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I am using a NodeMCU board and have already used 6 digital pins as output for interfacing with the LCD. I intend to use 2 more digital pins, one as input and other as output for interfacing an ultrasonic sensor.

D8, D4 and D3 are the only digital pins remaining. I understand that D8 (GPIO15), D4 (GPIO2) and D3 (GPIO0) pins of the NodeMCU have special boot-controlling uses. So how can I use those pins as generic digital pins. Any help is appreciated.

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    I would recommend to use an I2C LCD display – Juraj Mar 14 '19 at 10:59
  • you can get an I2C LCD controller for well under $1USD on ebay. – dandavis Mar 14 '19 at 16:28
  • I'm aware of this. I was looking for a way to implement D8, D4 and D3 as gpio pins. Thank you. – maheshmt Mar 14 '19 at 17:00
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The boot setup pins have pull-up on board (0, 2) and pull-down (15) to ensure the correct boot. io 0 (D3) is additionally wired to the reset circuit controlled over USB to put the esp8266 into bootloader a.k.a. flashing mode.

io 0 must be HIGH for normal boot, io 2 must not be LOW (can float according to spec).

If you attach a device to this pins, the device must respect the boot state. Some devices will pull-down or pull-up a wired pin. Usually it is not documented. You can't use such devices on boot config pin which requires the other state.

The pull-up and pull-down can influence the attached device or readings from the device. For some elements it can be advantageous.

Best is to use this pins for elements which would need pull-up or pull-down anyway. For example buttons/switches or sensors which behave like buttons or switches.

In my project with Wemos D1 R2 I had signal to relay module on io 0, speaker module (with amplifier) for tone() function on io 2. I could not use a relay or mosfet module on io 2. On io 15 I had a reed relay module control signal, which I didn't want to go HIGH at boot as most esp8266 io pins do.

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  • Thank you for the info. So how can interface an ultrasonic sensor to any of those 3 pins. Obviously both the trig and echo pins change states to high or low. – maheshmt Mar 14 '19 at 8:20
  • only the state at startup matters to esp8266. the distance sensor could be disturbed by the pull-ups or pull-downs. you know when you try it. try io 0 and io 15 first. – Juraj Mar 14 '19 at 8:39
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Not a direct answer to your question, but some other way that solves your problem, is to use a shift register like 74HC595 and control the 6 pins for the LCD with the 74HC595 which only uses 3 (digital GPIO) pins. That saves you 3 pins for others. Actually you can use 8 pins or even 32 when you daisy chain four 74HC595s.

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