222 lines
9.1 KiB
Markdown
222 lines
9.1 KiB
Markdown
# Mongoose OS Huzzah32 Featherwing
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## Introduction
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This project is a demonstration of the capabilities of Mongoose OS. It is
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build around two popular pieces of hardware, both available from Adafruit:
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* [Huzzah32 ESP32 Feather](https://learn.adafruit.com/adafruit-huzzah32-esp32-feather/)
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* [2.4" TFT/TouchScreen Featherwing](https://learn.adafruit.com/adafruit-2-4-tft-touch-screen-featherwing/)
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[](https://www.youtube.com/watch?v=d9CZ4VMgTGI)
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## Hardware: Moving Parts
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The TFT Featherwing features three components:
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* ILI9341 2.4" TFT screen driven by SPI
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* STMPE610 Resistive touchscreen driven by SPI
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* MicroSD storage device driven by SPI
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The Huzzah32 plugs right into the TFT Featherwing for a compact device
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without the need for breadboards, dupont wires and the like. It's really
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a great platform to showcase the power of Mongoose OS.
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### Hardware: Observations
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The Huzzah32 uses its SPI bus to communicate with the touch sensor and the
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TFT screen. Its `MOSI`, `MISO` and `SCLK` pins are shared with the other
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devices, and it selects which slave device to communicate with by means of
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three `CS` pins.
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Peculiarities of the hardware setup:
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* The TFT driver chip, ILI9341, has an additional pin called `DC`,
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which it uses to receive blobs of data from the microcontroller.
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* The Touchscreen driver chip, STMPE610, has an additional pin called
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`IRQ`, which it pulls low when a touch event has registered. The
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chip buffers the last 128 touch events, and the microcontroller,
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upon receipt of the interrupt, can read them. That means no polling
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or busy waiting!
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* The TFT Featherwing has an additional pin called `LITE`, on which it
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accepts a 10KHz PWM signal. Setting the duty cycle to 0% turns off the
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backlight entirely, setting it to 100% turns on the backlight, and
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values in between partially dim the backlight.
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* The Huzzah32 has a built in 3.7V LiPo, and charges it when the device
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is connected to USB. Adafruit have helpfully connected the battery
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output to an ADC pin (A13 / GPIO35) using a 1:1 voltage divider (so a
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full LiPo battery at 4.2V will read out at 2.1V on the ADC channel.
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#### Soldering Requirements
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The `LITE` and `IRQ` pads on the TFT Featherwing have to be soldered to
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connect them to the Huzzah32:
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* Solder the `IRQ` pad to pin 23, the top left pad.
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* Solder the `LITE` pad to pin 22, the second from the top left pad.
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Here's a picture to help you find your bearings:
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### General Design
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To showcase the idiomatic use of Mongoose OS, we will to do the following:
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1. Write drivers for the ILI9341 and STMPE610 chips. We've taken the native
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ESP32 SPI driver and checked it in to `libs/lobo-spi/`. There's an
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implementation of STMPE610 driver in `libs/stmpe610/`, and finally, there's
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an implementation of ILI9341 in `libs/ili9341/`.
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1. Install an interrupt handler for the touch screen events.
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1. Install a PWM driver for the backlight.
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1. Install an ADC reader on GPIO35.
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1. Create a UI container which can display `widgets`, both provided in C code,
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such as `src/widget_*.c`, but also provided by users in a JSON
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configuration.
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1. Store the JSON and image data on the provided `SPIFFS` filesystem in `fs/`.
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1. Interact with the user by performing actions on the `widgets`.
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1. Report on system statistics using Prometheus.
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## User Interface
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### Anatomy of the UI
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There are two main components: `widgets` and `screens`.
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#### Widgets
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A `widget` is an object that describes a user interface element (an image, a
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button, or a system `widget` with specific implementation behavior). System
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`widgets` can have timers associated with them, as such they can perform
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regular callbacks to redraw themselves. They are initialized with a few
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variables, notably their (x,y) coordinates and width and height. They also
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have a name and, optionally a `user_data` blob can be attached to them.
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##### Time widget
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This `widget` exposes `widget_time_ev()` which gets called every second, and
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displays the current NTP time.
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##### Battery widget
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This `widget` exposes `widget_battery_ev()` which gets called every now and
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again, measures the current voltage of the attached LiPo, and draws a battery
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icon in green (full), yellow (half full), or red (empty).
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##### WiFi widget
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This `widget` exposes `widget_wifi_ev()` which gets called every 5 seconds, and
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it retrieves the current WiFi signal strength (RSSI), mapping it to a value
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between 0 and 100%, and draws a WiFi icon in white.
