MAXIM MAX6954 Technical data

General Description

The MAX6954 is a compact display driver that interfaces microprocessors to a mix of 7-segment, 14-segment, and 16-segment LED displays through an SPI™or QSPI™-compatible 4-wire serial interface. The serial interface may be cascaded through multiple devices. The MAX6954 drives up to 16 digits 7-segment, 8 digits 14-segment, 8 digits 16-segment, or 128 discrete LEDs, while functioning from a supply voltage as low as

2.7V. The driver includes five I/O expander (or GPIO) lines, some or all or which may be configured as a keyswitch reader, which automatically scans and debounces a matrix of up to 32 switches.

Included on chip are full 14- and 16-segment ASCII 104-character fonts, a hexadecimal font for 7-segment displays, multiplex scan circuitry, anode and cathode drivers, and static RAM that stores each digit. The maximum segment current for the display digits is set using a single external resistor. Digit intensity can be independently adjusted using the 16-step internal digital brightness control. The MAX6954 includes a low-power shutdown mode, a scan-limit register that allows the user to display from 1 to 16 digits, segment blinking (synchronized across multiple drivers, if desired), and a test mode, which forces all LEDs on. The LED drivers are slew-rate limited to reduce EMI.

For a 2-wire interfaced version, refer to the MAX6955 data sheet. An evaluation kit (EV kit) for the MAX6955 is available.

Applications

Set-Top Boxes Automotive

Panel Meters Bar Graph Displays

White Goods Audio/Video Equipment

Features

♦ High-Speed 26MHz SPI/QSPI/MICROWIRE™-

Compatible Serial Interface

♦ 2.7V to 5.5V Operation

♦ Drives Up to 16 Digits 7-Segment, 8 Digits

14-Segment, 8 Digits 16-Segment, 128 Discrete LEDs, or a Combination of Digit Types

♦ Drives Common-Cathode Monocolor and Bicolor

LED Displays

♦ Built-In ASCII 104-Character Font for 14-Segment

and 16-Segment Digits and Hexadecimal Font for 7-Segment Digits

♦ Automatic Blinking Control for each Segment

♦ 10µA (typ) Low-Power Shutdown (Data Retained)

♦ 16-Step Digit-by-Digit Digital Brightness Control

♦ Display Blanked on Power-Up

♦ Slew-Rate Limited Segment Drivers for Lower EMI

♦ Five GPIO Port Pins Can Be Configured as Key-

Switch Reader to Scan and Debounce Up to 32 Switches with n-Key Rollover

♦ IRQ Output when a Key Input Is Debounced

♦ 36-Pin SSOP and 40-Pin DIP Packages

♦ Automotive Temperature Range Standard

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display

Driver with I/O Expander and Key Scan

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

19-2460; Rev 2; 8/05

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

PART

TEMP RANGE

PIN-

PKG CODE

MAX6954AAX

36 SSOP A36-2

MAX6954APL

40 PDIP P40-2

SPI and QSPI are trademarks of Motorola, Inc.

MICROWIRE is a trademark of National Semiconductor Corp.

Pin Configurations and Typical Operating Circuits appear at end of data sheet.

ISET

OSC

OSC_OUT

BLINK

CLK

DIN

DOUT

4-WIRE SERIAL INTERFACE

RAM

BLINK

CONTROL

CONFIGURATION

CHARACTER GENERATOR

ROM

CURRENT

SOURCE

DIVIDER/ COUNTER NETWORK

DIGIT

MULTIPLEXER

PWM

BRIGHTNESS

CONTROL

GPIO

AND KEY-SCAN

CONTROL

LED

DRIVERS

O0 TO O23

P0 TO P4

MAX6954

Functional Diagram

PACKAGE

-40°C to +125°C

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

(Voltage with respect to GND.)

