APPLICATION NOTE 3210

Quick-Start: Driving 7-Segment Displays with the MAX6954

© Jun 25, 2004, Maxim Integrated Products, Inc.

Abstract: This article is how-to guide, intended as a quick learning aid for engineers considering using the MAX6954 to drive 7-segment monocolor LEDs.

The MAX6954 is a versatile display driver, capable of controlling a mix of discrete, 7-segment, 14-segment, and 16-segment LED displays through a serial interface. This application note shows a typical application and configuration for driving sixteen mono-color, 7-segment LEDs using the MAX6954.

See the MAX6954 data sheet for additional information about MAX6954 features.

"Quick-Start: Driving 16-Segment Displays with the MAX6954" and "Quick-Start: Driving 14-Segment Displays with the MAX6954" are similar application notes that discuss configuring the MAX6954 for 16-segment and 14-segment applications, respectively.

Figure 1. MAX6954 7-segment application circuit.
For Larger Image

Figure 1. MAX6954 7-segment application circuit.

A common application for the MAX6954 is driving up to sixteen 7-segment mono-color LEDs. This application note guides the user through the process of connecting the MAX6954 to 7-segment displays and configuring the internal registers to control the displays using the included hexadecimal font map.

The MAX6954 utilizes a multiplex technique that reduces driver pin count by using ports alternately as cathode and anode drivers. This differs from the standard LED multiplex connection, which uses separate driver pins for anodes and cathodes. The technique is discussed in "Charlieplexing - Reduced Pin-Count LED Display Multiplexing". Table 1 lists the connection scheme for 7-segment digits that is compatible with the MAX6954 multiplex scheme and the built in hexadecimal font map. The letters in Table 1 correspond to the segment labels shown in Figure 2. Figure 1 illustrates a sixteen digit, 7-segment application circuit for the MAX6954.

Table 1. Connection Scheme for Sixteen 7-Segment Digits
Digit O0 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15 O16 O17 O18
0
0a
CC0   1a   1b 1c 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 2dp
1,
1a
  CC1 1a   1b 1c 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 2dp
2,
2a
1a   CC2   1b 1c 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 2dp
3
3a
1a     CC3 1b 1c 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 2dp
4
4a
1a   1b 1c CC4   1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 2dp
5
5a
1a   1b 1c   CC5 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 2dp
6,
6a
1a   1b 1c 1d 1dp CC6   1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 2dp
7,
7a
1a   1b 1c 1d 1dp   CC7 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g 2dp

Display test mode can be used to verify connections after connecting the MAX6954 to 7-segment displays. Display test mode overrides, but does not alter, all control and digit registers and turns all segments on. Enable display test mode by writing 0x01 to register 0x07. If a segment does not light in display test mode this indicates a connection problem that should be investigated. Write 0x00 to register 0x07 to exit display test mode.

Figure 2. Segment labeling for 7-aegment displays.
Figure 2. Segment labeling for 7-aegment displays.

After verification of the connections between the MAX6954 and the 7-segment displays the device must be configured for operation with 7-segment displays. Table 2 contains a list of the initial power-up states of important MAX6954 registers. When power is first applied to the MAX6954 the device is in shutdown mode, all digits are blanked, the 7-segment font map is selected and the device is set to scan the maximum number of digits. The default decode mode (0x01) and digit type (0x0C) register settings are correct for 7-segment applications.

Table 2. MAX6954 Initial Power-Up Register Status
REGISTER POWER-UP CONDITION ADDRESS
(HEX)
REGISTER DATA
D7 D6 D5 D4 D3 D2 D1 D0
Decode Mode 7-segment font 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 sixteen 7-segment digits: 0, 1, 2, 3, 4, 5, 6, 7, 0a, 1a, 2a, 3a, 4a, 5a, 6a, 7a 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 7 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
Intensity10a 1/16 (min on), digits 1a and 0a 0x14 0 0 0 0 0 0 0 0
Intensity32a 1/16 (min on), digits 3a and 2a 0x15 0 0 0 0 0 0 0 0
Intensity54a 1/16 (min on), digits 5a and 4a 0x16 0 0 0 0 0 0 0 0
Intensity76a 1/16 (min on), digits 7a and 6a 0x17 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
Digit 0a Blank digit, both planes 0x68 0 0 1 0 0 0 0 0
Digit 1a Blank digit, both planes 0x69 0 0 1 0 0 0 0 0
Digit 2a Blank digit, both planes 0x6A 0 0 1 0 0 0 0 0
Digit 3a Blank digit, both planes 0x6B 0 0 1 0 0 0 0 0
Digit 4a Blank digit, both planes 0x6C 0 0 1 0 0 0 0 0
Digit 5a Blank digit, both planes 0x6D 0 0 1 0 0 0 0 0
Digit 6a Blank digit, both planes 0x6E 0 0 1 0 0 0 0 0
Digit 7a Blank digit, both planes 0x6F 0 0 1 0 0 0 0 0

