ESP01 nodemcu d1-mini GPIO  boot

        D0     D0     16 (no intr, internal-pull-down-not-up)
        D1     D1      5                    i2c-sclk
        D2     D2      4                    i2c-sda
  ✓     D3(R)  D3(R)   0 up:RUN down:FLASH
  ✓     D4(R)  D4(R)   2     up             uart1-tx                   i2so-ws
        D5     D5     14                                      spi-sclk i2si-ws
        D6     D6     12                                      spi-miso i2si-data
        D7     D7     13                    uart0-swapped-rx  spi-mosi i2si-bck
 (R)    D8(R)  D8(R)  15    down            uart0-swapped-tx  spi-ss   i2so-bck
  ✓     D9     RX      3                    uart0-rx                   i2so-data
  ✓     D10    TX      1                    uart0-tx
                          ^^ boot ^^
d1_mini: R=12k, LED on D4/2       - pdf
nodemcu: R=12k, LED on D4/2+D0/16 - pdf

ESP01:
    ,---------------------------.
    | 1/TX    GND   .----.---.  |   /u: must be up at boot time
    | EN/u    2/u   `----'.--'  |   0:  down at boot time => flash mode
    | /RST/u  0/u    #  # `--.  |
    | 3.3v   3/RX    #  #  --'  |
    `---------------------------'