MAX7219.js

JavaScript abstraction for the MAX7219 display driver controller. Please read the datasheet for this chip here.

How to use

The "digits and segments" language isn't dissolved away by the API, to make your life easier when wiring the controller. Obviously, you can think of them as "cathodes and anodes" respectively when using this library. Multiplexing and persistence of vision is handled by the MAX7219, you only need to turn anodes on or off. Here's a quick example:

var disp = new MAX7219("/dev/spidev1.0");
disp.setDecodeNone();
disp.setScanLimit(8);
disp.startup();
disp.setDigitSegments(0, [0, 0, 1, 1, 0, 1, 1, 1]);
disp.setDigitSegments(1, [0, 1, 0, 0, 1, 1, 1, 1]);
disp.setDigitSegments(2, [0, 0, 0, 0, 1, 1, 1, 0]);
disp.setDigitSegments(3, [0, 1, 1, 0, 0, 1, 1, 1]);

Alternatively, using the BCD code font decoding is supported. Here's how to set it up:

var disp = new MAX7219("/dev/spidev1.0");
disp.setDecodeAll();
disp.setScanLimit(8);
disp.startup();
disp.setDigitSymbol(0, "H");
disp.setDigitSymbol(1, "E");
disp.setDigitSymbol(2, "L");
disp.setDigitSymbol(3, "P");

Prerequisites

The node-spi library is required. Get it from here, or via npm install spi.

You'll also need to make sure SPI devices are enabled (and you have a reasonably up-to-date Linux kernel). Here's how you can check this:

Ensure that the SPI driver is enabled:

$ dmesg | grep spi
[    3.769841] bcm2708_spi bcm2708_spi.0: master is unqueued, this is deprecated
[    3.793364] bcm2708_spi bcm2708_spi.0: SPI Controller at 0x20204000 (irq 80)

The devices are successfully installed in /dev:

$ ls -l /dev/spi*
crw------- 1 root root 153, 0 Jan  1  2000 /dev/spidev1.0
crw------- 1 root root 153, 1 Jan  1  2000 /dev/spidev2.0

Enabling SPI on the BeagleBone Black

The fastest way would be to use the BeagleBone-SPI-UART library.

Alternatively, you're going to have to compile some device tree overlays. It's pretty easy, just follow the steps described here.

Enabling SPI on the Raspberry PI

Easy as pie! Simply sudo nano /etc/modprobe.d/raspi-blacklist.conf and add a # character in front of the line spi-bcm2708. Use CTRL-X, then Y, then Return to save the file and exit. Reboot.

API

• Constructor MAX7219(device, count, callback)

Initializes an instance of the controller abstraction. The device argument is a string specifying the SPI device on which the controller is wired. For example, "/dev/spidev1.0". The optional count argument specifies the total number of chips when several MAX7219s are daisy-chained.

• setActiveController(index)

When daisy-chaining MAX7219s, specifies which chip is currently controlled. The index parameter is a number identifying the chip to control. By default, the chip at index 0 is controlled.

• getActiveController(index)

Returns which chip is currently controlled.

• startup(callback)

Sets this controller in normal operation mode. On initial power-up, all control registers are reset, the display is blanked, and the MAX7219 enters shutdown mode. This method sets the controller back in normal operation mode.

• shutdown(callback)

Sets this controller in shutdown mode. When the MAX7219 is in shutdown mode, the scan oscillator is halted, all segment current sources are pulled to ground, and the display is blanked.

• startDisplayTest(callback)

Sets this controller in display-test mode. Display-test mode turns all LEDs on by overriding, but not altering, all controls and digit registers (including the shutdown register).

• stopDisplayTest(callback)

Sets this controller back into the previous operation mode.

• setDecodeMode(modes, callback)

Sets this controller's decode mode, specifying how the segments controlled by the MAX7219 are set on/off. When no-decode is selected, data bits correspond to the segments directly. When decode mode is selected, certain symbols (only 0-9, E, H, L, P, and -) are encoded in a specific way. This is useful for BCD 7 segment displays. The modes argument is an array of decode/no-decode modes for each digit. E.g., to set decode mode for digits 0–3 and no-decode for 4–7, modes would be [1, 1, 1, 1, 0, 0, 0, 0].

• setDecodeNone(callback)

Shortcut for specifying that all digits are in no-decode mode.

• setDecodeAll(callback)

Shortcut for specifying that all digits are in decode mode.

• setDigitSegments(n, segments, callback)

Sets each segment in a digit on/off. For this to work properly, the digit should be in no-decode mode. The segments are identified as follows:

   _a_
 f|   |b
  |_g_|
  |   |
 e|___|c  dp (decimal point)
    d    *

The first parameter, n, is the digit number, from 0 up to and including 7. The segments parameter is a list specifying whether segments are on and off. E.g., to specify dp, c, d, e and g on, and a, b, f off, segments would be [1, 0, 0, 1, 1, 1, 0, 1], corresponding to the structure [dp, a, b, c, d, e, f, g].

• setDigitSegmentsByte(byte, callback)

Same as setDigitSegments, but it takes a byte instead of an array of bits.

• setDigitSymbol(n, symbol, callback)

Sets the symbol displayed in a digit. For this to work properly, the digit should be in decode mode. The first parameter, n is the digit number, from 0 up to and including 7. The symbol parameter is a string specifying the symbol do display: "0".."9", "E", "H", "L", "P", "-" or " ". The dp parameter is a boolean specifying whether the decimal point should be on or off.

• clearDisplay(callback)

Sets all segments for all digits off. This is a shortcut for manually calling setDigitSegments or setDigitSymbol with the appropriate params. If a decode mode wasn't specifically set beforehand, no-decode mode is assumed.

• setDisplayIntensity(brightness, callback)

Sets digital control of display brightness. The brightnesss parameter may be a number from 0 (dimmest) up to and including 15 (brightest).

• setScanLimit(limit, callback)

Sets how many digits are displayed, from 1 digit to 8 digits. The limit parameter specifies the number of digits displayed, counting from first to last. E.g., to display only the first digit, limit would be 1. Another e.g., to display only digits 0, 1 and 2, limit would be 3.

Callbacks

All methods that have a callback argument will invoke that function once the write to the SPI device finishes.

EOF

Thanks for reading. Happy multiplexing.