MicroPython nano-gui

A lightweight and minimal MicroPython GUI library for display drivers based on the FrameBuffer class. It is portable between a range of MicroPython hosts and display devices. Various display technologies are supported, including small color and monochrome OLED's, color TFT's, ePaper and Sharp units.

These images, most from OLED displays, fail to reproduce the quality of these displays. OLEDs are visually impressive displays with bright colors, wide viewing angle and extreme contrast. For some reason I find them hard to photograph.
The aclock.py demo.

Label objects in two fonts.

One of the demos running on an Adafruit 1.27 inch OLED. The colors change dynamically with low values showing green, intermediate yellow and high red.

The alevel.py demo. The Pyboard was mounted vertically: the length and angle of the vector arrow varies as the Pyboard is moved.

There is an optional graph plotting module for basic Cartesian and polar plots, also realtime plotting including time series.

A sample image from the plot module.

These images from a TFT display illustrate the new widgets.
The Scale widget. Capable of precision display of floats as the notionally very long scale moves behind its small window.

The Textbox widget for scrolling text with word wrap or clipping.

A mockup of a seismograph screen on an ePaper display.

Notes on Adafruit and other OLED displays including wiring details, pin names and hardware issues.

Contents

1. Introduction
   1.1 Change log
   1.2 Description
   1.3 Quick start
2. Files and Dependencies
   2.1 Files
        2.1.1 Core files
        2.1.2 Demo Scripts
        2.1.3 Fonts
        2.1.4 Hardware setup examples
   2.2 Dependencies
   2.3 Verifying hardware configuration
3. The nanogui module
   3.1 Application Initialisation Initial setup and refresh method.
        3.1.1 User defined colors
        3.1.2 Monochrome displays A slight "gotcha" with ePaper.
   3.2 Label class Dynamic text at any screen location.
   3.3 Meter class A vertical panel meter.
   3.4 LED class Virtual LED of any color.
   3.5 Dial and Pointer classes Clock or compass style display of one or more pointers.
   3.6 Scale class Linear display with wide dynamic range.
   3.7 Class Textbox Scrolling text display.
4. ESP8266 This can work. Contains information on minimising the RAM and flash footprints of the GUI.

Graph plotting module.

Device driver document.

1. Introduction

This library provides a limited set of GUI objects (widgets) for displays whose display driver is subclassed from the FrameBuffer class. Such drivers can be tiny as the graphics primitives are supplied by the FrameBuffer class. A range of device drivers is provided: the device driver doc provides guidance on selecting the right driver for your display, platform and application.

The GUI is cross-platform. The device driver doc explains how to configure it for a given display and MicroPython host by adapting a single small file. The GUI supports multiple displays attached to a single target, but bear in mind the RAM requirements for multiple frame buffers. The GUI has been tested on Pyboard 1.1, Pyboard D and on the ESP32 reference board without SPIRAM. Running on ESP8266 is possible but frozen bytecode must be used owing to its restricted RAM.

As of 14th March 2021 it runs on the Raspberry Pi Pico; on that target firmware must be of that date or later. The color15 demo fails because the firmware lacks uos.urandom() but hopefully it will be fixed soon.

It uses synchronous code but is compatible with uasyncio. Some demo programs illustrate this. Code is standard MicroPython, but some device drivers use the native and viper decorators.

The GUI is display-only and lacks provision for user input. Authors of applications requiring touch should consider the touch GUI's for the following displays:

• Official lcd160cr
• RA8875 large displays
• SSD1963 large displays

For historical reasons and to ensure consistency, code and documentation for my GUI's employ the American spelling of color.

1.1 Change log

25 Apr 2021 Support TTGO T-Display.
26 Mar 2021 Add ST7789. Alter uasyncio support on ili9341.
14 Mar 2021 Tested on Pi Pico.
17 Jan 2021
Add ePaper drivers. Ensure monochrome and color setup requirements are identical. Substantial update to docs.
16 Dec 2020
Add ILI9341 driver, 4-bit drivers and SPI bus sharing improvements. These mean that color_setup.py should now set SPI baudrate.
29 Nov 2020
Add ST7735R TFT drivers.
17 Nov 2020
Add Textbox widget. Scale constructor arg border replaced by bdcolor as per other widgets. 5 Nov 2020 - breaking change
This library has been refactored as a Python package. This reduces RAM usage: widgets are imported on demand rather than unconditionally. This has enabled the addition of new widgets with zero impact on existsing applications. Another aim was to simplify installation with dependencies such as writer included in the tree. Finally hardware configuration is contained in a single script: only this file needs to be customised to run all demo scripts or to port an application to different hardware.