V+ .........................................................................-0.3V to +6V

All Other Pins............................................-0.3V to (V+ + 0.3V)

Current

O0–O7 Sink Current ......................................................935mA

O0–O18 Source Current .................................................55mA

DIN, CLK,

CS, OSC, DOUT, BLINK, OSC_OUT, ISET ....20mA

P0, P1, P2, P3, P4 ...........................................................40mA

GND .....................................................................................1A

Continuous Power Dissipation (T

= +70°C)

36-Pin SSOP (derate at 11.8mW/°C above +70°C) .....941mW

40-Pin PDIP (derate at 16.7mW/°C above +70°C).....1333mW

Operating Temperature Range

MIN

to T

MAX

) ...............................................-40°C to +125°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +150°C

Lead Temperature (soldering, 10s) .................................+300°C

DC ELECTRICAL CHARACTERISTICS

(Typical Operating Circuits, V+ = 2.7V to 5.5V, TA= T

MIN

to T

MAX

, unless otherwise noted.) (Note 1)

PARAMETER

CONDITIONS

UNITS

Operating Supply Voltage V+ 2.7 5.5 V

TA = +25°C 10 35

Shutdown Supply Current I

SHDN

Shutdown mode, all digital inputs at V+ or GND

= T

MIN

to T

MAX

µA

TA = +25°C 22 30

Operating Supply Current I+

All segments on, all digits scanned, intensity set to full, internal oscillator, DOUT open circuit, no display or OSC_OUT load connected

= T

MIN

to T

MAX

OSC = RC oscillator, R

SET

= 56kΩ,

SET

= 22pF, V+ = 3.3V

Master Clock Frequency f

OSC

OSC driven externally 1 8

MHz

Dead Clock Protection Frequency

OSC

95 kHz

OSC Internal/External Detection Threshold

OSC

1.7 V

OSC High Time t

50 ns

OSC Low Time t

50 ns

Slow Segment Blink Period

OSC = RC oscillator, R

SET

= 56kΩ,

SET

= 22pF, V+ = 3.3V

Fast Segment Blink Period

OSC = RC oscillator, R

SET

= 56kΩ,

SET

= 22pF, V+ = 3.3V

0.5 s

Fast or Slow Segment Blink Duty Cycle

SYMBOL

MIN TYP MAX

SLOWBLINK

FASTBLINK

49.5 50.5

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display

Driver with I/O Expander and Key Scan

_______________________________________________________________________________________ 3

DC ELECTRICAL CHARACTERISTICS (continued)

(Typical Operating Circuits, V+ = 2.7V to 5.5V, TA= T

MIN

to T

MAX

, unless otherwise noted.) (Note 1)

PARAMETER

CONDITIONS

UNITS

Segment Drive Source Current I

SEG

LED

= 2.2V,

V+

= 3.3V

= +25°C

-40

Segment Current Slew Rate

TA = +25°C, V+ = 3.3V 11

mA/µs

Segment Drive Current Matching

∆I

SEGTA

= +25°C, V+ = 3.3V 5 10 %

LOGIC INPUTS AND OUTPUTS

Input Leakage Current DIN, CLK, CS, OSC, P0, P1, P2, P3, P4

, I

-1 +1 µA

4-Wire Logic-High Input Voltage DIN, CLK, CS

IHSPI

1.8 V

4-Wire Logic-Low Input Voltage DIN, CLK, CS

ILSPI

0.6 V

Port Logic-High Input Voltage P0, P1, P2, P3, P4

IHP

0.7 x V+

Port Logic-Low Input Voltage P0, P1, P2, P3, P4

ILP

0.3 x V+

Port Hysteresis Voltage P0, P1, P2, P3, P4

∆V

0.03 x V+

Port Input Pullup Current from V+

IPU

P0 to P3 configured as keyscan input, V+ =

3.3V

75 µA

Port Output Low Voltage V

OLP

SINK

= 8mA 0.3 0.5 V

Blink Output Low Voltage V

OLBKISINK

= 0.6mA 0.1 0.3 V

DOUT Output High Voltage

SOURCE

= 1.6mA

V+ -

0.2

DOUT Output Low Voltage

SINK

= 1.6mA 0.2 V

OSC_OUT Output High Voltage

SOURCE

= 1.6mA

V+ -

0.4

OSC_OUT Output Low Voltage

SINK

= 1.6mA 0.4 V

SYMBOL

∆I

SEG

OHDO

OLDO

OHOSC

OLOSC

/∆t

MIN TYP MAX

-34.5

-46.5

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan

4 _______________________________________________________________________________________

Typical Operating Characteristics

(V+ = 3.3V, LED forward voltage = 2.4V, typical application circuit, TA= +25°C, unless otherwise noted.)

