The Teensy supports persistent, permanent settings which are stored on an virtualized EEPROM realized on the Flash memory. In the LUCIDAC, this is used to store primarily settings around the digital networking I/O. In contrast, all analog settings, technically refered to as analog configuration or circuit configuration, is intentionally ephermal and lost when the Teensy or LUCIDAC looses power.

As a design decision, a change in permanent settings normally requires a reboot in order to take change. This makes sense for network-related settings which are typically only made use of at startup.

In the code, the settings is implemented by nvmconfig::PersistentSettingsWriter. In particular, there is a small registry at net::register_settings which collects all subsystems which store permanent settings.

Following the general idiom of the JSON-based LUCIDAC communication, also the permanent settings are just a JSON dictionary. In order to save space on the EEPROM area, which is only about 2kB in size, MessagePack can be used for serialization. In any way, the ArduinoJSON library is used the same way as in the JSONL protocol implementation. Therefore, the same conversion rules apply.

String casting rules

In case you want to write a minimal client which can modify settings, you might want to use the simple key-value idiom which assumes all (non-list/object) values to be strings, despite JSON also knows boolean and numbers. The casting is transparently performed by ArduinoJSON.

The following rules are handy to remember:

When trying to set a boolean value, do not pass the strings, as any string will be interpreted as truth value true. That is, "false", "0" and even the empty string "" evaluate to true. However, the numerical value 0 as well as the JSON Null type cast to false.
When trying to set a numeric value, string parsing does work. That is, ýou can pass a "123" and it is properly scanned as 123. This is because ArduinoJSON ships with a sophisticated string to number parser.

The other way around is untypical but anyway reported here: When you pass a number or boolean where a string is expected, the following happens:

numbers will be easily casted into strings, but
any boolean as well as the Null value results in an empty string.

Given these casting rules, it is suggested to do basic preprocessing of strings on the client side to deal with booleans.