MiPyAlpaca MicroPython Alpaca Driver

Introduction

MiPyAlpaca is an ASCOM Alpaca implementation for MicroPython. Currently it’s supporting only the device type “Switch” but can be extended to other devices (I have planned to add “CoverCalibrator” soon). It also supports the Alpaca discovery protocol. I have tested it on Raspberry Pi Pico W and ESP32-C3.

It can be used to make microcontroller-based projects accessible by astronomy software applications (like e.g. N.I.N.A.) or self-written applications via the standardized and operating system independent ASCOM Alpaca protocol.

Simple controlling of input/output pins, setting of PWM values and reading of ADC values can be done even without coding. As the provided switchExample1 demonstrates, you just have to edit a JSON configuration file. Configure the pin functionality, start the software and connect your application.

[
{"switchnr": 0,"name": "GPIO OUT","pincfg": {"pin": 15,"pinfct": "OUTP","initval": 1},"canwrite": true,"min": 0,"max": 1,"step": 1,"swfct": "MiPyPin","descr": "Output test pin"},
{"switchnr": 1,"name": "GPIO In","pincfg": {"pin": 16,"pinfct": "INP","pull": "PULL_DOWN"},"canwrite": false,"min": 0,"max": 1,"step": 1,"swfct": "MiPyPin","descr": "Input test pin"},
{"switchnr": 2,"name": "PWM test","pincfg": {"pin": 14,"pinfct": "PWM","initval": 19000,"freq": 1000},"canwrite": true,"min": 0,"max": 65535,"step": 1,"swfct": "MiPyPin","descr": "PWM test pin"},
{"switchnr": 3,"name": "ADC test","pincfg": {"pin": 28,"pinfct": "ADC"},"canwrite": false,"min": 0,"max": 65535,"step": 1,"swfct": "MiPyPin","descr": "ADC test pin"}
]

For the configuration shown above, N.I.N.A will display the following switch device (will be automatically adapted to your pin configuration):

Now you can set the output pins, read input pins, set PWM outputs and read ADC inputs from N.I.N.A..

Here is a simple breadboard setup testing the example above on a Raspberry Pico W. You can switch on/off one LED with GPIO output pin, read the state of the button, set the brightness of another LED via PWM and read the voltage at the trim potentiometer.

If you want to control more sophisticated “Switches”, you can derive a subclass of MiPySwitchDevice and just implement the required functionality in the get and set methods to read and write the switch values. See switchExample2 for details.

Installation

Copy the files from the GitHub repository on your device. Folders mipyalpaca and templates shall be subfolders of the folder containing the main application (like e.g. the example files).

MiPyAlpaca requires two additional packages:

• Microdot (https://github.com/miguelgrinberg/microdot)

• utemplate https://github.com/pfalcon/utemplate

For installation of these packages see the corresponding documentation. If you use the Thonny IDE (https://thonny.org/), you can also use the Tools/Manage packages function. In Thonny, the file system of my installation looks like that:

Usage

• The recommended starting point is to use one of the provided examples.

• Edit WLAN credentials in wlancred.py

• Assign a new Globally Unique ID (UID) for your device in the call argument of installDevice (not absolutely necessary, but recommended)

• If you want to use another discovery port than 32227 (default), or another port for the Alpaca server, edit the port numbers in servercfg.json.

• Edit switch0.json for your switch configuration (see below for details)

• Start the example program

Switch configuration

Attribute	Description
switchnr	Switch ID
name	Switch name
canwrite	true for writable switch false for read-only switch
min	Minumum value
max	Maximum value
step	Value step size
swfct	MiPyPin for MicroPython pin (see table below) UserDef  for user defined switch function
descr	switch description

If "swfct" has value "MiPyPin", attribute "pincfg" has to be defined with the following sub-attributes:

Attribute	Description	Remarks
pinfct	OUTP: Output pin INP: Input pin ADC: ADC input pin PWM: PWM output pin
initval	Initialisation value	for OUTP and PWM only optional
pull	PULL_UP: Pull up PULL_DOWN: Pull down	for INP only

If "swfct" has value "Userdef", create a new subclass derived from class SwitchDevice and overwrite the following methods with the required code:

• getswitchvalue

• getswitch

• setswitchvalue

• setswitch

Alpaca Management API

Main Setup Page

The Alpaca server main setup page can be called by

http://host:port/setup

where host is usually the ip address of your device.

Device Specific Setup Page(s)

The device specific setup pages can be called by

http://host:port/setup/vapiversion_number/device_type/device_number/setup

As this implementation currently supports only API version v1 and device type "switch", the correct URL for switch device 0 is

http://host:port/setup/api/v1/switch/0/setup

Examples

The following example applications are provided:

switchExample1

This is an example with 4 switches, each for one type of the available MicroPython pins:

• 1 Input pin

• 1 Output pin

• 1 ADC pin

• 1 PWM pin

The example was already described in the introduction above.

switchExample2

Example with the 4 switches from switchExample1 and 1 additional userdefined switch

The user defined switch is a read-only "switch" providing the temperature values of a DS18B20 temperature sensor.

This example code demonstrates (this is the additional code) how to implement support of a DS18B20 temperature sensor which can be read by the astronomy software:

class ExampleSwitchDevice(MiPySwitchDevice):
    def __init__(self, devnr, devname, uniqueid, config_file):
        super().__init__(devnr, devname, uniqueid, config_file) # get switch value

    def getswitchvalue(self, id):
        global curr_temp

        if id == 4:
            return curr_temp   # return current temperature for switch ID 4
        else:
            return super().getswitchvalue(id)

The temparature sensor is read cyclically in an Asyncio coroutine appGetTemp, started in parallel to the Alpaca server:

async def appGetTemp():
    global curr_temp
    while True:
        sensor_ds.convert_temp()                     # start temperature measurement
        await uasyncio.sleep_ms(1000)                # wait (at least 750ms)
        curr_temp = sensor_ds.read_temp(devices[0])  # read and store latest temperature
        await uasyncio.sleep_ms(2000)                # 2s until start of next measurement
    
    
# Asyncio coroutine
async def main():
    uasyncio.create_task(appGetTemp())   
    await AlpacaServer.startServer()

Here is the switch window of N.I.N.A. for switchExample2 with the temperature gauge:

This is a sample breadboard setup for this example:

ASCOM Alpaca compliance

MiPyAlpaca was successfully validated by the Conform Universal ConformU conformance validation tool to be Alpaca compliant.

Acknowledgements

Many thanks to Miguel Grinberg for providing his Microdot framework. He has also provided excellent and fast support to my questions and remarks.

Also many thanks to Peter Simpson from the ASCOM Driver and Application Development Support Forum for answering all my questions regarding the Alpaca protocol.