7-segment-display-max7219.md

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7 Segment Display with MAX7219 Backpack	text-width-sidebar	Using a MAX7219-driven 8-digit 7 Segment LED display module on the microbit.	true	jez	Use a 7-segment display to show digits on the microbit	external	7-Segment Display (SPI)	2016-12-23 10:20:00 UTC	2016-12-23 10:20:00 UTC

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This module will display 8 numerical digits on its display. Each digit is a 7-segment display comprised of 8 LEDs (7 for the number, and one for a decimal place).

It's possible to drive each of these component LEDs either directly, or through a shift register. However, these modules use the MAX7219 driver chip to simplify the process.

The microbit tells the MAX7219 chip which number to display over SPI. The chip then displays the digit.

Like the HT1633 8x8 module on this site, the LED displays are connected to a backpack containing the MAX7219 chip and wiring.

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Components

These 7-segment MAX7219 displays are available on eBay, Amazon and other sellers. Searching for MAX7219 7-segment display reveals the components. They're around £2 each.

Additionally, you will need a microbit breakout board to access the SPI pins of the microbit.

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Electronics

Purpose of Each Pin

There are five wires to connect to the display module. Each of the pins do the following:

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Pin Name	Purpose
VCC	for power
GND	for ground
CS / Chip Select	Goes low when data is being transmitted to the device. Goes high at the end of transmission. .write_digital() is used for this.
DIN	Data transmitted over this wire.
CLK	Tells the microbit when to transmit data.

Hookup Table

Connect the module to the microbit and its edge connector as below. Be aware the pin labels can change dependning on the manufacturer:

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My Module Label	Possible Label Names	microbit Pin	Micropython Doc Names for pin
VCC		3v
GND		GND
DIN	SOMI, SDI, DI, DIN, SI, MRST.	pin15	MOSI
CS	nCS, CSN, nSS, STE, SYNC	pin0	n/a, but frequently called chip select
CLK	SCLK	pin13	Serial Clock

• Your module might also have a DOUT pin. Ignore it!

To connect to these pins, you will need the microbit edge connector. Here's my module wired up to my microbit:

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Code

Download the Module

Download the Maxrix7seg module for the microbit from Github.. This contains the code used to communicate with the 7-segment display.

Import Module

The module is imported in the header of your Python script:

{% highlight python %} from microbit import spi

from matrix7seg.py import Matrix7seg class

from matrix7seg import Matrix7seg

Initialise an instance of the Matrix7seg class

and call it 'seg_display'.

It's connected to default SPI pins. pin0 is chip select

pin1 or pin2 etc could be used instead.

seg_display = matrix7seg(spi, pin0) {% endhighlight %}

Add the matrix7seg Module

Once your script is complete and has been flashed, the matrix7seg module can be copied to the microbit:

1. Flash your script.
2. Click 'Files' within in the mu editor.
3. From the /mu_code/ directory, copy drag the matrix7seg.py file across to the microbit.
4. Press reset on your microbit to reload the Python program with the matrix7seg module.

Using the Module

It's now possible to use the display module within the microbit:

{% highlight python %}

Number must be 8 or fewer digits

seg_display.write_number(1234)

Update display with '1234'

seg_display.show() {% endhighlight %}

Each time .write_number() is used, .show() must be used to update the display.

Code Examples

.write_number()

{% highlight python %}

display a number

seg_display.write_number(1234) {% endhighlight %}

.write_number(n, zeroPad=True)

{% highlight python %}

empty 7-segments filled with 0

seg_display.write_number(1234, zeroPad=True) {% endhighlight %}

`.write_number(n, leftJustify=True)

{% highlight python %}

justify numbers to left

seg_display.write_number(1234, leftJustify=True) {% endhighlight %}

Example Programs

Increment Numbers to 100

{% highlight python %}

sequentially write numbers to 100

for i in range(100) seg_display.write_number(i) seg_display.show() {% endhighlight %}

Show the Temperature

{% highlight python %}

read and display the temperature every second

while True: seg_display.write_number(temperature()) seg_display.show() sleep(1000) {% endhighlight %}

G-Force Game

{% highlight python %}

record total acceleration experienced by the microbit

if it's the highest value recorded by the microbit,

display the reading on the screen.

importing as little as possible to save memory

from microbit import spi, accelerometer, sleep, pin0 from math import sqrt from matrix7seg import Matrix7seg

def total_acceleration(): """ return total acceleration in milli-g across all 3 axes. """ x = accelerometer.get_x() y = accelerometer.get_y() z = accelerometer.get_z()

# root of sum of squares
total = sqrt(x**2 + y**2 + z**2)
return total

highest_reading = 0

segment = Matrix7seg(spi, pin0)

while True: reading = total_acceleration() if reading > highest_reading: highest_reading = reading segment.write_number(highest_reading) segment.show() {% endhighlight %}