TIMING CHARACTERISTICS

(Typical Operating Circuits, V+ = 2.7V to 5.5V, TA= T

MIN

to T

MAX

, unless otherwise noted.) (Note 1)

PARAMETER

CONDITIONS

UNITS

TIMING CHARACTERISTICS

CLK Clock Period t

CLK Pulse Width High t

16 ns

CLK Pulse Width Low t

16 ns

CS Fall to CLK Rise Setup Time t

CSS

9.5 ns

CLK Rise to CS Rise Hold Time t

CSH

0ns

DIN Setup Time t

9.5 ns

DIN Hold Time t

0ns

V+ = 3.0V to 5.5V 19

Output Data Propagation Delay t

V+ = 2.7V 25

DOUT Output Rise and Fall Times t

LOAD

= 10pF, V+ = 3.0V to 5.5V 10

Minimum CS Pulse High t

CSW

Note 1: All parameters tested at TA= +25°C. Specifications over temperature are guaranteed by design.

INTERNAL OSCILLATOR FREQUENCY

vs. TEMPERATURE

MAX6954 toc01

TEMPERATURE (°C)

OSCILLATOR FREQUENCY (MHz)

110805020-10

3.8

4.0

4.2

4.4

3.6

-40

SET

= 56kΩ

SET

= 22pF

INTERNAL OSCILLATOR FREQUENCY

vs. SUPPLY VOLTAGE

MAX6954 toc02

SUPPLY VOLTAGE (V)

OSCILLATOR FREQUENCY (MHz)

5.04.54.03.53.0

3.8

4.2

4.4

3.6

2.5 5.5

SET

= 56kΩ

SET

= 22pF

4.0

100ns/div

OSC: 500mV/div

OSC_OUT: 2V/div

MAX6954 toc03

OSC

OSC_OUT

INTERNAL OSCILLATOR WAVEFORM

AT OSC AND OSC_OUT PINS

SET

= 56k

Ω

SET

= 22pF

SYMBOL

MIN TYP MAX

38.4

19.5

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display

Driver with I/O Expander and Key Scan

_______________________________________________________________________________________ 5

Typical Operating Characteristics (continued)

(V+ = 3.3V, LED forward voltage = 2.4V, typical application circuit, TA= +25°C, unless otherwise noted.)

DEAD CLOCK OSCILLATOR FREQUENCY

vs. SUPPLY VOLTAGE

MAX6954 toc04

SUPPLY VOLTAGE (V)

OSCILLATOR FREQUENCY (kHz)

5.04.54.03.53.0

105

110

2.5 5.5

SET

= 56k

Ω

OSC = GND

100

SEGMENT SOURCE CURRENT

vs. SUPPLY VOLTAGE

MAX6954 toc05

SUPPLY VOLTAGE (V)

CURRENT NORMALIZED TO 40mA

5.04.54.03.53.0

0.94

0.98

1.00

1.02

0.92

2.5 5.5

LED

= 1.8V

0.96

1V/div

200µs/div

MAX6954 toc06

O18

WAVEFORM AT PINS O0 AND O18,

MAXIMUM INTENSITY

GPIO SINK CURRENT

vs. TEMPERATURE

MAX6954 toc07

TEMPERATURE (°C)

GPIO SINK CURRENT (mA)

11050 8020-10

-40

VCC = 3.3V

VCC = 2.5V

VCC = 5.5V

OUTPUT = LOW V

PORT

= 0.6V

PORT INPUT PULLUP CURRENT

vs. TEMPERATURE

MAX6954 toc08

TEMPERATURE (°C)

KEY-SCAN SOURCE CURRENT (mA)

11050 8020-10

0.05

0.10

0.15

0.20

0.25

0.30

-40

VCC = 3.3V

VCC = 2.5V

VCC = 5.5V

OUTPUT = HIGH V

PORT

= 1.4V

400µs/div

KEY_A: 1V/div

IRQ: 2V/div

MAX6954 toc09

KEY_A

IRQ

KEYSCAN OPERATION

(KEY_A AND IRQ)

MAX6954

Detailed Description

The MAX6954 is a serially interfaced display driver that can drive up to 16 digits 7-segment, 8 digits 14-segment, 8 digits 16-segment, 128 discrete LEDs, or a combination of these display types. Table 1 shows the drive capability of the MAX6954 for monocolor and bicolor displays.

