ESP-01 / ESP-01S (ESP8266)

Fig. 1. ESP-01 / ESP-01S (ESP8266), 2x4 2.54 mm, 24.8 x 14.3 mm. Hover or tap a signal to light up every pin that carries it.

• CH_PD must be pulled HIGH for the chip to run at all. A fresh ESP-01 wired up with CH_PD left floating does nothing, and this is the single most common reason one appears dead.
• Only GPIO0 and GPIO2 are brought out as general I/O, and both are strapping pins. GPIO0 low at boot enters flash mode; GPIO2 must be high at boot. That leaves very little room for a pull-down on either.
• No on-board regulator and no 5 V tolerance anywhere. Feed it a clean 3.3 V that can deliver 300 mA peaks. The 3.3 V pin on a USB-serial adapter usually cannot, and the symptom is random resets during WiFi association.
• TX and RX are 3.3 V. The module's RX is not 5 V tolerant, so a 5 V Arduino needs a divider on that line.
• No ADC pin is brought out, and neither is GPIO15 or GPIO16, so deep-sleep wake is not possible without soldering to the chip.
• The ESP-01S has 1 MB flash and moves the on-board LED to GPIO2. The original blue ESP-01 has 512 KB and its LED on the TX pin.
• The 2x4 header is on 2.54 mm pitch but the two rows are only 0.1 inch apart, so it does not straddle a breadboard center channel. Use an adapter or jumpers.

Table 1. Pin assignments.