MAX7219 I2C

Github Repo	C Header	C source	JS source
mongoose-os-libs/max7219-spi	mgos_max7219.h

A Mongoose library for MAX7219, an 8-digit LED Display Driver.

Implementation details

The chip is straight forwardly implemented, and allows for daisy chaining multiple devices (by tying DOUT of the first chip to DIN of the second chip, while connecting CS, CLK, VCC and GND to all chips).

After initialization, bytes can be written to so called digits, which are the individual lines of an 8x8 LED display, or the actual digits of a 7-segment display.

NOTE: The MAX7219 is a 5V device, and as such level shifters are necessary between the MOSI, SCLK and CS pins of the MCU and the pins on the chip(s). It may work without, but it's not recommended!

API Description

Create the object with mgos_max7219_create(), and if there are multiple devices connected, set the correct number with mgos_max7219_set_num_devices(). When you're done using the display(s), call mgos_max7219_destroy() to turn them off and clean them up.

mgos_max7219_set_intensity() Intensity of all displays will be set, where the argument is between 0 (very dim) to 15 (very bright).

mgos_max7219_set_mode() Setting the mode is done with a boolean mode argument. If true is specified, all connected devices are set into what is called Code B decoding, in which the values map internally to segments of a 7-segment display, for example writing 0 will light up the segments needed to draw a 0 on a 7-segment display. There are only 15 values in this system, see the header file for details. The mode is initialized as false, in other words do not use Code B, and now each bit on the 8-bit values correspond to one LED, typically on an 8x8 LED display.

Writes can be performed in three ways:

mgos_max7219_write_raw() writes an 8-bit value to a certain line (digit) of a certain device (deviceno). This will only work if the mode was set to false.

mgos_max7219_write_digit() writes a number to a certain position (digit) of a certain device (deviceno). This will only work if the mode was set to true.

mgos_max7219_write_line() writes an array of bytes, which must be of exactly num_devices length, to the given line (digit) of all connected displays.

Example application

#include "mgos.h"
#include "mgos_config.h"
#include "mgos_max7219.h"

const uint8_t program[] = {
  0,   153, 90,  60,  24,  24,  36,  66,  5,
  0,   24,  24,  255, 24,  24,  36,  66,  5,
  0,   24,  24,  60,  90,  153, 36,  66,  5,
  0,   24,  24,  255, 24,  24,  36,  66,  5,
};

void timer_cb(void *data) {
  struct mgos_max7219 *d        = (struct mgos_max7219 *)data;
  static uint8_t       waiting  = 0;
  static uint16_t      position = 0;

  if (!d) return;

  if (waiting > 0) {
    waiting--;
    return;
  }

  waiting = program[position * 9 + 8];
  LOG(LL_INFO, ("Writing program position %u, then waiting %.1f seconds", position, 0.1 * waiting));

  for (int i = 0; i < 8; i++) {
    mgos_max7219_write_raw(d, 0, i, program[position * 9 + i]);
    mgos_max7219_write_raw(d, 1, i, ~(program[position * 9 + i]));
  }

  position++;
  if (position == sizeof(program) / 9) position = 0;
}

enum mgos_app_init_result mgos_app_init(void) {
  struct mgos_max7219 *d = NULL;

  if (!(d = mgos_max7219_create(mgos_spi_get_global(), mgos_sys_config_get_max7219_cs_index())) {
    LOG(LL_ERROR, ("Could not create MAX7219 display"));
    return MGOS_APP_INIT_ERROR;
  }
  mgos_max7219_set_num_devices(d, 2);
  mgos_set_timer(100, true, timer_cb, d);

  return MGOS_APP_INIT_SUCCESS;
}

Disclaimer

This project is not an official Google project. It is not supported by Google and Google specifically disclaims all warranties as to its quality, merchantability, or fitness for a particular purpose.

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mgos_max7219_create

struct mgos_max7219 *mgos_max7219_create(struct mgos_spi *spi, uint8_t cs_index);

Initialize a MAX7219 on the SPI bus spi using the chipselect specified in cs_index parameter (see spi.cs*_gpio). Upon success a new struct mgos_max7219 is allocated and returned. If the device could not be found or initialized, NULL is returned.

mgos_max7219_destroy

bool mgos_max7219_destroy(struct mgos_max7219 **dev);

Destroy the data structure associated with a MAX7219 device. The reference to the pointer of the struct mgos_max7219 has to be provided, and upon successful destruction, its associated memory will be freed and the pointer set to NULL and true will be returned.

mgos_max7219_set_num_devices

bool mgos_max7219_set_num_devices(struct mgos_max7219 *dev, uint8_t num_devices);

Set the number of devices in the daisychain (1 or more). Returns true on success or false on failure.

mgos_max7219_set_intensity

bool mgos_max7219_set_intensity(struct mgos_max7219 *dev, uint8_t intensity);

Set display intensity (brightness) as a value from 0..15. Returns true on success or false on failure.

mgos_max7219_set_mode

bool mgos_max7219_set_mode(struct mgos_max7219 *dev, bool codeB_enabled);

Set display mode, either 'raw' or 'digit':

• codeB_enabled: set 'false' for 'raw', or 'true' to enable 7-segment digits. Returns true on success or false on failure.

mgos_max7219_write_raw

bool mgos_max7219_write_raw(struct mgos_max7219 *dev, uint8_t deviceno, uint8_t digit, uint8_t value);

Set display digit at position digit with a value

• deviceno: device to adddress, between 0 and num_devices-1;
• digit: must be between [0, 7]
• value: can be between [0, 255] Returns true on success or false on failure.

mgos_max7219_write_digit

bool mgos_max7219_write_digit(struct mgos_max7219 *dev, uint8_t deviceno, uint8_t digit, uint8_t value);

Set display digit at position digit with an 7-segment character value

• deviceno: device to adddress, between 0 and num_devices-1;
• digit: must be between [0, 7]
• value: must be between [0, 15]: 0-9 for numbers, , 10='-', 11='E', 12='H', 13='L', 14='P', 15=' ' called Code B decoding.

Returns true on success or false on failure.

mgos_max7219_write_line

bool mgos_max7219_write_line(struct mgos_max7219 *dev, uint8_t digit, uint8_t *value);

Write a line of values at position digit across all connected devices.

• digit: must be between [0, 7]
• value: An array of exactly num_devices bytes.

Note: If the device is in mode codeB, the values should be between [0, 15].

Returns true on success or false on failure.

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