The MAX6954 includes 104-character ASCII font maps for 14-segment and 16-segment displays, as well as the hexadecimal font map for 7-segment displays. The characters follow the standard ASCII font, with the addition of the following common symbols: £, A€ , ¥, °, µ, ±, ↑, and ↓. Seven bits represent the 104-character font

map; an 8th bit is used to select whether the decimal point (DP) is lit. Seven-segment LED digits may be controlled directly or use the hexadecimal font. Direct segment control allows the MAX6954 to be used to drive bar graphs and discrete LED indicators.

Tables 2, 3, and 4 list the connection schemes for 16-, 14-, and 7-segment digits, respectively. The letters in Tables 2, 3, and 4 correspond to the segment labels shown in Figure 1. (For applications that require mixed display types, see Tables 37–40.)

Serial Interface

The MAX6954 communicates through an SPI-compatible 4-wire serial interface. The interface has three

4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan

6 _______________________________________________________________________________________

Pin Description

SSOP

PDIP

NAME FUNCTION

1, 2,

34, 35, 36

1, 2,

P0–P4

General-Purpose I/O Ports (GPIOs). GPIO can be configured as logic inputs or open-drain outputs. Enabling key scanning configures some or all ports P0–P3 as key-switch matrix inputs with internal pullup and port P4 as IRQ output.

33CS

Chip-Select Input. Serial data is loaded into the shift register while CS is low. The most recent 16 bits of data latch on CS’s rising edge.

44DOUT

Serial-Data Output. The data into DIN is valid at DOUT 15.5 clock cycles later. Use this pin to daisy-chain several devices or allow data readback. Output is push-pull.

55CLK

Serial-Clock Input. On CLK’s rising edge, data shifts into the internal shift register. On CLK’s falling edge, data is clocked out of DOUT. CLK is active only while CS is low.

66DIN

Serial-Data Input. Data from DIN loads into the internal 16-bit shift register on CLK’s rising edge.

7–15,

22–31

7–15,

26–35

Digit/Segment Drivers. When acting as digit drivers, outputs O0 to O7 sink current from the display common cathodes. When acting as segment drivers, O0 to O18 source current to the display anodes. O0 to O18 are high impedance when not being used as digit or segment drivers.

16, 18

GND Ground

17 19 ISET

Segment Current Setting. Connect ISET to GND through series resistor R

SET

to set the peak

current.

19, 21

V+

Positive Supply Voltage. Bypass V+ to GND with a 47µF bulk capacitor and a 0.1µF ceramic capacitor.

20 22 OSC

Multiplex Clock Input. To use internal oscillator, connect capacitor C

SET

from OSC to GND.

To use external clock, drive OSC with a 1MHz to 8MHz CMOS clock.

32 36 BLINK Blink Clock Output. Output is open drain.

33 37

Clock Output. OSC_OUT is a buffered clock output to allow easy blink synchronization of multiple MAX6954s. Output is push-pull.

— 16, 25 N.C. Not Connected Internally

38, 39, 40

O0–O18

17, 18, 20

21, 23, 24

OSC_OUT

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display

Driver with I/O Expander and Key Scan

_______________________________________________________________________________________ 7

inputs: clock (CLK), chip select (CS), and data in (DIN), and one output, data out (DOUT). CS must be low to clock data into or out of the device, and DIN must be stable when sampled on the rising edge of CLK. DOUT is stable on the rising edge of CLK. Note that while the SPI protocol expects DOUT to be high impedance when the MAX6954 is not being accessed, DOUT on the MAX6954 is never high impedance.

