{"id":2172,"date":"2015-08-02T11:29:27","date_gmt":"2015-08-02T09:29:27","guid":{"rendered":"https:\/\/playembedded.org\/?p=2172"},"modified":"2021-07-29T11:00:33","modified_gmt":"2021-07-29T09:00:33","slug":"stm32-chibios-and-a-8x8-led-matrix","status":"publish","type":"post","link":"https:\/\/playembedded.org\/blog\/stm32-chibios-and-a-8x8-led-matrix\/","title":{"rendered":"STM32, ChibiOS and a 8×8 LED Matrix"},"content":{"rendered":"

A compact display driver: the MAX7219<\/h3>\n

The MAX7219<\/b> is a compact, serial input\/parallel output common-cathode display driver. Included on-chip there are a BCD code-B decoder, the multiplex scan circuitry, segment and digit drivers, and an 8×8 static RAM that stores each digit. Only one external resistor is required to set the segments current.<\/p>\n

Common uses<\/h4>\n
\"LED<\/a>
A 8×8 LED matrix DIY kit.<\/figcaption><\/figure>\n

This chip is often used to drive 7-segment display having up to 8 digits or up to 64 individual LEDs. It is not unusual find this chip in DIY kits to be mounted on a small PCBs like that used in this article (See Fig. 1).<\/p>\n

This chip could be found in different packages and uses a non-standard SPI (Data is clocked even if CS remains high but is needed a low to high transition to complete communication.If you are not familiar with SPI communication read Meeting SPI<\/a>). MAX7221<\/b> is similar to MAX7219<\/b> except for two parameters:<\/p>\n

    \n
  1. MAX7221<\/b> segment drivers are slew-rate limited to reduce electromagnetic interference;<\/li>\n
  2. its serial interface is fully SPI compatible.<\/li>\n<\/ol>\n

    In theory our example could work with MAX7221<\/b> as well but it has been tested with MAX7219<\/b> only.<\/p>\n

    Documentation<\/h4>\n

    The next document is the datasheet for both MAX7219<\/b> and MAX7221<\/b>.<\/p>\n

    MAX7219 Datasheet<\/a><\/p>\n

    Features<\/h3>\n

    Design and DIY kit connections<\/h4>\n
    \"LED<\/a>
    A schematic of a LED matrix 8×8.<\/figcaption><\/figure>\n

    The MAX7219<\/b> can work with a voltage between 4.0~5.5V, and could be configured writing in some registers using a SPI. DIY kit we are using in this example is almost standard. the 8×8 LED Matrix provides 8 common cathodes and 8 common anodes identifying rows and columns of our matrix (See Fig. 2). Anodes are connected to segment outputs and cathodes to digits outputs. In MAX7219<\/b>, when a segment driver is turned on it is pulled to V+<\/b> and when a digit driver is turned on it is pulled to GND<\/b>.<\/p>\n

    If we consider the matrix orientation as that from Fig.1, segments (i.e. anodes) are representative of row indexes and digits (i.e. cathodes) are representative of column indexes. So if segment C and digit 0 are turned on the first LED on the third row is on. Combining the possible values for digits and segments MAX7219<\/b> can address each LED individually. Note that we have not to perform some kind of multiplexing or scan as it is already performed internally by MAX7219<\/b>.<\/p>\n

    On the PCB we should find some capacitors to minimize power-supply ripple and a resistor to setup output current. In our case we have a 10μF electrolytic and a 0.1μF ceramic capacitor (between V+<\/b> and GND<\/b> pins from MAX7219<\/b>) and a 10kΩ resistor (between V+<\/b> and ISET<\/b> pins from MAX7219<\/b>). Current for a segment driver (i.e. current for an entire row) is typically 100 times the RSET<\/b> current (that could be computed as V+<\/b>\/RSET<\/b> = 5V \/ 10kΩ = 0.5mA).<\/p>\n

    Pin description<\/h4>\n
    \"LED<\/a>
    The MAX7219 based driver circuit with pin labels.<\/figcaption><\/figure>\n

    Not every pin from MAX7219<\/b> is connected to the external connector. These devices could be connected in chain so we have two connectors. For the input one we have starting from top:<\/p>\n

      \n
    1. VCC<\/b>, power supply 4.0~5.5V DC<\/li>\n
    2. GND<\/b>, connection to ground<\/li>\n
    3. DIN<\/b>, SPI master output slave input<\/li>\n
    4. SCL<\/b>, SPI serial clock<\/li>\n
    5. CS<\/b>, SPI chip select<\/li>\n<\/ol>\n

      For the output one we have starting from top:<\/p>\n

        \n
      1. VCC<\/b>, power supply 4.0~5.5V DC<\/li>\n
      2. GND<\/b>, connection to ground<\/li>\n
      3. DOUT<\/b>, SPI master input slave output<\/li>\n
      4. SCL<\/b>, SPI serial clock<\/li>\n
      5. CS<\/b>, SPI chip select<\/li>\n<\/ol>\n

        Note that DOUT<\/b> could be connected to SPI MISO closing daisy chain or to DIN<\/b> of another MAX7219<\/b>.<\/p>\n

        Operation mode<\/h3>\n

        Communication scheme and timing<\/h4>\n

        Communication occurs through SPI bus. Summarizing most important info about communication are:<\/p>\n