Users of versions prior to this refactor should re-install from scratch. In existing applications, import statements will need to be adapted as per the demos. The GUI API is otherwise unchanged.

1.2 Description

Compatible and tested display drivers include:

• The official SSD1306 driver.
• The PCD8544/Nokia 5110.
• The Adafruit 0.96 inch color OLED with this driver.
• A driver for Adafruit 1.5 inch OLED and Adafruit 1.27 inch OLED is documented here.
• A driver for Sharp ultra low power consumption monochrome displays such as 2.7 inch 400x240 pixels is documented here.
• Drivers for Adafruit ST7735R based TFT's: 1.8 inch and 1.44 inch documented here.
• Drivers for Adafruit ST7789 TFT's and TTGO T-Display ESP32 with TFT display: 1.3 inch and 1.54 inch. TTGO Product page
• Drivers for ILI9341 such as Adafruit 3.2 inch documented here.
• Adafruit 2.9 inch ePaper display documented here.
• Waveshare 2.7 inch ePaper HAT documented here. Please note the warning regarding poor quality suspected clone units.

Widgets are intended for the display of data from physical devices such as sensors. They are drawn using graphics primitives rather than icons to minimise RAM usage. It also enables them to be effciently rendered at arbitrary scale by hosts with restricted processing power. The approach also enables widgets to maximise information in ways that are difficult with icons, in particular using dynamic color changes in conjunction with moving elements.

Copying the contents of the frame buffer to the display is relatively slow. The time depends on the size of the frame buffer and the interface speed, but the latency may be too high for applications such as games. For example the time to update a 128x128x8 color ssd1351 display on a Pyboard 1.0 is 41ms.

Drivers based on FrameBuffer must allocate contiguous RAM for the buffer. To avoid 'out of memory' errors it is best to instantiate the display before importing other modules. The example color_setup files illustrate this.

1.3 Quick start

A GUI description can seem daunting because of the number of class config options. Defaults can usually be accepted and meaningful applications can be minimal. Installation can seem difficult. To counter this, this session using rshell installed and ran a demo showing analog and digital clocks.

Clone the repo to your PC, wire up a Pyboard (1.x or D) to an Adafruit 1.27" OLED as per color_setup.py, move to the root directory of the repo and run rshell.

> cp -r drivers /sd
> cp -r gui /sd
> cp color_setup.py /sd
> repl ~ import gui.demos.aclock

Note also that the gui.demos.aclock.py demo comprises 38 lines of actual code. This stuff is easier than you might think.

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2. Files and Dependencies

Firmware should be V1.13 or later. On the Pi Pico firmware should be V1.15 or later.

Installation comprises copying the gui and drivers directories, with their contents, plus a hardware configuration file, to the target. The directory structure on the target must match that in the repo. This consumes about 300KiB of flash.

Filesystem space may be conserved by copying only the required driver from drivers, but the directory path to that file must be retained. For example, for SSD1351 displays only the following are actually required:
drivers/ssd1351/ssd1351.py, drivers/ssd1351/__init__.py.

The small color_setup.py file contains all hardware definitions (for color or monochrome displays). This is the only file which will require editing to match the display and its wiring. For information on how to do this, see the drivers document.

2.1 Files

2.1.1 Core files

The root directory contains an example setup file color_setup.py for a color OLED display. Other examples may be found in the setup_examples directory. These are templates for adaptation: only one file is copied to the target. On the target the file should be named color_setup.py and put in the root of the filesystem.

The chosen template will need to be edited to match the display in use, the MicroPython target and the electrical connections between display and target. Electrical connections are detailed in the template source.

