Quick-Start: Driving 14-Segment Displays with the MAX6954
Abstract:This article is how-to guide, intended as a quick learning aid for engineers considering using the MAX6954 to drive 14-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 eight mono-color, 14-segment LEDs.
See the MAX6954 data sheet for additional information about MAX6954 features.
For Larger Image Figure 1. MAX6954 14-segment application circuit.
Table 1. Connection Scheme for Eight 14-Segment Digits
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 labeling for 14-segment displays.
Figure 3. 14-segment display font map.
A common application for the MAX6954 is driving up to eight 14-segment mono-color LEDs. This application note guides the user through the process of connecting the MAX6954 to 14-segment displays and configuring the internal registers to control the displays using the included 104-character 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 AppNote 1880. Table 1 lists the connection scheme for 14-segment digits that is compatible with the MAX6954 multiplex scheme and the built in 104-character font map. The letters in Table 1 correspond to the segment labels shown in Figure 2. Figure 1 illustrates an eight digit, 14-segment application circuit for the MAX6954.
Display test mode can be used to verify connections after connecting the MAX6954 to 14-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.
After verification of the connections between the MAX6954 and the 14-segment displays the device registers must be configured for operation with 14-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, and the device is set to scan the maximum eight digits. Configure the MAX6954 for 14-segment operation by writing 0xFF to register 0x0C. The default decode mode (0x01) register setting is correct for 14-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
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 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).
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 14-segment displays remain blank, as the power-up state of digit control registers 0x60 to 0x67 is 0x20 or no segments lit (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. Example Configurations for Display Digit Combinations
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 includes a 104-character ASCII font map for 14-segment displays. The characters follow the standard ASCII font, with the addition of the following common symbols: £, €, ¥, °, µ, ±, , and . Seven bits represent the 104-character font map; an 8th bit is used to select whether the decimal point (DP) is lit (Table 5). 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 8). The digit registers are mapped so that a digit's data can be updated in plane P0 (registers 0x20 through 0x27), plane P1 (registers 0x40 through 0x47), or both planes simultaneously (registers 0x60 through 0x67). The data in the digit registers does not control the digit segments directly for 16-segment displays. Instead, the register data is used to address a character generator that stores the data for the 14-segment 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 5. Digit Type 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 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 eight characters, "MAXIM-IC," using the MAX6954. We disabled the blink functionality when we programmed the configuration register, so data can be written to the 0x20 through 0x27 registers to control digit 0 through digit 7, respectively. Using the font map in Figure 3 determine the code equivalent for each character:
Table 6. MAXIM-IC Example
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 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 7. Quick-Start Example Command Summary
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
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, and 
. Seven bits represent the 104-character font map; an 8th bit is used to select whether the decimal point (DP) is lit (Table 5). 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 8). The digit registers are mapped so that a digit's data can be updated in plane P0 (registers 0x20 through 0x27), plane P1 (registers 0x40 through 0x47), or both planes simultaneously (registers 0x60 through 0x67). The data in the digit registers does not control the digit segments directly for 16-segment displays. Instead, the register data is used to address a character generator that stores the data for the 14-segment 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.