CLK and DIN may be used to transmit data to other peripherals. The MAX6954 ignores all activity on CLK and DIN except when CS is low.

Control and Operation Using the 4-Wire Interface

Controlling the MAX6954 requires sending a 16-bit word. The first byte, D15 through D8, is the command, and the second byte, D7 through D0, is the data byte (Table 5).

Connecting Multiple MAX6954s to the 4-Wire Bus

Multiple MAX6954s may be daisy-chained by connecting the DOUT of one device to the DIN of the next, and driving CLK and CS lines in parallel (Figure 2). Data at DIN propagates through the internal shift registers and appears at DOUT 15.5 clock cycles later, clocked out on the falling edge of CLK. When sending commands to daisy-chained MAX6954s, all devices are accessed at the same time. An access requires (16 x n) clock cycles, where n is the number of MAX6954s connected

together. To update just one device in a daisy-chain, the user can send the no-op command (0x00) to the others. Figure 3 is the MAX6954 timing diagram.

The MAX6954 is written to using the following sequence:

1) Take CLK low.

2) Take CS low. This enables the internal 16-bit shift

3) Clock 16 bits of data into DIN, D15 first to D0 last, observing the setup and hold times. Bit D15 is low, indicating a write command.

4) Take CS high (while CLK is still high after clocking in the last data bit).

5) Take CLK low.

Figure 4 shows a write operation when 16 bits are transmitted.

If fewer or greater than 16 bits are clocked into the MAX6954 between taking CS low and taking CS high again, the MAX6954 stores the last 16 bits received, including the previous transmission(s). The general case is when n bits (where n > 16) are transmitted to the MAX6954. The last bits are comprising bits {n-15} to {n}, are retained, and are parallel loaded into the 16-bit latch as bits D15 to D0, respectively (Figure 5).

DISPLAY TYPE

7 SEGMENT

(16-CHARACTER

HEXADECIMAL FONT)

14 SEGMENT/

16 SEGMENT

(104-CHARACTER ASCII FONT MAP)

DISCRETE LEDs

(DIRECT CONTROL)

Monocolor 16 8 128

Bicolor 8 4 64

Table 1. MAX6954 Drive Capability

1dp

2dp

g1 g2

dp dp

1e 1c

2f 2b

2e 2c

g1 g2

Figure 1. Segment Labeling for 7-Segment Display, 14-Segment Display, and 16-Segment Display

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan

8 _______________________________________________________________________________________

Reading Device Registers

Any register data within the MAX6954 may be read by sending a logic high to bit D15. The sequence is:

1) Take CLK low.

2) Take CS low. This enables the internal 16-bit shift

3) Clock 16 bits of data into DIN, D15 first to D0 last.

D15 is high, indicating a read command and bits D14 through D8 contain the address of the register

DIGIT

O18

Table 2. Connection Scheme for Eight 16-Segment Digits

Table 3. Connection Scheme for Eight 14-Segment Digits

D IG IT *

O0O1O2O3O4O5O6O7O8O9O10

O11

O12

O13

O14

O15

O16

O17

O18

0, 0a

2dp

1, 1a

2dp

2, 2a

2dp

3, 3a

2dp

4, 4a

2dp

5, 5a

2dp

6, 6a

2dp

7, 7a

2dp

Table 4. Connection Scheme for Sixteen 7-Segment Digits

D15

D14

D13

D12

D11

D10D9D8D7D6D5D4D3D2D1D0

R/W

ADDRESS

DATA

LSB

Table 5. Serial-Data Format (16 Bits)

*Each cathode driver output (CC0-CC7) connects to two digit common cathode pins.