The configuration register (0x01) is used to enter and exit shutdown, control the blink function, globally clear the digit data and to select between global or digit-by-digit control of intensity. The configuration register contains 7 bits (Table 3):
  • S bit selects shutdown or normal operation (read/write).
  • B bit selects the blink rate (read/write).
  • E bit globally enables or disables the blink function (read/write).
  • T bit resets the blink timing (data is not stored-transient bit).
  • R bit globally clears the digit data for both planes P0 and P1 for ALL digits (data is not stored-transient bit).
  • I bit selects between global or digit-by-digit control of intensity (read/write).
  • P bit returns the current phase of the blink timing (read only-a write to this bit is ignored).
Table 3. Configuration Register Format
CONFIGURATION
REGISTER
DATA BIT
DATA BIT LABEL STATE FUNCTION
D7 P 0 P1 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 Intensity76a 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 active-low CS
D4 T 0 Blink timing counters are unaffected.
1 Blink timing counters are reset on the rising edge of active-low 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

Enable the display driver with global intensity control and without blinking by writing 0x01 to register 0x04. The internal oscillator powers as the MAX6954 exits shutdown and the oscillator output is available at OSC_OUT. Notice that the 7-segment displays remain blank, as the power-up state of digit control registers 0x60 to 0x6F is 0x20 or no segments lit (see Table 2 and Figure 3).

Figure 3. 7-segment display font map.
Figure 3. 7-segment display font map.

The MAX6954 includes a hexadecimal font map for 7-segment displays. Seven bits represent the hexadecimal font map; an 8th bit is used to select whether the decimal point (DP) is lit (Table 4). Digit registers are implemented with two planes, P0 and P1. Each digit is represented by 2 bytes of memory, 1 byte in plane P0 and the other in plane P1 (see Table 7). The digit registers are mapped so that a digit's data can be updated in plane P0 (registers 0x20 through 0x2F), plane P1 (registers 0x40 through 0x4F), or both planes simultaneously (registers 0x60 through 0x6F). The data in the digit registers does not control the digit segments directly for 7-segment displays. Instead, the register data is used to address a character generator that stores the data for the font (Figure 3). The lower 7 bits of the digit data (D6 to D0) select the character from the font. The most significant bit of the register data (D7) controls the DP segment of the digits; it is set to 1 to light DP, and to 0 to leave DP unlit.

Table 4. Digit Type Data Register Format
MODE ADDRESS CODE (HEX) REGISTER DATA
D7 D6 D5 D4 D3 D2 D1 D0
7-segment mode, writing digit data to use font map data with decimal place unlit 0x20 to 0x2F
0x40 to 0x4F
0x60 to 0x6F
0 Bits D6 to D0 select font character
7-segment mode, writing digit data to use font map data with decimal place lit 0x20 to 0x2F
0x40 to 0x4F
0x60 to 0x6F
1 Bits D6 to D0 select font character

If the blink function is disabled through the Blink Enable Bit E (Table 3) in the configuration register, then the digit register data in plane P0 is used to multiplex the display. The digit register data in P1 is not used. If the blink function is enabled, then the digit register data in both plane P0 and plane P1 are alternately used to multiplex the display. Blinking is achieved by multiplexing the LED display using data plane P0 and plane P1 on alternate phases of the blink clock.

As an example let's display the phone number for Maxim/Dallas Direct: 1 800 629 4642, using the MAX6954. We disabled the blink functionality when we programmed the configuration register, so data can be written to the 0x20 through 0x2F registers to control digit 0 through digit 7a, respectively. Using the font map in Figure 3 determine the code equivalent for each character:

Table 5. Call 800.629.4642 Example
REGISTER ADDRESS (HEX) DIGIT CHARACTER FONT MAP EQUIVALENT CODE
DECIMAL HEX
0x20 0 C 0000 1100 0x0C
0x28 0a A 0000 1001 0x0A
0x21 1 1 0000 0001 0x01
0x29 1a 1 0000 0001 0x01
0x23 3 8 0000 1000 0x08
0x2B 3a 0 0000 0000 0x00
0x24 4 0 1000 0000 0x80
0x2C 4a 6 0000 0110 0x06
0x25 5 2 0000 0010 0x02
0x2D 5a 9 1000 1001 0x89
0x26 6 4 0000 0100 0x04
0x2E 6a 6 0000 0110 0x06
0x27 7 4 0000 0100 0x04
0x2F 7a 2 0000 0010 0x02

Adjust the intensity of the displays by writing to register 0x02, the global intensity control register. Intensity may be adjusted between 0x00 (minimum on, 1/16 current) and 0xFF (maximum on, 15/16 current). Digital control of display brightness can also be managed individually for each digit. For full details about adjusting digit intensity and other advanced features such as GPIO and key-scan, refer to the MAX6954 data sheet.