• color_setup.py Hardware setup for the display. As written supports an SSD1351 display connected to a Pyboard.

The gui/core directory contains the GUI core and its principal dependencies:

• nanogui.py The library.
• writer.py Module for rendering Python fonts.
• fplot.py The graph plotting module.
• colors.py Color constants.
• framebuf_utils.mpy Accelerator for the CWriter class. This optional file is compiled for STM hardware. It is specific to Pyboards (1.x and D) and will be ignored on other ports. Details may be found here.

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2.1.2 Demo scripts

The gui/demos directory contains test/demo scripts.

Demos for small displays:

• mono_test.py Tests/demos using the official SSD1306 driver for a monochrome 128*64 OLED display.
• color96.py Tests/demos for the Adafruit 0.96 inch color OLED.

Demos for larger displays.

• color15.py Demonstrates a variety of widgets. Cross platform.
• aclock.py Analog clock demo. Cross platform.
• alevel.py Spirit level using Pyboard accelerometer.
• fpt.py Plot demo. Cross platform.
• scale.py A demo of the Scale widget. Cross platform. Uses uasyncio.
• asnano_sync.py Two Pyboard specific demos using the GUI with uasyncio.
• asnano.py Could readily be adapted for other targets.
• tbox.py Demo Textbox class. Cross-platform.

Demos for ePaper displays:

• waveshare_test.py For the Waveshare eInk Display HAT 2.7" 176*274 display.
• epd29_sync.py Demo for Adafruit 2.9" eInk display: emulates a seismograph.
• epd29_async.py Asynchronous demo for Adafruit 2.9" eInk display.
• epd29_lowpower.py Micropower demo for Adafruit 2.9" eInk display. This doc Micropower use should be read before attempting to run this.

Demos for Sharp displays:

• sharptest.py Basic functionality check.
• clocktest.py Digital and analog clock demo.
• clock_batt.py Low power demo of battery operated clock.

Usage with uasyncio is discussed here. In summary the GUI works well with uasyncio but the blocking which occurs during transfer of the framebuffer to the display may affect more demanding applications. Some display drivers have an additional asynchronous refresh method. This may optionally be used to mitigate the resultant latency.

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2.1.3 Fonts

Python font files are in the gui/fonts directory. The easiest way to conserve RAM is to freeze them which is highly recommended. In doing so the directory structure must be maintained: the ESP8266 provides an illustration.

To create alternatives, Python fonts may be generated from industry standard font files with font_to_py.py. The -x option for horizontal mapping must be specified. If fixed pitch rendering is required -f is also required. Supplied examples are:

• arial10.py Variable pitch Arial. 10 pixels high.
• arial35.py Arial 35 high.
• arial_50.py Arial 50 high.
• courier20.py Fixed pitch Courier, 20 high.
• font6.py FreeSans 14 high.
• font10.py FreeSans 17 high.
• freesans20.py FreeSans 20 high.

2.1.4 Hardware setup examples

The setup_examples directory contains example setup files for various hardware. These are templates which may be adapted to suit the hardware in use, then copied to the hardware root as color_setup.py. Example files:

• ssd1306_pyb.py Setup file for monochrome displays using the official driver. Supports hard or soft SPI or I2C connections.
• ssd1351_esp32.py As written supports an ESP32 connected to a 128x128 SSD1351 display. After editing to match the display and wiring, it should be copied to the target as /pyboard/color_setup.py.
• ssd1351_esp8266.py Similar for ESP8266. Usage is somewhat experimental.
• st7735r_pyb.py Assumes a Pyboard with an Adafruit 1.8 inch TFT display.
• st7735r144_pyb.py For a Pyboard with an Adafruit 1.44 inch TFT display.
• ili9341_esp32.py A 240*320 ILI9341 display on ESP32.
• st7789_pico.py Example with SSD7789 driver and Pi Pico host.
• st7789_ttgo.py Setup for the TTGO T-Display device.
• waveshare_pyb.py 176*274 ePaper display on Pyboard.
• epd29_pyb_sync.py Adafruit 2.9 inch ePaper display for synchronous code.
• epd29_pyb_async.py Adafruit 2.9 inch ePaper display: uasyncio applications.