O0 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15 O16 O17

CCO — a1 a2 b c d1 d2 e

— CC1 a1 a2 b c d1 d2 e

a1 a2 CC2 — b c d1 d2 e

a1 a2 — CC3 b c d1 d2 e

a1 a2 b c CC4 — d1 d2 e

a1 a2 b c — CC5 d1 d2 e

a1 a2 b c d1 d2 CC6 — e

a1 a2 b c d1 d2 — CC7 e

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—CC1a—b c d—e fg1 g2 h i j k l m dp

2a—CC2 — b c d — e f g1 g2 h i j k l m dp

3a——CC3 b c d — e f g1 g2 h i j k l m dp

4a—bcCC4 — d — e f g1 g2 h i j k l m dp

5a—bc—CC5 d — e f g1 g2 h i j k l m dp

6a—bcd—CC6 — e f g1 g2 h i j k l m dp

7a—bcd——CC7 e f g1 g2 h i j k l m dp

g1 g2 h

C C 0— 1a — 1b 1c 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g

— CC1 1a — 1b 1c 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g

1a — CC2 — 1b 1c 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g

1a — — CC3 1b 1c 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g

1a — 1b 1c CC4 — 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g

1a — 1b 1c — CC5 1d 1dp 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g

1a — 1b 1c 1d 1dp CC6 — 1e 1f1g2a2b2c2d2e 2f2g

1a — 1b 1c 1d 1dp — CC7 1e 1f 1g 2a 2b 2c 2d 2e 2f 2g

MSB

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display

Driver with I/O Expander and Key Scan

_______________________________________________________________________________________ 9

to read. Bits D7 to D0 contain dummy data, which is discarded.

4) Take CS high (while CLK is still high after clocking in

the last data bit), positions D7 through D0 in the shift register are now loaded with the register data addressed by bits D15 through D8.

5) Take CLK low.

6) Issue another read or write command (which can be a no-op), and examine the bit stream at DOUT; the second 8 bits are the contents of the register addressed by bits D14 through D8 in step 3.

Digit Type Registers

The MAX6954 uses 32 digit registers to store the characters that the user wishes to display. These 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. The digit registers

are mapped so that a digit’s data can be updated in plane P0, plane P1, or both planes at the same time (Table 6).

If the blink function is disabled through the Blink Enable Bit E (Table 19) 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 (Table 20).

The data in the digit registers does not control the digit segments directly for 14- and 16-segment displays. Instead, the register data is used to address a character generator that stores the data for the 14- and 16segment fonts (Tables 7 and 8). The lower 7 bits of the digit data (D6 to D0) select the character from the font.

CSS

CSH

CSW

D15

CLK

DIN

D14 D1 D0

D15

DOUT

Figure 3. Timing Diagram

MAX6954

DOUT

MICROCONTROLLER

CLK

DIN

MAX6954

CLK

DIN

DOUT

CLK

DIN

DOUT

CLK

DIN

DOUT

Figure 2. MAX6954 Daisy-Chain Connection

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan

10 ______________________________________________________________________________________

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 zero to leave DP unlit (Table 9).

For 7-segment displays, the digit plane data register can be used to address a character generator, which contains the data of a 16-character font containing the hexadecimal font. The decode mode register can be used to disable the character generator and allow the segments to be controlled directly. Table 10 shows the one-to-one pairing of each data bit to the appropriate segment line in the digit plane data registers. The hexadecimal font is decoded according to Table 11.

The digit-type register configures the display driver for various combinations of 14-segment digits, 16-segment digits, and/or pairs, or 7-segment digits. The function of this register is to select the appropriate font for each digit and route the output of the font to the appropriate MAX6954 driver output pins. The MAX6954 has four digit drive slots. A slot can be filled with various combinations of monocolor and bicolor 16-segment displays, 14-segment displays, or two 7-segment displays. Each pair of bits in the register corresponds to one of the four

digit drive slots, as shown in Table 12. Each bit also corresponds to one of the eight common-cathode digit drive outputs, CC0 to CC7. When using bicolor digits, the anode connections for the two digits within a slot are always the same. This means that a slot correctly drives two monocolor or one bicolor 14- or 16-segment digit. The digit type register can be written, but cannot be read. Examples of configuration settings required for some display digit combinations are shown in Table 13.

7-Segment Decode-Mode Register

In 7-segment mode, the hexadecimal font can be disabled (Table 14). The decode-mode register selects between hexadecimal code or direct control for each of eight possible pairs of 7-segment digits. Each bit in the register corresponds to one pair of digits. The digit pairs are {digit 0, digit 0a} through {digit 7, digit 7a}. Disabling decode mode allows direct control of the 16 LEDs of a dual 7-segment display. Direct control mode can also be used to drive a matrix of 128 discrete LEDs.

