I have a moderately complex circuit with a Teensy, an ESP8266, a key pad, and an LCD display.
Prior to connecting the LCD display, everything was working fine. Once I connected the LCD display (which ran perfect when tested by itself), I was getting backlight flicker. Enough to be quite annoying.
Then I realized the power LED for the 3.3V was flickering slightly too, just nowhere near as noticeable.
Putting a scope on the power supply, sure enough I was seeing short voltage sags. My 3.3V power supply would drop to close to 3.0 volts.
After lots of probing, I found the culprit was the ESP8266. If I held the reset pin high (with a 1000K pullup), no noise. As soon as the ESP8266 fires up, lots of noise.
I have followed the standards of placing caps on the ESP8266 VCC and the power regulator VCC. Made no difference. I proceeded to try about every combination of cap sizes and locations trying to solve the problem.
Searching the interweb didn’t lead to a solution.
I decided to put the ESP8266 on its own dedicated LM3940 regulator. That helped a lot, but there was still some flicker.
After a lot more trial and error I discovered that simply tying the new 3.3V regulator’s GND into the GND bus wasn’t enough. I actually need to place the ESP8266’s ground near the new regulator’s ground as well (though it is still tied to the common ground as well).
In reviewing the problem, having a larger regulator (more current) might have helped, but for reasons I won’t go into here, that would have been difficult. If you are having this problem, I suggest that first.
If that doesn’t resolve the problem, I suspect adding a dedicated regulator for the ESP8266 will work for most people. Any voltage drops caused by the ESP8266 are isolated to it.
If that still doesn’t help, then try connecting the regulator’s ground near to the ESP8266. I suspect I needed this additional step only because of my particular project, but maybe others will find they need it too.
As of this fix, the main power supply is clean and the LCD is flicker free.
Sep 2015 Update:
Still had some problems with this. Though 2 power supplies worked at first, then they stopped. I’ve ended up using one power supply and the ESP8266 has both Vcc and ground connected close to the regulator, and everything else needing 3.3V is further downstream. So far so good.
My project also has an audio component, and I’ve noticed some clicking in the background which goes away if I hold the ESP8266’s reset line low. A 470uF cap on the source power made it better, but it is still there.