ESP32-S2 USB CDC regression in Arduino-ESP32 3.3.5: Serial prints stop after 1–2 lines

[NGL3324]

Board

ESP32-S2 (Adafruit QT-PY)

Device Description

Board

Adafruit QT Py ESP32-S2

Arduino IDE

Arduino IDE 2.3.7
CLI version 1.3.1

Arduino-ESP32 core versions tested

• 3.3.4 → works correctly
• 3.3.5 → regression

USB configuration

• Native USB CDC
• “USB CDC on boot” enabled

Hardware Configuration

• GPIO 40/41 (I2C SDA1/SCL1) on StemmaQT port connected

Version

v3.3.5

Type

Bug

IDE Name

Arduino IDE 2.3.7

Operating System

Windows 11

Flash frequency

80MHz

PSRAM enabled

yes

Upload speed

921600

Description

Expected behavior

Serial over USB CDC works continuously.
CSV telemetry and command parsing continue indefinitely.

Actual behavior (Arduino-ESP32 3.3.5)

• Serial prints 1–2 lines after boot, then stops completely
• Serial command input (e.g. typing commands) also stops responding
• Board does NOT reset (USB port remains enumerated; no disconnect sound)
• Downgrading core to 3.3.4 immediately restores correct behavior

Notes

• Reproduces with old, previously known-good sketches
• Appears unrelated to application logic changes
• Suspect regression in USB CDC / TinyUSB handling or Serial backend

Minimal reproduction sketch

• not for distribution unfortunately

Sketch

Serial.print(now / 1000.0/60.0, 3);  // xxsecondsXX mins since boot, force floating point division since "now" is an integer
      Serial.print(',');
      Serial.print(Tj_F, 3);
      Serial.print(',');
      Serial.print(Tw_F, 3);
      Serial.print(',');
      Serial.print(Ta_F, 3);
      Serial.print(',');
      Serial.print(Tw_sp_F, 3);
      Serial.print(',');
      Serial.print(duty_pct, 3);
      Serial.print(',');
      Serial.print(I_j, 3);  
      Serial.print(',');
      Serial.print(Delta_FF, 3);
      Serial.print(',');
      if (isnan(dTj_dt)) Serial.println("nan");
      else Serial.println(dTj_dt, 5);

Debug Message

there is none.  Serialjust freezes after 1.5 lines of serial printed to Arduino IDE serial monitor or CoolTerm.  Code compiles fine

Other Steps to Reproduce

No response

I have checked existing issues, online documentation and the Troubleshooting Guide

• I confirm I have checked existing issues, online documentation and Troubleshooting guide.

[NGL3324]

/*
Minimal USB-CDC Serial freeze reproducer (ESP32-S2, Arduino-ESP32 core regression test)

Intended board/settings:
- ESP32-S2 (e.g., Adafruit QT Py ESP32-S2)
- Tools -> USB CDC On Boot: Enabled
- Monitor baud: 115200
- Core versions:
3.3.4: OK
3.3.5: regression suspected (Serial prints stop after 1–2 lines; input stops too)

Prints:
CSV_HEADER,sec,Tj_F,Tw_F,Ta_F,Twsp_F,duty_pct,I_j,Delta_FF,dTj_dt_Fps
then periodic CSV lines mimicking the project's format, including "nan" occasionally.
*/

#include <Arduino.h>
#include <math.h>

static const uint32_t LOG_PERIOD_MS = 200;
static const float Delta_FF = 5.5f;

static uint32_t last_csv_ms = 0;
static float prev_Tj = NAN;

static void handleSerialLine(const String &line) {
if (line.length() == 0) return;

if (line == "help" || line == "HELP") {
Serial.println("OK: commands: help, ping");
Serial.println("This is a minimal USB CDC repro sketch.");
} else if (line == "ping") {
Serial.println("pong");
} else {
Serial.print("unknown: ");
Serial.println(line);
}
}

static void pollSerialCommands() {

static String buf;
while (Serial.available()) {
char c = (char)Serial.read();
if (c == '\r') continue;
if (c == '\n') {
handleSerialLine(buf);
buf = "";
} else if (buf.length() < 128) {
buf += c;
}
}
}

void setup() {
Serial.begin(115200);
delay(200);
uint32_t t0 = millis();
while (!Serial && (millis() - t0) < 2000) {
delay(10);
}

Serial.println(F("CSV_HEADER,sec,Tj_F,Tw_F,Ta_F,Twsp_F,duty_pct,I_j,Delta_FF,dTj_dt_Fps"));
}

void loop() {
pollSerialCommands();

const uint32_t now = millis();
if ((uint32_t)(now - last_csv_ms) >= LOG_PERIOD_MS) {
const float dt_s = (last_csv_ms == 0) ? NAN : ((now - last_csv_ms) / 1000.0f);
last_csv_ms = now;

// Synthetic signals that vary smoothly, but include enough float formatting to stress dtostrf/printf paths.
const float t_s = now / 1000.0f;
const float Tj_F   = 75.0f + 0.6f * sinf(0.02f * t_s) + 0.08f * sinf(0.7f * t_s);
const float Tw_F   = 83.0f + 0.8f * sinf(0.015f * t_s + 0.3f);
const float Ta_F   = 68.0f + 0.2f * sinf(0.005f * t_s);
const float Twsp_F = 80.0f + 0.7f * sinf(0.011f * t_s + 0.5f);

// duty in [0..100], with occasional small values
float duty_cmd = 0.12f + 0.10f * sinf(0.09f * t_s);
if ((now / 5000) % 2 == 0) duty_cmd *= 0.25f;  // every 5 s, shrink duty to make tiny numbers
if (duty_cmd < 0.0f) duty_cmd = 0.0f;
if (duty_cmd > 1.0f) duty_cmd = 1.0f;
const float duty_pct = duty_cmd * 100.0f;

// simple synthetic integrator-like value
static float I_j = 0.0f;
I_j += 0.0005f * (75.0f - Tj_F);

// dTj/dt over the CSV interval
float dTj_dt = NAN;
if (!isnan(prev_Tj) && !isnan(dt_s) && dt_s > 0.0f) {
  dTj_dt = (Tj_F - prev_Tj) / dt_s;
}
prev_Tj = Tj_F;

// Occasionally force dTj_dt to NaN to exercise the "println(nan)" branch
if ((now / 7000) % 3 == 1) dTj_dt = NAN;

// Print CSV line (matches failing structure)
Serial.print(now / 1000.0f / 60.0f, 3);
Serial.print(',');
Serial.print(Tj_F, 3);
Serial.print(',');
Serial.print(Tw_F, 3);
Serial.print(',');
Serial.print(Ta_F, 3);
Serial.print(',');
Serial.print(Twsp_F, 3);
Serial.print(',');
Serial.print(duty_pct, 3);
Serial.print(',');
Serial.print(I_j, 3);
Serial.print(',');
Serial.print(Delta_FF, 3);
Serial.print(',');
if (isnan(dTj_dt)) Serial.println("nan");
else Serial.println(dTj_dt, 5);

}
}