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

Key Debounced Registers

The Key_A debounced, Key_B debounced, Key_C

debounced, and Key_D debounced read-only registers

(Table 35) show which keys have been detected as

debounced by the key-scanning circuit.

Each bit in the register corresponds to one key switch.

The bit is set if the switch has been correctly

debounced since the register was read last. Reading a

debounced register clears that register (after the data

has been read) so that future keys pressed can be

identified. If the debounced registers are not read, the

key-scan data accumulates. However, as there is no

FIFO in the MAX6955, the user is not able to determine

key order, or whether a key has been pressed more

than once, unless the debounced key status registers

are read after each interrupt, and before the next key-

scan cycle.

Reading any of the four debounced registers clears the

P4/IRQ output. If a key is pressed and held down, the

key is reported as debounced (and IRQ issued) only

once. The key must be detected as released by the key-

scanning circuit, before it debounces again. If the

debounced registers are being read in response to the

P4/IRQ being asserted, then the user should generally

read all four registers to ensure that all the keys that were

detected by the key-scanning circuit are discovered.

Key Pressed Registers

The Key_A pressed, Key_B pressed, Key_C pressed,

and Key_D pressed read-only registers (Table 36)

show which keys have been detected as pressed by

the key-scanning circuit during the last test.

Each bit in the register corresponds to one key switch.

The bit is set if the switch has been detected as

pressed by the key-scanning circuit during the last test.

The bit is cleared if the switch has not been detected

as pressed by the key-scanning circuit during the last

test. Reading a pressed register does not clear that

register or clear the P4/IRQ output.

Display Test Register

The display test register (Table 37) operates in two

modes: normal and display test. Display test mode

turns all LEDs on (including DPs) by overriding, but not

altering, all controls and digit registers (including the

shutdown register), except for the digit-type register

and the GPIO configuration register. The duty cycle,

while in display test mode, is 7/16 (see the Choosing

Supply Voltage to Minimize Power Dissipation section).

External Components RSET and

CSET to Set Oscillator Frequency and

Peak Segment Current

The RC oscillator uses an external resistor, RSET, and

an external capacitor, CSET, to set the frequency, fOSC.

The allowed range of fOSC is 1MHz to 8MHz. RSET also

sets the peak segment current. The recommended val-

ues of RSET and CSET set the oscillator to 4MHz, which

makes the blink frequencies selectable between 0.5Hz

and 1Hz. The recommended value of RSET also sets the

peak current to 40mA, which makes the segment cur-

rent adjustable from 2.5mA to 37.5mA in 2.5mA steps.

ISEG = KL/RSET mA

fOSC = KF/(RSET x CSET) MHz

where:

KL = 2240

KF = 10K (typ)

RSET = external resistor in kΩ

CSET = external capacitor in pF

CSTRAY = stray capacitance from OSC pin to GND in pF

The recommended value of RSET is 56kΩ and the rec-

ommended value of CSET is 22pF.

The recommended value of RSET is the minimum

allowed value, since it sets the display driver to the

maximum allowed peak segment current. RSET can be

set to a higher value to set the segment current to a

lower peak value where desired. The user must also

ensure that the peak current specifications of the LEDs

connected to the driver are not exceeded.

The effective value of CSET includes not only the actual

external capacitor used, but also the stray capacitance

from OSC to GND. This capacitance is usually in the

1pF to 30pF range, depending on the layout used.

Applications Information

Driving Bicolor LEDs

Bicolor digits group a red and a green die together for

each display element, so that the element can be lit red

or green (or orange), depending on which die (or both)

is lit. The MAX6955 allows each segment’s current to

be set individually from the 1/16th (minimum current

and LED intensity) to 15/16th (maximum current and

LED intensity), as well as off (zero current). Thus, a

bicolor (red-green) segment pair can be set to 256

color/intensity combinations.

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