2.2 Dependencies

The source tree now includes all dependencies. These are listed to enable users to check for newer versions:

• writer.py Provides text rendering of Python font files.

Optional feature:

• An STM32 implementation of this optimisation.

A copy of the official driver for OLED displays using the SSD1306 chip is provided. The official file is here:

• SSD1306 driver.

Displays based on the Nokia 5110 (PCD8544 chip) require this driver. It is not in this repo but may be found here:

• PCD8544/Nokia 5110

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2.3 Verifying hardware configuration

This script performs a basic check that the color_setup.py file matches the hardware, that (on color units) all three primary colors can be displayed and that pixels up to the edges of the display can be accessed.

from color_setup import ssd  # Create a display instance
from gui.core.colors import RED, BLUE, GREEN
from gui.core.nanogui import refresh
refresh(ssd, True)  # Initialise and clear display.
# Uncomment for ePaper displays
# ssd.wait_until_ready()
ssd.fill(0)
ssd.line(0, 0, ssd.width - 1, ssd.height - 1, GREEN)  # Green diagonal corner-to-corner
ssd.rect(0, 0, 15, 15, RED)  # Red square at top left
ssd.rect(ssd.width -15, ssd.height -15, 15, 15, BLUE)  # Blue square at bottom right
ssd.show()

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3. The nanogui module

The GUI supports a variety of widgets, some of which include text elements. The coordinates of a widget are those of its top left corner. If a border is specified, this is drawn outside of the limits of the widgets with a margin of 2 pixels. If the widget is placed at [row, col] the top left hand corner of the border is at [row-2, col-2].

When a widget is drawn or updated (typically with its value method) it is not immediately displayed. To update the display nanogui.refresh is called: this enables multiple updates to the FrameBuffer contents before once copying the buffer to the display. Postponement enhances performance providing a visually instant update.

Text components of widgets are rendered using the Writer (monochrome) or CWriter (colour) classes.

3.1 Application Initialisation

The GUI is initialised by issuing:

from color_setup import ssd

This defines the hardware as described in the drivers document.

A typical application then imports nanogui modules and clears the display:

from gui.core.nanogui import refresh
from gui.widgets.label import Label  # Import any widgets you plan to use
from gui.widgets.dial import Dial, Pointer
refresh(ssd, True)  # Initialise and clear display.

Initialisation of text display follows. For each font a CWriter instance is created (for monochrome displays a Writer is used):

from gui.core.writer import CWriter  # Renders color text
import gui.fonts.arial10  # A Python Font
from gui.core.colors import *  # Standard color constants

CWriter.set_textpos(ssd, 0, 0)  # In case previous tests have altered it
 # Instantiate any CWriters to be used (one for each font)
wri = CWriter(ssd, arial10, GREEN, BLACK, verbose=False)  # Colors are defaults
wri.set_clip(True, True, False)

Calling nanogui.refresh on startup sets up and clears the display. The method will subsequently be called whenever a refresh is required. It takes two args:

1. device The display instance (the GUI supports multiple displays).
2. clear=False If set True the display will be blanked; it is also blanked when a device is refreshed for the first time.

3.1.1 User defined colors

The file gui/core/colors.py defines standard color constants which may be used with any display driver. This section describes how to change these or to create additional colors.

Most of the color display drivers define colors as 8-bit or larger values. In such cases colors may be created and assigned to variables as follows:

from color_setup import SSD
PALE_YELLOW = SSD.rgb(150, 150, 0)

The GUI also provides drivers with 4-bit color to minimise RAM use. Colors are assigned to a lookup table having 16 entries. The frame buffer stores 4-bit color values, which are converted to the correct color depth for the hardware when the display is refreshed.

Of the possible 16 colors 13 are assigned in gui/core/colors.py, leaving color numbers 12, 13 and 14 free. Any color can be assigned as follows:

from gui.core.colors import *  # Imports the create_color function
PALE_YELLOW = create_color(12, 150, 150, 0)

This creates a color rgb(150, 150, 0) assigns it to "spare" color number 12 then sets PALE_YELLOW to 12. Any color number in range 0 <= n <= 15 may be used (implying that predefined colors may be reassigned). It is recommended that BLACK (0) and WHITE (15) are not changed. If code is to be ported between 4-bit and other drivers, use create_color() for all custom colors: it will produce appropriate behaviour. See the vari_fields function in the demo color15.py for an example.

