Some wifi controllers can also boot like that. In particular ESP8089 chip that shipped with some android tablets circa 2012-2014. Later, Espressif took that ch

Some wifi controllers can also boot like that. In particular ESP8089 chip that shipped with some android tablets circa 2012-2014. Later, Espressif took that chip, modified bootrom to be able to boot from an SPI flash as well, and marketed that variant as "ESP8266". Serial bootloader was kept as a debug/programming interface, and that was inherited to ESP32 and later chips. All of which can boot directly from serial.