I recently ordered a set of 10 new ESP-01’s from China. After the usual few weeks, they arrived. I pulled one out, plugged it into the socket on my little interface board – which has buttons for RST and GPIO0, a 3.3V regulator, and pins to hook my USB-to-TTL cable to. That little board makes it really easy to program these minimal-feature ESP8266 boards.
So, I go through my process of setting the board options in the Arduino IDE, clicking “Program”, and then holding down the GPIO0 button, while momentarily pressing RST. I immediately noticed that I didn’t see the usual short flash of the LED (which is connect to the TX pin on ESP-01 boards). It then failed to write to flash. After some experimenting, I could get it to write to flash by holding GPIO0 down while plugging the USB cable in. However, my sketch didn’t seem to execute. I hit RST again and nothing happened. Reset was definitely not doing anything, and no matter what I did, I couldn’t get my sketch to run (I knew it worked, because I could flash it to an older ESP-01 and it ran fine).
So finally, after googling abit, I discovered that people are sometimes now receiving “V3” versions of the ESP-01. This seems to vary a bit between different vendors – some still ship V2’s, some send V3’s. And the difference on the V3 is that there is now a 6K resistor inline with the pin on the ESP-01 and the RST pin on the actual chip. This seems to prevent RST from working and prevents the chip from entering run mode on power-up. The post I found included the following picture illustrating where the resistor is. With some relatively fine soldering skills, you can remove the resistor and solder a fine jumper wire in place, which solves the problem. DON’T FORGET that you also have to pull CH_PD HIGH for the chip to enter run mode – I usually solder another jumper directly onto the ESP-01 to do this, except in cases where I what to have control over the chip entering low-power mode.
This is also documented on the ESP-10 wiki here: https://www.letscontrolit.com/wiki/index.php/ESP-01#ESP-01_v3