3.1.2 Monochrome displays

Most widgets work on monochrome displays if color settings are left at default values. If a color is specified, drivers in this repo will convert it to black or white depending on its level of saturation. A low level will produce the background color, a high level the foreground. This can produce a surprise on ePaper units where the foreground is white.

At the bit level 1 represents the foreground. This is white on an emitting display such as an OLED. On a Sharp display it indicates reflection. On an ePaper display it represents black. Given that 1 is the foreground color, explicitly specifying BLACK on an ePaper will produce 0 as black has (very) low saturation. In this context the resultant physically white background color may come as a surprise.

In general the solution is to leave color settings at default.

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3.2 Label class

The purpose of a Label instance is to display text at a specific screen location.

Text can be static or dynamic. In the case of dynamic text the background is cleared to ensure that short strings cleanly replace longer ones.

Labels can be displayed with an optional single pixel border.

Colors are handled flexibly. By default the colors used are those of the Writer instance, however they can be changed dynamically; this might be used to warn of overrange or underrange values. The color15.py demo illustrates this.

Constructor args:

1. writer The Writer instance (font and screen) to use.
2. row Location on screen.
3. col
4. text If a string is passed it is displayed: typically used for static text. If an integer is passed it is interpreted as the maximum text length in pixels; typically obtained from writer.stringlen('-99.99'). Nothing is dsplayed until .value() is called. Intended for dynamic text fields.
5. invert=False Display in inverted or normal style.
6. fgcolor=None Optionally override the Writer colors.
7. bgcolor=None
8. bdcolor=False If False no border is displayed. If None a border is shown in the Writer forgeround color. If a color is passed, it is used.

The constructor displays the string at the required location.

Methods:

1. value Redraws the label. This takes the following args:
   • text=None The text to display. If None displays last value.
   • invert=False If true, show inverse text.
   • fgcolor=None Foreground color: if None the Writer default is used.
   • bgcolor=None Background color, as per foreground.
   • bdcolor=None Border color. As per above except that if False is passed, no border is displayed. This clears a previously drawn border.
     Returns the current text string.
2. show No args. (Re)draws the label. Primarily for internal use by GUI.

If populating a label would cause it to extend beyond the screen boundary a warning is printed at the console. The label may appear at an unexpected place. The following is a complete "Hello world" script.

from color_setup import ssd  # Create a display instance
from gui.core.nanogui import refresh
from gui.core.writer import CWriter
from gui.core.colors import *

from gui.widgets.label import Label
import gui.fonts.freesans20 as freesans20

refresh(ssd)  # Initialise and clear display.
CWriter.set_textpos(ssd, 0, 0)  # In case previous tests have altered it
wri = CWriter(ssd, freesans20, GREEN, BLACK, verbose=False)
wri.set_clip(True, True, False)

# End of boilerplate code. This is our application:
Label(wri, 2, 2, 'Hello world!')
refresh(ssd)

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3.3 Meter class

This provides a vertical linear meter display of values scaled between 0.0 and 1.0.

Constructor positional args:

1. writer The Writer instance (font and screen) to use.
2. row Location on screen.
3. col

Keyword only args:

4. height=50 Height of meter.
5. width=10 Width.
6. fgcolor=None Foreground color: if None the Writer default is used.
7. bgcolor=None Background color, as per foreground.
8. ptcolor=None Color of meter pointer or bar. Default is foreground color.
9. bdcolor=False If False no border is displayed. If None a border is shown in the Writer forgeround color. If a color is passed, it is used.
10. divisions=5 No. of graduations to show.
11. label=None A text string will cause a Label to be drawn below the meter. An integer will create a Label of that width for later use.
12. style=Meter.LINE The pointer is a horizontal line. Meter.BAR causes a vertical bar to be displayed. Much easier to read on monochrome displays.
13. legends=None If a tuple of strings is passed, Label instances will be displayed to the right hand side of the meter, starting at the bottom. E.G. ('0.0', '0.5', '1.0')
14. value=None Initial value. If None the meter will not be drawn until its value() method is called.

