The Cardputer documentation describes a “Download Mode” which is not clear what it’s for.
Is Download Mode used for uLisp operation? I wonder whether the mode needs to be activated, say, each time the Cardputer is connected via USB such as for accessing it with the Serial Monitor.
As far as I know, this isn’t anything specific to the Cardputer. If pin 0 (ie the G0 button on the Cardputer) is LOW when the ESP32 boots it will not run the code in the flash, instead it will wait for a firmware update via serial port. Most boards (including mine) have a couple of transistors that are able to activate download mode automatically, because esptool toggles the DTR and RTS lines in a special way when it is trying to download a new program. However they all still have a button on pin 0 so you can activate it manually if something doesn’t work.
If a firmware update of the Cardputer doesn’t proceed automatically when activated (e.g. when installing uLisp), I’ll try pressing the G0 button.