Espressif ESP8266 Technical Reference page 79

!
U0RTS: pin26(u0txd)
As the transceiver feet of UART0,hardware pin13 and pin12 won't print out duing
booting,but be attention to ensure pin13 (mtdo) can not be pulled up by external in
ESP8266 is booting.
Espressif 70 86 ! /!
11. UART Introduction
2016.05