Methods:

1. value Args: n=None, color=None.
   • n should be a float in range 0 to 1.0. Causes the meter to be updated. Out of range values are constrained. If None is passed the meter is not updated.
   • color Updates the color of the bar or line if a value is also passed. None causes no change.
     Returns the current value.
2. text Updates the label if present (otherwise throws a ValueError). Args:
   • text=None The text to display. If None displays last value.
   • invert=False If true, show inverse text.
   • fgcolor=None Foreground color: if None the Writer default is used.
   • bgcolor=None Background color, as per foreground.
   • bdcolor=None Border color. As per above except that if False is passed, no border is displayed. This clears a previously drawn border.
3. show No args. (Re)draws the meter. Primarily for internal use by GUI.

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3.4 LED class

This is a virtual LED whose color may be altered dynamically.

Constructor positional args:

1. writer The Writer instance (font and screen) to use.
2. row Location on screen.
3. col

Keyword only args:

4. height=12 Height of LED.
5. fgcolor=None Foreground color: if None the Writer default is used.
6. bgcolor=None Background color, as per foreground.
7. bdcolor=False If False no border is displayed. If None a border is shown in the Writer forgeround color. If a color is passed, it is used.
8. label=None A text string will cause a Label to be drawn below the LED. An integer will create a Label of that width for later use.

Methods:

1. color arg c=None Change the LED color to c. If c is None the LED is turned off (rendered in the background color).
2. text Updates the label if present (otherwise throws a ValueError). Args:
   • text=None The text to display. If None displays last value.
   • invert=False If true, show inverse text.
   • fgcolor=None Foreground color: if None the Writer default is used.
   • bgcolor=None Background color, as per foreground.
   • bdcolor=None Border color. As per above except that if False is passed, no border is displayed. This clears a previously drawn border.
3. show No args. (Re)draws the LED. Primarily for internal use by GUI.

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3.5 Dial and Pointer classes

A Dial is a circular display capable of displaying a number of vectors; each vector is represented by a Pointer instance. The format of the display may be chosen to resemble an analog clock or a compass. In the CLOCK case a pointer resembles a clock's hand extending from the centre towards the periphery. In the COMPASS case pointers are chevrons extending equally either side of the circle centre.

In both cases the length, angle and color of each Pointer may be changed dynamically. A Dial can include an optional Label at the bottom which may be used to display any required text.

In use, a Dial is instantiated then one or more Pointer objects are instantiated and assigned to it. The Pointer.value method enables the Dial to be updated affecting the length, angle and color of the Pointer. Pointer values are complex numbers.

Dial class

Constructor positional args:

1. writer The Writer instance (font and screen) to use.
2. row Location on screen.
3. col

Keyword only args:

4. height=50 Height and width of dial.
5. fgcolor=None Foreground color: if None the Writer default is used.
6. bgcolor=None Background color, as per foreground.
7. bdcolor=False If False no border is displayed. If None a border is shown in the Writer forgeround color. If a color is passed, it is used.
8. ticks=4 No. of gradutions to show.
9. label=None A text string will cause a Label to be drawn below the meter. An integer will create a Label of that width for later use.
10. style=Dial.CLOCK Pointers are drawn from the centre of the circle as per the hands of a clock. Dial.COMPASS causes pointers to be drawn as arrows centred on the control's centre. Arrow tail chevrons are suppressed for very short pointers.
11. pip=None Draws a central dot. A color may be passed, otherwise the foreground color will be used. If False is passed, no pip will be drawn. The pip is suppressed if the shortest pointer would be hard to see.

When a Pointer is instantiated it is assigned to the Dial by the Pointer constructor.

Pointer class

Constructor arg:

1. dial The Dial instance on which it is to be dsplayed.

