External port expander module

[falkoschindler]

To increase the number of available pins and to simplify wiring, we've been thinking about creating a bus system for external port expanders. Here is a short sketch of our current idea:

• The existing Expander module remains unchanged.
• We introduce a core.ota(wifi_ssid, wifi_password, ota_url) method for firmware updates "over the air".
• After an OTA the ESP calls the server again to verify that an OTA would work with the new firmware. Otherwise the ESP rolls back.
• We introduce a new ExternalExpander(serial, address) module for communicating with external port expanders, which is very similar to the existing Expander module.
• When flashing the external ESP, we call a new core.store(key, value) command to write its address (key __address) to the NVS.
• We introduce an ExternalExpander.ota(...) method which calls core.ota(...) on the external ESP using its address (see syntax below).

Usage example:

serial = Serial(26, 27, 11500, 1)
expander = Expander(serial, 32, 33)
led = expander.Output(15)
led.on()

serial2 = Serial(42, 43, 11500, 2)
external = ExternalExpander(serial2, 0x3)
led = external.Output(15)
led.on()

Syntax for inter-ESP communication with leading two-digit hex address

!00core.print(42)@f0
!01core.print(1 + 1)@87
...
!0Fcore.print("foo")@09

[rodja]

We should try to make the address-schema and baud rates compatible with modbus.

[Johannes-Thiel]

Modbus messages will start with a colon : with this difference, there should be no interference with our proposed protocol.

[JensOgorek]

I created a pull request (#44) containing the OTA functionality via HTTPS OTA update. It is controlled by the core module with core.ota(wifi_ssid, wifi_password, ota_url) . There is also an example of an HTTP server receiving the request and providing the binary.