CLK

DIN

D15 D14

D13

D12

D11 D10

D4 D3 D2

DOUT

D15 = 0

Figure 4. Transmission of 16 Bits to the MAX6954

CLK

DIN

BIT1BIT

N-9 N-8 N-7 N-6 N-5 N-4 N-3 N-2

DOUT

N-15

N-15 N-14 N-13 N-12 N-11 N-10 N-1

N-31 N-30 N-29 N-28 N-27 N-26 N-25 N-24

N-23

N-22 N-21 N-20 N-19

N-18

N-17 N-16

Figure 5. Transmission of More than 16 Bits to the MAX6954

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display

Driver with I/O Expander and Key Scan

______________________________________________________________________________________ 11

A logic high selects hexadecimal decoding, while a logic low bypasses the decoder. When direct control is selected, the data bits D7 to D0 correspond to the segment lines of the MAX6954. Write x0010000 to blank all segments in hexadecimal decode mode.

Display Blink Mode

The display blinking facility, when enabled, makes the driver flip automatically between displaying the digit register data in planes P0 and P1. If the digit register data for any digit is different in the two planes, then that digit appears to flip between two characters. To make a character appear to blink on or off, write the character to one plane, and use the blank character (0x20) for the other plane. Once blinking has been configured, it continues automatically without further intervention.

Blink Speed

The blink speed is determined by the frequency of the multiplex clock, OSC, and by the setting of the Blink Rate Selection Bit B (Table 18) in the configuration register. The Blink Rate Selection Bit B sets either fast or slow blink speed for the whole display.

Initial Power-Up

On initial power-up, all control registers are reset, the display is blanked, intensities are set to minimum, and shutdown is enabled (Table 15).

Configuration Register

The configuration register is used to enter and exit shutdown, select the blink rate, globally enable and disable the blink function, globally clear the digit data, select between global or digit-by-digit control of intensity, and reset the blink timing (Tables 16–19 and 21–24).

The configuration register contains 7 bits:

•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).

Character Generator Font Mapping

The font is composed of 104 characters in ROM. The lower 7 bits of the 8-bit digit register represent the character selection. The most significant bit, shown as x in the ROM map of Tables 7 and 8, is 1 to light the DP segment and zero to leave the DP segment unlit.

The character map follows the standard ASCII font for 96 characters in the x0101000 through x1111111 range. The first 16 characters of the 16-segment ROM map cover 7-segment displays. These 16 characters are numeric 0 to 9 and characters A to F (i.e., the hexadecimal set).

Multiplex Clock and Blink Timing

The OSC pin can be fitted with capacitor C

SET

to GND to use the internal RC multiplex oscillator, or driven by an external clock to set the multiplex clock frequency and blink rate. The multiplex clock frequency determines the frequency that the complete display is updated. With OSC at 4MHz, each display digit is enabled for 200µs.

The internal RC oscillator uses an external resistor, R

SET

, and an external capacitor, C

SET

, to set the oscil-

lator frequency. The suggested values of R

SET

(56kΩ)

and C

SET

(22pF) set the oscillator at 4MHz, which

makes the blink frequency 0.5Hz or 1Hz.

The external clock is not required to have a 50:50 duty cycle, but the minimum time between transitions must be 50ns or greater and the maximum time between transitions must be 750ns.

The on-chip oscillator may be accurate enough for applications using a single device. If an exact blink rate is required, use an external clock ranging between 1MHz and 8MHz to drive OSC. The OSC inputs of multiple MAX6954s can be tied together to a common external clock to make the devices blink at the same rate. The relative blink phasing of multiple MAX6954s can be synchronized by setting the T bit in the control register for all the devices in quick succession. If the serial interfaces of multiple MAX6954s are daisy-chained by connecting the DOUT of one device to the DIN of the next, then synchronization is achieved automatically by updating the configuration register for all devices simultaneously. Figure 6 is the multiplex timing diagram.