Methods:

1. value Args:
   • v=None The value is a complex number. A magnitude exceeding unity is reduced (preserving phase) to constrain the Pointer within the unit circle.
   • color=None By default the pointer is rendered in the foreground color of the parent Dial. Otherwise the passed color is used.
     Returns the current value.
2. text Updates the label if present (otherwise throws a ValueError). Args:
   • text=None The text to display. If None displays last value.
   • invert=False If true, show inverse text.
   • fgcolor=None Foreground color: if None the Writer default is used.
   • bgcolor=None Background color, as per foreground.
   • bdcolor=None Border color. As per above except that if False is passed, no border is displayed. This clears a previously drawn border.
3. show No args. (Re)draws the control. Primarily for internal use by GUI.

Typical usage (ssd is the device and wri is the current Writer):

def clock(ssd, wri):
    # Border in Writer foreground color:
    dial = Dial(wri, 5, 5, ticks = 12, bdcolor=None)
    hrs = Pointer(dial)
    mins = Pointer(dial)
    hrs.value(0 + 0.7j, RED)
    mins.value(0 + 0.9j, YELLOW)
    dm = cmath.exp(-1j * cmath.pi / 30)  # Rotate by 1 minute
    dh = cmath.exp(-1j * cmath.pi / 1800)  # Rotate hours by 1 minute
    # Twiddle the hands: see aclock.py for an actual clock
    for _ in range(80):
        utime.sleep_ms(200)
        mins.value(mins.value() * dm, RED)
        hrs.value(hrs.value() * dh, YELLOW)
        refresh(ssd)

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3.6 Scale class

This displays floating point data having a wide dynamic range. It is modelled on old radios where a large scale scrolls past a small window having a fixed pointer. This enables a scale with (say) 200 graduations (ticks) to readily be visible on a small display, with sufficient resolution to enable the user to interpolate between ticks. Default settings enable estimation of a value to within about +-0.1%.

Legends for the scale are created dynamically as it scrolls past the window. The user may control this by means of a callback. The example lscale.py illustrates a variable with range 88.0 to 108.0, the callback ensuring that the display legends match the user variable. A further callback enables the scale's color to change over its length or in response to other circumstances.

The scale displays floats in range -1.0 <= V <= 1.0.

Constructor positional args:

1. writer The Writer instance (font and screen) to use.
2. row Location on screen.
3. col

Keyword only arguments (all optional):

• ticks=200 Number of "tick" divisions on scale. Must be divisible by 2.
• legendcb=None Callback for populating scale legends (see below).
• tickcb=None Callback for setting tick colors (see below).
• height=0 Pass 0 for a minimum height based on the font height.
• width=200
• bdcolor=None Border color. If None, fgcolor will be used.
• fgcolor=None Foreground color. Defaults to system color.
• bgcolor=None Background color defaults to system background.
• pointercolor=None Color of pointer. Defaults to .fgcolor.
• fontcolor=None Color of legends. Default fgcolor.

Method:

• value=None Set or get the current value. Always returns the current value. A passed float is constrained to the range -1.0 <= V <= 1.0 and becomes the Scale's current value. The Scale is updated. Passing None enables reading the current value, but see note below on precision.

Callback legendcb

The display window contains 20 ticks comprising two divisions; by default a division covers a range of 0.1. A division has a legend at the start and end whose text is defined by the legendcb callback. If no user callback is supplied, legends will be of the form 0.3, 0.4 etc. User code may override these to cope with cases where a user variable is mapped onto the control's range. The callback takes a single float arg which is the value of the tick (in range -1.0 <= v <= 1.0). It must return a text string. An example from the lscale.py demo shows FM radio frequencies:

def legendcb(f):
    return '{:2.0f}'.format(88 + ((f + 1) / 2) * (108 - 88))

The above arithmetic aims to show the logic. It can (obviously) be simplified.

Callback tickcb

This callback enables the tick color to be changed dynamically. For example a scale might change from green to orange, then to red as it nears the extremes. The callback takes two args, being the value of the tick (in range -1.0 <= v <= 1.0) and the default color. It must return a color. This example is taken from the scale.py demo:

def tickcb(f, c):
    if f > 0.8:
        return RED
    if f < -0.8:
        return BLUE
    return c

Increasing the ticks value

This increases the precision of the display.