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##### Network widget
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This `widget` exposes `widget_network_ev()` which gets draws two arrows, one
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pointed up (for Send traffic), and one pointed down (for Recv traffic). It
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has two helper functions: `widget_network_send()` and `widget_network_recv()`
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which redraw the arrows in yellow, setting a 100ms timer that will redraw the
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arrows in grey again.
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Users can call the send and recv functions to show network activity.
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##### Name widget
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This `widget` exposes `widget_name_ev()` which prints a string based on some
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local state it keeps. Its event handler implements `TOUCH_DOWN` and `TOUCH_UP`
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Which cycles between the `app.hostname` system configuration string (see
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`mos.yml` for its defintion), the IP address, the associated WiFi SSID, and
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the `screen` name (see below).
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#### Screens
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A `screen` is an object that holds the `widgets`. The API for screens is
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meant to be simplistic: both to be able to fit in the available compute
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resources of smaller micro controllers, but also to show readers how these
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things work without bogging them down in overly complex code to wade through.
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Users typically create a screen by:
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```c
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struct screen_t *screen;
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struct widget_t *w;
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screen=screen_create_from_file("/screen_home.json", widget_default_ev, NULL);
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// Add a custom widget
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w = widget_create("time", 240, 0, 80, 20);
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widget_set_handler(w, widget_time_ev, NULL);
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widget_set_timer(w, 1000);
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screen_widget_add(screen, w);
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```
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#### Touch Handler
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The main app installs the interrupt handler for STMPE610, which is where all
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the action is. When users interact with the device, the interrupt handler
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calls a callback with an event number and additional data (such as the
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(x,y) coordinates, pressure, direction of the touch (`TOUCH_DOWN` and `TOUCH_UP`)
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and duration of the touch.
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The handler `touch_handler()` then looks if any `widgets` are covering the
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(x,y) coordinates in the current `screen`, and if so, passes the event to a
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hander for the `widget` (in our example above, `widget_default_ev` for
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`widgets` we read from the JSON file, and `widget_time_ev` for the manually
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added time `widget`.
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#### Backlight
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The Featherwing has a PWM based backlight. The implementaion in
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`src/backlight.c` shows a way to dim the screen when it is not in use.
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The way this works, is by means of `backlight_keepalive()` which sets the
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backlight on, and installs a timer (by default for 10 seconds). Each time
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the user touches the screen, the timer is re-initialized. If the user does
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not touch the screen, the timer invokes the callback `backlight_keepalive_cb()`
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which will set turn the screen off: it will install a target duty cycle and
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time to get to that target (usually 1000ms). Then it'll start a repeating
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a 10ms timer that dims the backlight until the target it reached, after
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which it deinstalls itself. See `backlight_fader_cb()` for details. Very slick!
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If the screen is off, `backlight_active()` will return false. The main
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`touch_handler()` (the one that gets interrupts from the STMPE610), will
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ignore the first `TOUCH_DOWN` and `TOUCH_UP` events in that case, but it
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will wake up the screen again by setting the backlight back on.
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### Other tricks
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The application includes [Prometheus Metrics](https://github.com/mongoose-os-libs/prometheus-metrics)
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which allows users to export metrics to a monitoring system. The library comes
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with several Mongoose OS specific metrics (such as memory, build platform,
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MQTT statistics, etc), but also allows users to add handlers of their own.
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For example, the battery `widget` installs a callback adding one such metric
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(the measured battery voltage).
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## Unit Tests
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Unit tests are an incredibly important tool for any software engineer. The
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author wrote the `widget` and `screen` implementations by means of _test based
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engineering_, in which the code is written by first authoring tests, and then
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making the tests pass. This is a wonderful way to prove non-trivial
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implementations.
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See the `unittest/Makefile` for a compilable target (on Linux at least). It
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runs tests against the code, ensuring that timers are set and removed,
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object creation and destruction are working, and getters/setters and other
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code operates as designed.
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## Acknowledgements
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Several pieces of code were borrowed from other authors. In particular, kudos
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go to the following fine individuals:
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* ***Espressif Systems*** for the SPI driver
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* ***LoBo*** (loboris@GitHub) for parts of the ILI9341 driver
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* ***Adafruit*** for inspiration on the STMPE610 driver (which the author
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rewrote to support interrupts).
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* ***Lode Vandevenne*** and ***Sean Middleditch*** for the uPNG code to
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handle PNG images.
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* ***Cesanta*** for Mongoose OS, Mongoose, and the JSON `frozen` library.
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