Using MAX6954 to drive a 14-segment LED display

　　The MAX6954 is a universal display driver capable of controlling multiple discrete, 7-segment, 14-segment, or 16-segment LED displays through a single serial port. This application note introduces typical applications and configurations for driving 8-bit monochrome, 14-segment LED displays. For detailed information about the MAX6954, please refer to the MAX6954 data sheet.

　　The application notes "Using the MAX6954 to Drive a 7-Segment LED Display" and "Using the MAX6954 to Drive a 16-Segment LED Display" are similar to this article and describe how to configure the MAX6954 to drive a 7-segment and 16-segment display respectively.

　　　Figure 1. MAX6954 driving 14-segment display circuit

　　　Table 1. Connection table with eight-bit 14-segment display

　　Figure 2. Segment identification for a 14-segment display

　　　　　　　　　　Figure 3. 14-segment display font

　　A common application for the MAX6954 is to drive up to 8 bits of 14-segment single-color LEDs. This application note guides users on how to connect the MAX6954 to a 14-segment display, and how to configure its internal registers to control the display of 104 characters in the built-in font.

　　The MAX6954 uses a multiplexing technique that alternates the polarity of the driver ports to reduce pin count. This is different from standard LED multiplexing connections where the driver's cathode and anode pins are used separately. Application Note AppNote 1880 discusses this technique. Table 1 is the connection diagram between the MAX6954 and the 14-segment display. This table is compatible with the multiplexing configuration of the MAX6954 and its 104-character built-in font. The letters in Table 1 correspond to the segment identifiers in Figure 2. Figure 1 is a schematic circuit diagram of the MAX6954 driving an eight-digit 14-segment display.

　　After completing the connection between the MAX6954 and the 14-segment display, you can use the display test mode to check the correctness of the connection. The display test mode is not affected by the control and data registers. It lights up all segments of the display but does not change the contents of any register. Writing 0x01 to Register 0x07 enables display test mode. If there are unlit segments on the display in test mode, it means the connection is incorrect. Write 0x00 to register 0x07 to exit display test mode.

　　After completing the connection check, you need to set the MAX6954 register value according to the requirements for driving a 14-segment display. Table 2 includes the initial power-on values ​​of some important registers of the MAX6954. When the MAX6954 is first powered on, it is in shutdown mode, each display is blanked, and the number of scan digits is set to the maximum, which is eight digits. The MAX6954 can be set to drive a 14-segment display by writing 0xFF to Register 0x0C. The default value of the decoding mode register (0x01) is suitable for a 14-segment display.

　　Table 2. MAX6954 register initial power-on state

　　The configuration register (0x04) (the original text was mistaken for 0x01) is used to set the entry and exit of shutdown mode, control the flashing function, globally clear each bit of data, and select whether to control the brightness of each bit globally or individually. The configuration register contains 7 bits (see Table 3).

　　The S bit is used to select shutdown mode or normal operating mode (read/write).
　　Bit B is used to select the blink rate (read/write).
　　The E bit is used to globally enable or disable the flashing function (read/write).
　　The T bit is used to reset the flash sequence (this data is not stored - transient bit).
　　The R bit is used to globally clear the P0 and P1 two-level registers corresponding to all display bits (this data is not stored - transient bit).
　　The I bit is used to select whether to control the brightness of each bit globally or individually (read/write).
　　The P bit returns the current blink timing phase (read-only—writing a value to this bit will be ignored).

　　Start the display driver by writing 0x01 to register 0x04, and set the working mode to global brightness control and disable flickering. The internal oscillator starts when the MAX6954 exits shutdown mode, and the oscillator output pin is OSC_OUT. Note that when the power-on status of data control registers 0x60 to 0x67 is 0x20 or no segments are lit, the 14-segment display remains blank (see Table 2 and Figure 3).

　　Table 3. Configuration register format

　　Table 4. Display data combination configuration example

　　　　?

　　The MAX6954 has a built-in 104-character ASCII font pattern for a 14-segment display. The characters are standard ASCII fonts plus ￡, €, ￥, °, μ, ±, , and these common symbols. These 104 characters are represented by 7 bits, and the 8th bit is used to select whether to light the decimal point (DP) (see Table 5). The data register has two levels, P0 and P1. Each display bit is represented by 2 bytes in memory, one byte is located in P0 and the other is located in P1 (see Table 8). The data register is a mapped address, so each bit of data can be updated in P0 (register 0x20 to 0x27), or in P1 (register 0x40 to 0x47), or both levels can be updated simultaneously (register 0x60 to 0x67). The data in the data register does not directly control the display of each segment in the 14-segment display. Instead, it is used to address a character generator that stores 14 segments of word pattern data (see Figure 3). The lower 7 bits of register data (D6 to D0) are used to select characters from the font. The highest bit (D7) controls the decimal point (DP) display; setting it to 1 will display DP, and setting it to 0 will not display DP. Welcome to Wuyou Electronic Development Network (online shopping mall for online reading of technical articles)

　　Table 5. Data register format

　　If the flash function is disabled by the flash enable bit E of the configuration register (see Table 3), the data in the data register in P0 is used for display multiplexing. The data in the data register in P1 is not used. If the blink function is enabled, the data multiplexing displays in P0 and P1 are used alternately. The data in P0 and P1 are respectively applied to different phases of the alternating changes of the flashing clock, thereby multiplexing the LED display to achieve the flashing function.

　　The following is an example of using the MAX6954 driver to display eight characters, "MAXIM-IC". Set the configuration register to disable the flash function, so that data bits 0 to 7 can be controlled respectively by writing data to registers 0x20 to 0x27. Use the font pattern in Figure 3 to get the code corresponding to each character.

　Table 6. Examples of MAXIM-IC

　　Adjust the display brightness by writing data to register 0x02 (global brightness control register). The brightness can be adjusted between 0x00 (minimum brightness, 1/16 current) and 0xFF (maximum brightness, 15/16 current). The brightness of each bit can also be controlled individually. Please refer to the MAX6954 data sheet for details on display brightness adjustment and other advanced features such as GPIO and key scanning.

　　Table 7. Summary of commands for the quick start example

　　Table 8. Partial register address map of MAX6954/MAX6955