# USB Serial Blocking Issue - Root Cause and Fix

**Date:** 2026-01-28  
**Issue:** Device blocking/hanging when USB is not connected at boot

---

## Problem Description

The device would hang or behave unpredictably when booted without USB connected. This was traced to improper Serial handling on ESP32-C3 with USB CDC.

## Root Cause Analysis

### Factor A: `checkForFlashCommand()` Called Without Serial Initialization

The most critical issue was in `checkForFlashCommand()`, which is called at the start of every `loop()` iteration:

```cpp
void loop() {
  checkForFlashCommand();  // Called EVERY loop iteration
  // ...
}

void checkForFlashCommand() {
  while (Serial.available()) {  // Called even when Serial.begin() was never called!
    char c = Serial.read();
    // ...
  }
}
```

When USB is not connected at boot, `Serial.begin()` is never called. Then in `loop()`, `checkForFlashCommand()` calls `Serial.available()` and `Serial.read()` on an uninitialized Serial object. On ESP32-C3 with USB CDC, this causes undefined behavior or blocking.

### Factor B: Removed `while (!Serial)` Wait Loop

The upstream 0.16.0 code included a 3-second wait loop after `Serial.begin()`:

```cpp
if (isUsbConnected()) {
  Serial.begin(115200);
  unsigned long start = millis();
  while (!Serial && (millis() - start) < 3000) {
    delay(10);
  }
}
```

This wait loop was removed in an earlier attempt to fix boot delays, but it may be necessary for proper USB CDC initialization.

### Factor C: `Serial.setTxTimeoutMs(0)` Added Too Early

`Serial.setTxTimeoutMs(0)` was added immediately after `Serial.begin()` to make TX non-blocking. However, calling this before the Serial connection is fully established may interfere with USB CDC initialization.

---

## The Fix

### 1. Guard `checkForFlashCommand()` with Serial Check

```cpp
void checkForFlashCommand() {
  if (!Serial) return;  // Early exit if Serial not initialized
  while (Serial.available()) {
    // ... rest unchanged
  }
}
```

### 2. Restore Upstream Serial Initialization Pattern

```cpp
void setup() {
  t1 = millis();

  // Only start serial if USB connected
  pinMode(UART0_RXD, INPUT);
  if (isUsbConnected()) {
    Serial.begin(115200);
    // Wait up to 3 seconds for Serial to be ready to catch early logs
    unsigned long start = millis();
    while (!Serial && (millis() - start) < 3000) {
      delay(10);
    }
  }
  // ... rest of setup
}
```

### 3. Remove `Serial.setTxTimeoutMs(0)`

This call was removed entirely as it's not present in upstream and may cause issues.

### 4. Remove Unnecessary `if (Serial)` Guards

The 15 `if (Serial)` guards added to `EpubReaderActivity.cpp` were removed. `Serial.printf()` is safe to call when Serial isn't initialized (it simply returns 0), so guards around output calls are unnecessary.

**Key distinction:**
- `Serial.printf()` / `Serial.println()` - Safe without guards (no-op when not initialized)
- `Serial.available()` / `Serial.read()` - **MUST** be guarded (undefined behavior when not initialized)

---

## Files Changed

| File | Change |
|------|--------|
| `src/main.cpp` | Removed `Serial.setTxTimeoutMs(0)`, restored `while (!Serial)` wait, added guard to `checkForFlashCommand()` |
| `src/activities/reader/EpubReaderActivity.cpp` | Removed all 15 `if (Serial)` guards |

---

## Testing Checklist

After applying fixes, verify:

1. ✅ Boot with USB connected, serial monitor open - should work
2. ✅ Boot with USB connected, NO serial monitor - should work (3s delay then continue)
3. ✅ Boot without USB - should work immediately (no blocking)
4. ✅ Sleep without USB, plug in USB during sleep, wake - should work
5. ✅ Sleep with USB, unplug during sleep, wake - should work

---

## Lessons Learned

1. **Always guard Serial input operations**: `Serial.available()` and `Serial.read()` must be guarded with `if (Serial)` or `if (!Serial) return` when Serial initialization is conditional.

2. **Serial output is safe without guards**: `Serial.printf()` and similar output functions are safe to call even when Serial is not initialized - they simply return 0.

3. **Don't remove initialization waits without understanding why they exist**: The `while (!Serial)` wait loop exists for proper USB CDC initialization and shouldn't be removed without careful testing.

4. **Upstream patterns exist for a reason**: When diverging from upstream behavior, especially around low-level hardware initialization, be extra cautious and test thoroughly.