OSC_OUT Output

The OSC_OUT output is a buffered copy of either the internal oscillator clock or the clock driven into the OSC pin if the external clock has been selected. The feature is useful if the internal oscillator is used, and the user wishes to synchronize other MAX6954s to the same blink frequency. The oscillator is disabled while the MAX6954 is in shutdown.

MAX6954

4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan

12 ______________________________________________________________________________________

Scan-Limit Register

The scan-limit register sets how many 14-segment digits or 16-segment digits or pairs of 7-segment digits are displayed, from 1 to 8. A bicolor digit is connected as two monocolor digits. The scan register also limits the number of keys that can be scanned.

Since the number of scanned digits affects the display brightness, the scan-limit register should not be used to blank portions of the display (such as leading-zero suppression). Table 25 shows the scan-limit register format.

Intensity Registers

Digital control of display brightness is provided and can be managed in one of two ways: globally or individually. Global control adjusts all digits together. Individual control adjusts the digits separately.

The default method is global brightness control, which is selected by clearing the global intensity bit (I data bit D6) in the configuration register. This brightness setting applies to all display digits. The pulse-width modulator is then set by the lower nibble of the global intensity register, address 0x02. The modulator scales the average segment current in 16 steps from a maximum of 15/16 down to 1/16 of the peak current. The minimum interdigit blanking time is set to 1/16 of a cycle. When using bicolor digits, 256 color/brightness combinations are available.

Individual brightness control is selected by setting the global intensity bit (I data bit D6) in the configuration register. The pulse-width modulator is now no longer set by the lower nibble of the global intensity register, address 0x02, and the data is ignored. Individual digital control of display brightness is now provided by a separate pulse-width modulator setting for each digit. Each digit is controlled by a nibble of one of the four intensity registers: intensity10, intensity32, intensity54, and intensity76 for all display types, plus intensity10a, intensity32a, intensity54a, and intensity76a for the extra eight digits possible when 7-segment displays are used. The data from the relevant register is used for each digit as it is multiplexed. The modulator scales the average segment current in 16 steps in exactly the same way as global intensity adjustment.

Table 26 shows the global intensity register format, Table 27 shows individual segment intensity registers, Table 28 is the even individual segment intensity format, and Table 29 is the odd individual segment intensity format.

GPIO and Key Scanning

The MAX6954 feature five general-purpose input/output (GPIO) ports: P0 to P4. These ports can be individually enabled as logic inputs or open-drain logic outputs. The GPIO ports are not debounced when configured as inputs. The ports can be read and the outputs set using the 4-wire interface.

Some or all of the five ports can be configured to perform key scanning of up to 32 keys. Ports P0 to P4 are renamed Key_A, Key_B, Key_C, Key_D, and IRQ, respectively, when used for key scanning. The full keyscanning configuration is shown in Figure 7. Table 30 is the GPIO data register.

One diode is required per key switch. These diodes can be common-anode dual diodes in SOT23 packages, such as the BAW56. Sixteen diodes would be required for the maximum 32-key configuration.

The MAX6954 can only scan the maximum 32 keys if the scan-limit register is set to scan the maximum eight digits. If the MAX6954 is driving fewer digits, then a maximum of (4 x n) switches can be scanned, where n is the number of digits set in the scan-limit register. For example, if the MAX6954 is driving four 14-segment digits cathode drivers O0 to O3 are used. Only 16 keys can be scanned in this configuration; the switches shown connected to O4 through O7 are not read.

If the user wishes to scan fewer than 32 keys, then fewer scan lines can be configured for key scanning. The unused Key_x ports are released back to their original GPIO functionality. If key scanning is enabled, regardless of the number of keys being scanned, P4 is always configured as IRQ (Table 31).

The key-scanning circuit utilizes the LEDs’ commoncathode driver outputs as the key-scan drivers. O0 to 07 go low for nominally 200µs (with OSC = 4MHz) in turn as the displays are multiplexed. By varying the oscillator frequency, the debounce time changes, though key scanning still functions. Key_x inputs have internal pullup resistors that allow the key condition to be tested. The Key_x input is low during the appropriate digit multiplex period when the key is pressed. The timing diagram of Figure 8 shows the normal situation where all eight LED cathode drivers are used.

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