Quick Start: Using MAX6954 to drive a 14-segment LED display

Abstract: This article aims to help engineers quickly master the use of MAX6954 to drive 14-segment monochrome LED displays.

The MAX6954 is a universal display driver that can control multiple discrete, 7-segment, 14-segment, or 16-segment LED displays via a serial port. This application note describes typical applications and configurations for driving 8-bit monochrome, 14-segment LED displays.

For more information about MAX6954, please refer to the MAX6954 data sheet.

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

Figure 1. The MAX6954 drives a 14-segment display circuit
Click to enlarge Figure 1. MAX6954 drives a 14-segment display circuit

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

DIGIT O0 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15 O16 O17 O18
0 CCO - a - b c d - e f g1 g2 h i j k l m dp
1 - CC1 a - b c d - e f g1 g2 h i j k l m dp
2 a - CC2 - b c d - e f g1 g2 h i j k l m dp
3 a - - CC3 b c d - e f g1 g2 h i j k l m dp
4 a - b c CC4 - d - e f g1 g2 h i j k l m dp
5 a - b c - CC5 d - e f g1 g2 h i j k l m dp
6 a - b c d - CC6 - e f g1 g2 h i j k l m dp
7 a - b c d - - CC7 e f g1 g2 h i j k l m dp

Figure 2. Segment identification of the 14-segment display
Figure 2. Segment identification of the 14-segment display

Figure 3. 14-segment display font
Figure 3. 14-segment display font

A common application of the MAX6954 is to drive up to 8-bit 14-segment monochrome LEDs. This application note guides the user 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 a built-in font.

The MAX6954 uses a multiplexing technique that alternates the polarity of the driver port to reduce the number of pins. This is different from the standard LED multiplexing connection used separately for the cathode and anode pins of the driver. Application note 1880 discusses this technique. Table 1 is the connection table between MAX6954 and 14-segment display. This table is compatible with the MAX6954 multiplexing configuration 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 diagram of the MAX6954 driving an eight-bit 14-segment display.

After the MAX6954 is connected to the 14-segment display, the display test mode can be used to check the correctness of the connection. The display test mode is not affected by the control and data registers, and lights all segments of the display, but does not change any register contents. Write 0x01 to register 0x07 to start the display test mode. In the test mode, if there is an unlit segment on the display, 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 as required to drive the 14-segment display. Table 2 includes the power-on initial values ​​of some important registers of MAX6954. The MAX6954 is in power-down mode when it is first powered on. Every bit of display is blanked, and the number of scanning digits is set to the maximum, that is, eight digits. The MAX6954 can be set to drive the 14-segment display by writing 0xFF to register 0x0C. The default value of the decoding mode register (0x01) is suitable for 14-segment display.

Table 2. MAX6954 register power-on initial state

REGISTER POWER-UP
CONDITION
ADDRESS
(HEX)
REGISTER DATA
D7 D6 D5 D4 D3 D2 D1 D0
Decode Mode Font map enabled 0x01 1 1 1 1 1 1 1 1
Global Intensity 1/16 (minimum intensity) 0x02 X X X X 0 0 0 0
Scan Limit Display eight digits: 0, 1, 2, 3, 4, 5, 6, 7 0x03 X X X X X 1 1 1
Control Register Shutdown enabled, blink disabled, blink
speed is slow
0x04 0 0 X X 0 0 0 0
Display Test Normal operaTIon (display test disabled) 0x07 X X X X X X X 0
Digit Type Digits 0 through 7 are 16 segment digits 0x0C 0 0 0 0 0 0 0 0
Intensity10 1/16 (min on), digits 1 and 0 0x10 0 0 0 0 0 0 0 0
Intensity32 1/16 (min on), digits 3 and 2 0x11 0 0 0 0 0 0 0 0
Intensity54 1/16 (min on), digits 5 and 4 0x12 0 0 0 0 0 0 0 0
Intensity76 1/16 (min on), digits 7 and 6 0x13 0 0 0 0 0 0 0 0
Digit 0 Blank digit, both planes 0x60 0 0 1 0 0 0 0 0
Digit 1 Blank digit, both planes 0x61 0 0 1 0 0 0 0 0
Digit 2 Blank digit, both planes 0x62 0 0 1 0 0 0 0 0
Digit 3 Blank digit, both planes 0x63 0 0 1 0 0 0 0 0
Digit 4 Blank digit, both planes 0x64 0 0 1 0 0 0 0 0
Digit 5 Blank digit, both planes 0x65 0 0 1 0 0 0 0 0
Digit 6 Blank digit, both planes 0x66 0 0 1 0 0 0 0 0
Digit 7 Blank digit, both planes 0x67 0 0 1 0 0 0 0 0


