  g_gpsPathReady = true;
+  return true;
+}
+
+static bool appendDisciplineLog(const DateTime& gpsUtc, int64_t rtcMinusGpsSeconds) {
+  if (!ensureGpsLogPathReady()) {
+    logf("SD not mounted, skipping append to gps/discipline_rtc.log");
+    return false;
+  }
+
+  File f = SD.open("/gps/discipline_rtc.log", FILE_APPEND);
+  if (!f) {
+    logf("Could not open /gps/discipline_rtc.log for append");
+    return false;
+  }
+
+  char ts[32];
+  formatUtcCompact(gpsUtc, ts, sizeof(ts));
+
+  char line[220];
+  snprintf(line,
+           sizeof(line),
+           "%s\t set RTC to GPS using 1PPS pulse-per-second discipline\trtc-gps drift=%+lld s",
+           ts,
+           (long long)rtcMinusGpsSeconds);
+
+  size_t wrote = f.println(line);
+  f.close();
+  if (wrote == 0) {
+    logf("Append write failed: /gps/discipline_rtc.log");
+    return false;
+  }
+  return true;
+}
+
+static bool gpsUtcIsFresh() {
+  if (!g_gps.hasValidUtc) {
+    return false;
+  }
+  return (uint32_t)(millis() - g_gps.lastUtcMs) <= 2000;
+}
+
+static bool waitForNextPps(uint32_t timeoutMs) {
+  uint32_t startCount = g_ppsEdgeCount;
+  uint32_t startMs = millis();
+  while ((uint32_t)(millis() - startMs) < timeoutMs) {
+    pollGpsSerial();
+    g_sd.update();
+    if (g_ppsEdgeCount != startCount) {
+      return true;
+    }
+    delay(2);
+  }
+  return false;
+}
+
+static void waitWithUpdates(uint32_t delayMs) {
+  uint32_t start = millis();
+  while ((uint32_t)(millis() - start) < delayMs) {
+    pollGpsSerial();
+    g_sd.update();
+    delay(10);
+  }
+}
+
+static void showNoTimeAndDelay() {
+  uint8_t sats = bestSatelliteCount();
+  char l3[24];
+  snprintf(l3, sizeof(l3), "Satellites: %u", (unsigned)sats);
+  oledShowLines("GPS time unavailable", "RTC NOT disciplined", l3, "Retry in 30 seconds");
+  logf("GPS UTC unavailable. satellites=%u. Waiting 30 seconds.", (unsigned)sats);
+  waitWithUpdates(kNoTimeDelayMs);
+}
+
+static bool disciplineRtcToGps() {
+  if (!gpsUtcIsFresh()) {
+    showNoTimeAndDelay();
+    return false;
+  }
+
+  DateTime priorRtc{};
+  bool lowV = false;
+  bool havePriorRtc = rtcRead(priorRtc, lowV);
+  if (havePriorRtc && (lowV || !isValidDateTime(priorRtc))) {
+    havePriorRtc = false;
+  }
+
+  DateTime gpsSnap = g_gps.utc;
+  if (!waitForNextPps(kPpsWaitTimeoutMs)) {
+    oledShowLines("GPS 1PPS missing", "RTC NOT disciplined", "Retry in 30 seconds");
+    logf("No 1PPS edge observed within timeout. Waiting 30 seconds.");
+    waitWithUpdates(kNoTimeDelayMs);
+    return false;
+  }
+
+  DateTime target = gpsSnap;
+  addOneSecond(target);
+
+  if (!rtcWrite(target)) {
+    oledShowLines("RTC write failed", "Could not set from GPS");
+    logf("RTC write failed");
+    return false;
+  }
+
+  int64_t driftSec = 0;
+  if (havePriorRtc) {
+    driftSec = toEpochSeconds(priorRtc) - toEpochSeconds(target);
+  }
+
+  (void)appendDisciplineLog(target, driftSec);
+
+  char utcLine[36];
+  char driftLine[36];
+  formatUtcHuman(target, utcLine, sizeof(utcLine));
+  if (havePriorRtc) {
+    snprintf(driftLine, sizeof(driftLine), "rtc-gps drift: %+lld s", (long long)driftSec);
+  } else {
+    snprintf(driftLine, sizeof(driftLine), "rtc-gps drift: RTC_unset");
+  }
+
+  oledShowLines("RTC disciplined to GPS", utcLine, "Method: 1PPS", driftLine);
+
+  logf("RTC disciplined to GPS with 1PPS. %s drift=%+llds lowV=%s",
+       utcLine,
+       (long long)driftSec,
+       lowV ? "yes" : "no");
+  return true;
+}
+
+void setup() {
+  Serial.begin(115200);
+  delay(kSerialDelayMs);
+
+  Serial.println("\r\n==================================================");
+  Serial.println("Exercise 11: Set RTC to GPS with 1PPS discipline");
+  Serial.println("==================================================");
+
+  if (!tbeam_supreme::initPmuForPeripherals(g_pmu, &Serial)) {
+    logf("PMU init failed");
+  }
+
+  Wire.begin(OLED_SDA, OLED_SCL);
+  g_oled.setI2CAddress(OLED_ADDR << 1);
+  g_oled.begin();
+  oledShowLines("Exercise 11", "RTC <- GPS (1PPS)", "Booting...");
+
+  SdWatcherConfig sdCfg{};
+  if (!g_sd.begin(sdCfg, nullptr)) {
+    logf("SD startup manager begin() failed");
+  }
+  (void)ensureGpsLogPathReady();
+
+#ifdef GPS_WAKEUP_PIN
+  pinMode(GPS_WAKEUP_PIN, INPUT);
+#endif
+#ifdef GPS_1PPS_PIN
+  pinMode(GPS_1PPS_PIN, INPUT);
+  attachInterrupt(digitalPinToInterrupt(GPS_1PPS_PIN), onPpsEdge, RISING);
+#endif
+
+  g_gpsSerial.setRxBufferSize(1024);
+  g_gpsSerial.begin(GPS_BAUD, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN);
+  logf("GPS UART started: RX=%d TX=%d baud=%u", GPS_RX_PIN, GPS_TX_PIN, (unsigned)GPS_BAUD);
+
+  oledShowLines("GPS startup probe", "Checking UTC + 1PPS");
+  initialGpsProbe();
+
+  g_nextDisciplineMs = millis();
+}
+
+void loop() {
+  pollGpsSerial();
+  g_sd.update();
+
+  if (g_sd.consumeMountedEvent()) {
+    g_gpsPathReady = false;
+    (void)ensureGpsLogPathReady();
+  }
+  if (g_sd.consumeRemovedEvent()) {
+    g_gpsPathReady = false;
+  }
+
+  uint32_t now = millis();
+  if ((int32_t)(now - g_nextDisciplineMs) >= 0) {
+    bool ok = disciplineRtcToGps();
+    g_nextDisciplineMs = now + (ok ? kLoopMsDiscipline : kNoTimeDelayMs);
+  }
+
+  delay(5);
+}