It does this by lengthening the scale while keeping the window the same size, with 20 ticks displayed. If the scale becomes 10x longer, the value diference between consecutive large ticks and legends is divided by 10. This means that the tickcb callback must return a string having an additional significant digit. If this is not done, consecutive legends will have the same value.

Precision

For performance reasons the control stores values as integers. This means that if you set value and subsequently retrieve it, there may be some loss of precision. Each visible division on the control represents 10 integer units.

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3.7 Class Textbox

Displays multiple lines of text in a field of fixed dimensions. Text may be clipped to the width of the control or may be word-wrapped. If the number of lines of text exceeds the height available, scrolling will occur. Access to text that has scrolled out of view may be achieved by calling a method. The widget supports fixed and variable pitch fonts.

from gui.widgets.textbox import Textbox

Constructor mandatory positional arguments:

1. writer The Writer instance (font and screen) to use.
2. row Location on screen.
3. col
4. width Width of the object in pixels.
5. nlines Number of lines of text to display. The object's height is determined from the height of the font:
   height in pixels = nlines*font_height
   As per most widgets the border is drawn two pixels beyond the control's boundary.

Keyword only arguments:

• bdcolor=None Border color. If None, fgcolor will be used.
• fgcolor=None Color of border. Defaults to system color.
• bgcolor=None Background color of object. Defaults to system background.
• clip=True By default lines too long to display are right clipped. If False is passed, word-wrap is attempted. If the line contains no spaces it will be wrapped at the right edge of the window.

Methods:

• append Args s, ntrim=None, line=None Append the string s to the display and scroll up as required to show it. By default only the number of lines which will fit on screen are retained. If an integer ntrim=N is passed, only the last N lines are retained; ntrim may be greater than can be shown in the control, hidden lines being accessed by scrolling.
  If an integer (typically 0) is passed in line the display will scroll to show that line.
• scroll Arg n Number of lines to scroll. A negative number scrolls up. If scrolling would achieve nothing because there are no extra lines to display, nothing will happen. Returns True if scrolling occurred, otherwise False.
• value No args. Returns the number of lines of text stored in the widget.
• clear No args. Clears all lines from the widget and refreshes the display.
• goto Arg line=None Fast scroll to a line. By default shows the end of the text. 0 shows the start.

Fast updates:
Rendering text to the screen is relatively slow. To send a large amount of text the fastest way is to perform a single append. Text may contain newline ('\n') characters as required. In that way rendering occurs once only.

ntrim__ If text is regularly appended to a Textbox its buffer grows, using RAM. The value of ntrim sets a limit to the number of lines which are retained, with the oldest (topmost) being discarded as required.

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4. ESP8266

Some personal observations on successful use with an ESP8266.

I chose an Adafruit 128x128 OLED display to represent the biggest display I thought the ESP8266 might support. I reasoned that, if this can be made to work, smaller or monochrome displays would present no problem.

The ESP8266 is a minimal platform with typically 36.6KiB of free RAM. The framebuffer for a 128*128 OLED requires 16KiB of contiguous RAM (the display hardware uses 16 bit color but my driver uses an 8 bit buffer to conserve RAM). The 4-bit driver halves this size.

A further issue is that, by default, ESP8266 firmware does not support complex numbers. This rules out the plot module and the Dial widget. It is possible to turn on complex support in the build, but I haven't tried this.

I set out to run the scale.py and textbox.py demos as these use uasyncio to create dynamic content, and the widgets themselves are relatively complex.

I froze a subset of the drivers and the gui directories. A subset minimises the size of the firmware build and eliminates modules which won't compile due to the complex number issue. The directory structure in my frozen modules directory matched that of the source. This was the structure of my frozen directory before I added the 4 bit driver:

I erased flash, built and installed the new firmware. Finally I copied setup_examples/esp8266_setup.py to /pyboard/color_setup.py. This could have been frozen but I wanted to be able to change pins if required.

Both demos worked perfectly.

I modified the demos to regularly report free RAM. scale.py reported 10480 bytes, tbox.py reported 10512 bytes, sometimes more, as the demo progressed. With the 4 bit driver scale.py reported 18112 bytes. In conclusion I think that applications of moderate complexity should be feasible.

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