ESP-01 / ESP-01S “Survival Guide” (ESP8266) — Wiring, Boot Modes, Flashing, and Real-World Gotchas

The ESP-01 (and ESP-01S) is the smallest, most annoying, and most misunderstood ESP8266 module. It can be incredibly useful (Wi-Fi bridge, tiny sensor node, UART→Wi-Fi device), but only if you wire it correctly and respect its boot pins.

This guide is the “no excuses” reference: pinout, power, boot straps, flashing, and cookbook examples.

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1) ESP-01 vs ESP-01S (what changed)

• ESP-01: usually exposes 2 GPIO (GPIO0 and GPIO2).
• ESP-01S: often exposes more stable components and sometimes an extra GPIO (GPIO1/3 are UART, still not “free”), but functionally you should treat it the same: GPIO0 + GPIO2 are your real I/O.

Both are ESP8266 modules and behave the same for boot and flashing.

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2) ESP-01 pinout (8 pins, and every one matters)

Typical ESP-01 header (top view, antenna at the top, pins down):

Pin	Label (common)	What it is	Notes
1	GND	Ground	Must be solid ground
2	GPIO2	I/O + boot strap	Must be HIGH at boot
3	GPIO0	I/O + boot strap	LOW at boot = flashing mode
4	RX	UART0 RX	Used for flashing and serial
5	TX	UART0 TX	Used for flashing and serial
6	CH_PD / EN	Chip enable	Must be HIGH (3.3V) to run
7	RST	Reset	Active LOW
8	VCC	3.3V power	3.3V only

3) Power: the #1 reason ESP-01 “doesn’t work”

3.1 Minimum power requirements (real life)

ESP8266 pulls high current spikes during Wi-Fi TX. If your 3.3V supply is weak, you’ll see:

• random resets
• boot loops
• flaky Wi-Fi
• “works on USB, fails on battery” behaviour

Do this:

• Use a proper 3.3V regulator rated at 500 mA (or at least 300 mA with good caps).
• Add caps close to the module:
  • 100 µF electrolytic (or tantalum) across VCC↔GND
  • 0.1 µF ceramic across VCC↔GND (as close as possible)

3.2 Don’t do this

• Don’t power ESP-01 from a random FTDI 3.3V pin unless you know it can deliver burst current.
• Don’t use a cheap AMS1117 board without caps and expect stability (sometimes it works, often it doesn’t).

4) Boot modes (strap pins) — the rules that decide everything

ESP8266 checks three strap pins at reset:

• GPIO0
• GPIO2
• GPIO15 (not exposed on ESP-01 header, but it exists internally on the module)

Normal boot (run your sketch)

You must have at reset:

• GPIO0 = HIGH
• GPIO2 = HIGH
• GPIO15 = LOW (handled internally on ESP-01, but don’t fight it)

Flashing mode (UART download mode)

You must have at reset:

• GPIO0 = LOW
• GPIO2 = HIGH
• GPIO15 = LOW

Practical meaning:

• GPIO0 decides “normal vs flashing”
• GPIO2 must be HIGH at boot, so don’t connect it to something that drags it LOW during reset (like an LED to GND without thinking).

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5) “Minimum stable wiring” (ESP-01 that boots every time)

This is the wiring you want for a stable ESP-01 setup:

Required connections

• VCC → 3.3V
• GND → GND
• EN/CH_PD → 3.3V (pull-up)
• RST → 3.3V via pull-up (optional but recommended)

Recommended pull-ups (especially on bare modules)

• EN/CH_PD: pull up to 3.3V (e.g., 10k)
• RST: pull up to 3.3V (e.g., 10k)
• GPIO0: pull up to 3.3V (e.g., 10k) so it boots normally by default
• GPIO2: pull up to 3.3V (e.g., 10k) so it boots reliably

Many ESP-01 boards already have some of these resistors. Many clones don’t. If your ESP-01 is flaky, add them.

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6) Flashing ESP-01 (wiring to USB-Serial adapter)

6.1 USB-Serial requirements

• Must support 3.3V logic (TX/RX at 3.3V).
• 5V TX will kill ESP8266 RX (or degrade it).

6.2 Wiring (USB-Serial ↔ ESP-01)

USB-Serial	ESP-01
TX	RX
RX	TX
GND	GND
3.3V (if strong enough)	VCC (only if your adapter can supply enough current)

If your adapter power is weak: use an external 3.3V regulator for VCC and keep common GND.