The configuration register (0x01) is used to set the entry and exit of shutdown mode, control the flashing function, globally clear the data of each bit, 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 the shutdown mode or normal operation mode (read / write). Bit B is used to select the blinking 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 flashing 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 the brightness of each bit is controlled globally or individually (read / write). The P bit is used to return to the current flash timing phase (read-only-writing to this bit will be ignored).

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

Table 3. Configuration register format

CONFIGURATION
REGISTER
DATA BIT
DATA BIT LABEL STATE FUNCTION
D7 P 0 P1 Blink Phase
1 P0 Blink Phase
D6 I 0 Intensity for all digits is controlled by one setting in the Global
Intensity Register
1 Intensity for digits is controlled by the individual settings in the
Intensity10 through Intensity76 registers.
D5 R 0 Digit data for both planes P0 and P1 are unaffected
1 Digit data for both planes P0 and P1 are cleared on the rising edge
of / CS \
D4 T 0 Blink timing counters are unaffected.
1 Blink timing counters are reset on the rising edge of / CS \.
D3 E 0 Blink function disabled
1 Blink function enabled
D2 B 0 Slow Blinking. Segments blink on for 1s, off for 1s with fOSC = 4MHz.
1 Fast Blinking. Segments blink on for 0.5s, off for 0.5s with fOSC = 4MHz.
D1 X X Don't care
D0 S 0 Shutdown
1 Normal Operation


Table 4. Display data combination configuration examples
DIGIT TYPE REGISTER SETTING ADDRESS
CODE (HEX)
REGISTER DATA
D7 D6 D5 D4 D3 D2 D1 D0
Digits 7-0 are 14-segment digits 0x0C 1 1 1 1 1 1 1 1


The MAX6954 has a built-in 104-character ASCII font for 14-segment displays. The characters are standard ASCII fonts plus these commonly used symbols. The 104 characters are represented by 7 digits, and the 8th digit 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 the memory, one byte is at P0 and the other is at P1 (see Table 8). The data register is a mapped address mode, so each bit of data can be updated in P0 (registers 0x20 to 0x27) or P1 (registers 0x40 to 0x47), and it can also be updated in two stages (registers 0x60 to 0x67). The data in the data register does not directly control the display of each segment in the 14 (original error is 16) segment display. Instead, it is used to address a character generator that stores 14 segments of font data (see Figure 3). The lower 7 bits of the register data (D6 to D0) are used to select characters from the font. The highest digit (D7) controls the decimal point (DP) display; set to 1 to display DP, and set to 0 to not display DP.

Table 5. Data register format
MODE ADDRESS
CODE
(HEX)
REGISTER DATA
D7 D6 D5 D4 D3 D2 D1 D0
14-segment mode, writing digit data to use font map data
with decimal place unlit
0x20 to 0x27
0x40 to 0x47
0x60 to 0x67
0 Bits D6 to D0 select font character 0 to
127
14-segment mode, writing digit data to use font map data
with decimal place lit
0x20 to 0x27
0x40 to 0x47
0x60 to 0x67
1 Bits D6 to D0 select font character 0 to
127


If the flash function is disabled by the configuration register flash enable bit E (see Table 3), the data in the data register in P0 is used as the display multiplex. The data in the data register in P1 is not used. If the blinking function is enabled, the data multiplexing display in P0 and P1 is used alternately. The data in P0 and P1 are applied to different phases of the blinking clock alternately changing, thereby multiplexing the LED display to achieve the blinking function.

