user48846: I am trying to power an ESP8266 with a 3.3V regulator (this one, can supply ~ 1A). I've been struggling for days to get the ESP to work – it will occasionally boot, occasionally connect, but then not work again for hours. What I mean by "not working" is that it will often have trouble communicating over UART; or, upload my code, but not run it; or, run my code and crash on an Exception 0.
Now I have noticed that my voltage rail is dipping severely, sometimes below 3V. If I press my reset button, the level returns to 3.3V, but then when the ESP "boots" (but often not running any code, or crashing), it dips back to 2.9V. It is also fine while programming.
Does this suggest my ESP is simply fried? It's possible I briefly misconnected a wire when I first got the unit.

user48846: It was a power issue. Everyone tells you the ESP8266 needs a lot of current, and I didn't believe them.
In case anyone else runs in to this issue, I was powering the ESP via a 3.3V regulator (fine), being fed from and Arduino's 5V (fine). However: I was plugging my Arduino directly into my MacBook. It seems that my Mac was limiting USB power. Using a powered USB hub (max current ~2A) worked fine, but you could probably also get away with directly powering VIN on the Arduino (or a standalone 3.3V supply).

user47766: The ESP8266 will do an analog calibration when it is running the user code. This will typically be the worst case power requirement. The peak current requirement can be as much as 1/2 an amp. While your regulator probably can supply that I would bet your interconnecting wiring, and maybe the power source, are causing too much voltage drop. 
The fact that you can see the voltage go down to 2.9 volts makes me thin that you have serious problems.
Early on when I started with the ESP8266 I was getting flaky operation. I put it down to bad code. But eventually I ruled that out. I took my setup to work (where I design electronic products) and looked at the power connections with a digital scope at the module. Right at the last point before the traces run under the shield. (I use the ESP-12 modules) It dipped just under 3.0 volts for two microseconds, and that was enough to cause a reset. 
These things are not tolerant to any power dips. And this is why I recommend anyone starting with the ESP8266 to buy a Nodemcu or a Wemos type module. Something that has the regulator right at the processor. 

48846: <p>I am trying to power an ESP8266 with a 3.3V regulator (<a href="https://www.mouser.com/datasheet/2/389/ld1117a-974076.pdf" rel="nofollow noreferrer">this one</a>, can supply ~ 1A). I've been struggling for days to get the ESP to work – it will occasionally boot, occasionally connect, but then not work again for hours. What I mean by "not working" is that it will often have trouble communicating over UART; or, upload my code, but not run it; or, run my code and crash on an Exception 0.</p>

<p>Now I have noticed that my voltage rail is dipping severely, sometimes below 3V. If I press my reset button, the level returns to 3.3V, but then when the ESP "boots" (but often not running any code, or crashing), it dips back to 2.9V. It is also fine while programming.</p>

<p>Does this suggest my ESP is simply fried? It's possible I briefly misconnected a wire when I first got the unit.</p>
 48846: <p>It was a power issue. Everyone tells you the ESP8266 needs a lot of current, and I didn't believe them.</p>

<p>In case anyone else runs in to this issue, I was powering the ESP via a 3.3V regulator (fine), being fed from and Arduino's 5V (fine). <strong>However:</strong> I was plugging my Arduino directly into my MacBook. It seems that my Mac was limiting USB power. Using a powered USB hub (max current ~2A) worked fine, but you could probably also get away with directly powering VIN on the Arduino (or a standalone 3.3V supply).</p>
 47766: <p>The ESP8266 will do an analog calibration when it is running the user code. This will typically be the worst case power requirement. The peak current requirement can be as much as 1/2 an amp. While your regulator probably can supply that I would bet your interconnecting wiring, and maybe the power source, are causing too much voltage drop. </p>

<p>The fact that you can see the voltage go down to 2.9 volts makes me thin that you have serious problems.</p>

<p>Early on when I started with the ESP8266 I was getting flaky operation. I put it down to bad code. But eventually I ruled that out. I took my setup to work (where I design electronic products) and looked at the power connections with a digital scope <strong>at the module</strong>. Right at the last point before the traces run under the shield. (I use the ESP-12 modules) It dipped just under 3.0 volts for two microseconds, and that was enough to cause a reset. </p>

<p>These things are <strong>not tolerant</strong> to any power dips. And this is why I recommend anyone starting with the ESP8266 to buy a Nodemcu or a Wemos type module. Something that has the regulator right at the processor. </p>
