At the heart of the ESP-01 module is the ESP8266 chip with at least two boot modes — the normal mode and the programming/flash mode. The latter refers to uploading any software (or firmware) to the ...

Arguably, the problems started when [Miguel] picked up an ESP-01 module from a local electronics ... inadvertently putting the ESP8266 into programming mode, since it was holding the pin LOW ...

and the correct programming mode must be selected for a given hardware setup. These modes are known as DIO and QIO, meaning “dual IO” and “quad IO” respectively. This refers to the number ...