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

Table 6. Examples of MAXIM-IC
REGISTER
ADDRESS (HEX)
DIGIT CHARACTER FONT MAP EQUIVALENT CODE
DECIMAL HEX
0x20 0 M 0100 1101 0x4D
0x21 1 A 0100 0001 0x41
0x22 2 X 0101 1000 0x58
0x23 3 I 0100 1001 0x49
0x24 4 M 0100 1101 0x4D
0x25 5 - 0010 1101 0x2D
0x26 6 I 0100 1001 0x49
0x27 7 C 0100 0011 0x43


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). You can also individually control the brightness of each bit. For detailed information about display brightness adjustment and other advanced features (such as GPIO and key scan), please refer to the MAX6954 data sheet.

Table 7. Command summary for quick start examples
REGISTER (HEX) COMMAND (HEX) FUNCTION
0x07 0x01 Enter display test mode
0x07 0x00 Exit display test mode
0x0C 0xFF Set all eight digits to use the 14-segment font map
0x04 0x01 Exit shutdown mode, disable blinking and select global
intensity control
0x20 0x4D Write "M" to digit 0
0x21 0x41 Write "A" to digit 1
0x22 0x58 Write "X" to digit 2
0x23 0x49 Write "I" to digit 3
0x24 0x4D Write "M" to digit 4
0x25 0x2D Write "-" to digit 5
0x26 0x49 Write "I" to digit 6
0x27 0x43 Write "C" to digit 7
0x02 0xFF Set global intensity to full-scale


Table 8. MAX6954 / MAX6955 partial register address map
REGISTER ADDRESS
(COMMAND BYTE)
ADDRESS (HEX
CODE)
D15 D14 D13 D12 D11 D10 D9 D8
Decode Mode R // W \ 0 0 0 0 0 0 1 0x01
Global Intensity R // W \ 0 0 0 0 0 1 0 0x02
Scan Limit R // W \ 0 0 0 0 0 1 1 0x03
Configuration R // W \ 0 0 0 0 1 0 0 0x04
Display Test R // W \ 0 0 0 0 1 1 1 0x07
Digit Type R // W \ 0 0 0 1 1 0 0 0x0C
Intensity 10 R // W \ 0 0 1 0 0 0 0 0x10
Intensity 32 R // W \ 0 0 1 0 0 0 1 0x11
Intensity 54 R // W \ 0 0 1 0 0 1 0 0x12
Intensity 76 R // W \ 0 0 1 0 0 1 1 0x13
Digit 0 Plane P0 R // W \ 0 1 0 0 0 0 0 0x20
Digit 1 Plane P0 R // W \ 0 1 0 0 0 0 1 0x21
Digit 2 Plane P0 R // W \ 0 1 0 0 0 1 0 0x22
Digit 3 Plane P0 R // W \ 0 1 0 0 0 1 1 0x23
Digit 4 Plane P0 R // W \ 0 1 0 0 1 0 0 0x24
Digit 5 ​​Plane P0 R // W \ 0 1 0 0 1 0 1 0x25
Digit 6 Plane P0 R // W \ 0 1 0 0 1 1 0 0x26
Digit 7 Plane P0 R // W \ 0 1 0 0 1 1 1 0x27
Digit 0 Plane P1 R // W \ 1 0 0 0 0 0 0 0x40
Digit 1 Plane P1 R // W \ 1 0 0 0 0 0 1 0x41
Digit 2 Plane P1 R // W \ 1 0 0 0 0 1 0 0x42
Digit 3 Plane P1 R // W \ 1 0 0 0 0 1 1 0x43
Digit 4 Plane P1 R // W \ 1 0 0 0 1 0 0 0x44
Digit 5 ​​Plane P1 R // W \ 1 0 0 0 1 0 1 0x45
Digit 6 Plane P1 R // W \ 1 0 0 0 1 1 0 0x46
Digit 7 Plane P1 R // W \ 1 0 0 0 1 1 1 0x47



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