Why does the LCD impede the ESP8266 NodeMCU initialization?

Question

I have a very simple setup on a breadboard that connects a Lolin v3 NodeMCU ESP8266 with a 16x02 LCD. No I2C communication, I'm doing with directly with Arduino IDE and the LiquidCrystal library.

For some reason when I disconnect the LCD Vdd and connect it after the MCU has booted everything works fine. My power supply indicates it draws ~70mA during boot and then lowers the current. However if I leave the LCD connected and turn on the power supply, the MCU won't boot and the circuit is stuck drawing 40mA with the LCD showing blocks on the first line at 50% contrast.

How can I fix this?

Here's how I setup my wiring:

And my code:

#include <LiquidCrystal.h>

LiquidCrystal lcd(D8, D7, D6, D5, D4, D3);

void setup(void) {
  lcd.begin(16, 2);
}

void loop(void) {
  lcd.setCursor(0, 0);
  lcd.print("hello, world!");
  delay(10000);
}

Answer 1

Certains pins of the ESP are boot mode selection straps, and are internally weakly pulled high or low to boot normally, but these can be overridden externally to select other boot modes (to load firmware etc).

The LCD data, R/W and RS pins all have internal weak pull-ups to LCD supply voltage. These weak pull-ups of the LCD module override the weak pull-downs of the ESP, and select a boot mode where it does not boot normally.

What is even worse, is that the ESP pins are rated for 3.3V IO voltages, and the LCD pull-ups try to pull them to 5V. The current is very small due to the pull-up resistances are generally quite high, but still this should not be done for long-term reliability.

Answer 2

Add diode before VDD of the LCD (Anode in +5V and Cathode in VDD of LCD)