6.3 Enter flashing mode

1. Make sure GPIO2 is HIGH
2. Pull GPIO0 LOW
3. Reset the module (toggle RST LOW then HIGH, or power-cycle)
4. Upload firmware

After flashing:

• Release GPIO0 back to HIGH
• Reset again to run normally

6.4 Common upload errors (and what they usually mean)

• “Failed to connect” / “Timed out”
  → GPIO0 wasn’t LOW at reset, wiring RX/TX swapped, wrong COM port, bad power.
• Garbage serial output
  → wrong baud rate or unstable power.

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7) Using GPIO0 and GPIO2 safely (because they’re also boot pins)

You only have two real GPIO and they’re both boot-sensitive. So you must design external circuits that don’t mess up boot levels.

Safe-ish uses

• GPIO2: onboard LED (if wired correctly), reading a sensor input, I²C SDA/SCL (yes, possible but tight)
• GPIO0: a button input (to GND) if you understand it can force flashing mode if held at boot

Rule: avoid circuits that force LOW at boot

• If you connect an LED from GPIO2 to GND, it may pull the pin LOW at boot (depending on resistor/value and leakage) → boot failure.
• Better: connect LED from 3.3V to GPIO2 so the pin sinks current when LOW (inverted logic) — and you still keep boot HIGH default.

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8) Cookbook: quick Arduino examples (ESP8266 core)

Assume Arduino IDE with ESP8266 board package installed.
Select board: “Generic ESP8266 Module” (common for ESP-01) and set:

• Flash size: match your module (often 1M or 4M)
• Upload speed: 115200 (start conservative)

Recipe 0: Serial “hello” (always start here)

void setup() {
  Serial.begin(115200);
  delay(200);
  Serial.println();
  Serial.println("ESP-01 serial OK");
}

void loop() {
  delay(1000);
  Serial.println("tick");
}

Recipe 1: Blink LED on GPIO2 (with note about wiring)

const int LED = 2; // GPIO2

void setup() {
  pinMode(LED, OUTPUT);
}

void loop() {
  digitalWrite(LED, HIGH);
  delay(300);
  digitalWrite(LED, LOW);
  delay(300);
}

If your LED is wired 3.3V → resistor → GPIO2, the logic is inverted (LOW = ON).

Recipe 2: Wi-Fi connect + print IP

#include <ESP8266WiFi.h>

const char* SSID = "YOUR_SSID";
const char* PASS = "YOUR_PASS";

void setup() {
  Serial.begin(115200);
  delay(200);

  WiFi.begin(SSID, PASS);
  Serial.print("Connecting");

  for (int i = 0; i < 40 && WiFi.status() != WL_CONNECTED; i++) {
    delay(250);
    Serial.print(".");
  }
  Serial.println();

  if (WiFi.status() == WL_CONNECTED) {
    Serial.print("IP: ");
    Serial.println(WiFi.localIP());
  } else {
    Serial.println("WiFi failed (power or credentials)");
  }
}

void loop() {}

Recipe 3: Minimal web server (useful ESP-01 demo)

#include <ESP8266WiFi.h>
#include <ESP8266WebServer.h>

const char* SSID = "YOUR_SSID";
const char* PASS = "YOUR_PASS";
ESP8266WebServer server(80);

void setup() {
  Serial.begin(115200);
  delay(200);

  WiFi.begin(SSID, PASS);
  while (WiFi.status() != WL_CONNECTED) { delay(300); Serial.print("."); }
  Serial.println();
  Serial.println(WiFi.localIP());

  server.on("/", []() {
    server.send(200, "text/plain", "ESP-01 says hi");
  });

  server.begin();
}

void loop() {
  server.handleClient();
}

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9) ESP-01 as a “Wi-Fi modem” (AT firmware use case)

A huge amount of ESP-01 usage is still:

• leave Espressif AT firmware on it
• connect it to another MCU via UART
• use AT commands to join Wi-Fi / TCP / HTTP

If you go this route:

• UART wiring and stable power matter more than anything
• Use 115200 or 9600 depending on the AT build
• Don’t share the UART with other chatty debug output

(If you want, I can write a dedicated “ESP-01 AT firmware + examples” article too.)

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10) Troubleshooting checklist (fast)

“It doesn’t boot”

• EN/CH_PD must be HIGH
• GPIO2 must be HIGH at reset
• GPIO0 must be HIGH at reset (unless flashing)
• Power must be stable (add caps)

“It flashes but won’t run”

• You forgot to release GPIO0 back HIGH after flashing
• GPIO2 is being pulled LOW by your circuit
• Brownout/weak supply during Wi-Fi start

“It connects to Wi-Fi then resets”

• Power supply or regulator can’t handle burst current
• Add 100 µF + 0.1 µF near VCC/GND
• Try powering from a real 3.3V regulator, not USB-serial 3.3V pin