Table 6. Quick-Start Example Command Summary
REGISTER
(HEX)
COMMAND
(HEX)
FUNCTION
0x07 0x01 Enter display test mode
0x07 0x00 Exit display test mode
0x04 0x01 Exit shutdown mode, disable blinking and select global intensity control
0x20 0x0C Write "C" to digit 0
0x28 0x0A Write "A" to digit 0a
0x21 0x01 Write "1" to digit 1
0x29 0x01 Write "1" to digit 1a
0x23 0x08 Write "8" to digit 3
0x2B 0x00 Write "0" to digit 3a
0x24 0x80 Write "0." to digit 4
0x2C 0x06 Write "6" to digit 4a
0x25 0x02 Write "2" to digit 5
0x2D 0x89 Write "9." to digit 5a
0x26 0x04 Write "4" to digit 6
0x2E 0x06 Write "6" to digit 6a
0x27 0x04 Write "4" to digit 7
0x2F 0x02 Write "2" to digit 7a
0x02 0xFF Set global intensity to full-scale

Table 7. Partial MAX6954/6955 Register Address Map
REGISTER ADDRESS (COMMAND BYTE) ADDRESS
(HEX CODE)
D15 D14 D13 D12 D11 D10 D9 D8
Decode Mode R/active-low W 0 0 0 0 0 0 1 0x01
Global Intensity R/active-low W 0 0 0 0 0 1 0 0x02
Scan Limit R/active-low W 0 0 0 0 0 1 1 0x03
Configuration R/active-low W 0 0 0 0 1 0 0 0x04
Display Test R/active-low W 0 0 0 0 1 1 1 0x07
Digit Type R/active-low W 0 0 0 1 1 0 0 0x0C
Intensity 10 R/active-low W 0 0 1 0 0 0 0 0x10
Intensity 32 R/active-low W 0 0 1 0 0 0 1 0x11
Intensity 54 R/active-low W 0 0 1 0 0 1 0 0x12
Intensity 76 R/active-low W 0 0 1 0 0 1 1 0x13
Intensity 10a R/active-low W 0 0 1 0 1 0 0 0x14
Intensity 32a R/active-low W 0 0 1 0 1 0 1 0x15
Intensity 54a R/active-low W 0 0 1 0 1 1 0 0x16
Intensity 76a R/active-low W 0 0 1 0 1 1 1 0x17
Digit 0 Plane P0 R/active-low W 0 1 0 0 0 0 0 0x20
Digit 1 Plane P0 R/active-low W 0 1 0 0 0 0 1 0x21
Digit 2 Plane P0 R/active-low W 0 1 0 0 0 1 0 0x22
Digit 3 Plane P0 R/active-low W 0 1 0 0 0 1 1 0x23
Digit 4 Plane P0 R/active-low W 0 1 0 0 1 0 0 0x24
Digit 5 Plane P0 R/active-low W 0 1 0 0 1 0 1 0x25
Digit 6 Plane P0 R/active-low W 0 1 0 0 1 1 0 0x26
Digit 7 Plane P0 R/active-low W 0 1 0 0 1 1 1 0x27
Digit 0a Plane P0 R/active-low W 0 1 0 1 0 0 0 0x28
Digit 1a Plane P0 R/active-low W 0 1 0 1 0 0 1 0x29
Digit 2a Plane P0 R/active-low W 0 1 0 1 0 1 0 0x2A
Digit 3a Plane P0 R/active-low W 0 1 0 1 0 1 1 0x2B
Digit 4a Plane P0 R/active-low W 0 1 0 1 1 0 0 0x2C
Digit 5a Plane P0 R/active-low W 0 1 0 1 1 0 1 0x2D
Digit 6a Plane P0 R/active-low W 0 1 0 1 1 1 0 0x2E
Digit 7a Plane P0 R/active-low W 0 1 0 1 1 1 1 0x2F
Digit 0 Plane P1 R/active-low W 1 0 0 0 0 0 0 0x40
Digit 1 Plane P1 R/active-low W 1 0 0 0 0 0 1 0x41
Digit 2 Plane P1 R/active-low W 1 0 0 0 0 1 0 0x42
Digit 3 Plane P1 R/active-low W 1 0 0 0 0 1 1 0x43
Digit 4 Plane P1 R/active-low W 1 0 0 0 1 0 0 0x44
Digit 5 Plane P1 R/active-low W 1 0 0 0 1 0 1 0x45
Digit 6 Plane P1 R/active-low W 1 0 0 0 1 1 0 0x46
Digit 7 Plane P1 R/active-low W 1 0 0 0 1 1 1 0x47
Digit 0a Plane P1 R/active-low W 1 0 0 1 0 0 0 0x48
Digit 1a Plane P1 R/active-low W 1 0 0 1 0 0 1 0x49
Digit 2a Plane P1 R/active-low W 1 0 0 1 0 1 0 0x4A
Digit 3a Plane P1 R/active-low W 1 0 0 1 0 1 1 0x4B
Digit 4a Plane P1 R/active-low W 1 0 0 1 1 0 0 0x4C
Digit 5a Plane P1 R/active-low W 1 0 0 1 1 0 1 04D
Digit 6a Plane P1 R/active-low W 1 0 0 1 1 1 0 0x4E
Digit 7a Plane P1 R/active-low W 1 0 0 1 1 1 1 0x4F



Related Parts
MAX6954 4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan Free Samples  
MAX6955 2-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan Free Samples  


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APP 3210: Jun 25, 2004
APPLICATION NOTE 3210, AN3210, AN 3210, APP3210, Appnote3210, Appnote 3210