MAX6955 데이터 시트보기 (PDF) - Maxim Integrated

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MAX6955

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

Maxim Integrated 

MAX6955

2-Wire Interfaced, 2.7V to 5.5V

LED Display Driver with I/O

Expander and Key Scan

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 hexa-

decimal set).

Multiplex Clock and Blink Timing

The OSC pin can be fitted with capacitor CSET 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,

RSET, and an external capacitor, CSET, to set the oscil-

lator frequency. The suggested values of RSET (56kΩ)

and CSET (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 multi-

ple MAX6955s can be connected to a common external

clock to make the devices blink at the same rate. The

relative blink phasing of multiple MAX6955s can be syn-

chronized by setting the T bit in the control register for

all the devices in quick succession. If the serial inter-

faces of multiple MAX6955s are daisy-chained by con-

necting the DOUT of one device to the DIN of the next,

then synchronization is achieved automatically by

updating the configuration register for all devices simul-

taneously. Figure 10 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 MAX6955s to the same

blink frequency. The oscillator is disabled while the

MAX6955 is in shutdown.

Scan-Limit Register

The scan-limit register sets how many 14-segment dig-

its 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 sup-

pression). Table 26 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 individ-

ually. 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 aver-

age 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 sep-

arate 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 inten-

sity76 for all display types, plus intensity10a, intensi-

ty32a, 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 27 shows the global intensity register format. Table

28 shows individual segment intensity registers. Table 29

shows the even individual segment intensity format. Table

30 shows the odd individual segment intensity format.

GPIO and Key Scanning

The MAX6955 features five general-purpose input/out-

put (GPIO) ports: P0 to P4. These ports can be individ-

ually enabled as logic inputs or open-drain logic

outputs. The GPIO ports are not debounced when con-

figured as inputs. The ports can be read and the out-

puts set using the 2-wire interface.

Some or all of the five ports can be configured to per-

form 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 key-

scanning configuration is shown in Figure 11. Table 31

is the GPIO data register.

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