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

Table 7. Register Address Map (continued)

REGISTER

ADDRESS (COMMAND BYTE)

D15 D14 D13 D12 D11 D10 D9 D8

Write Digit 2a Planes P0 and P1 with Same

Data (7 Segment Only), Reads as 0x00

X

1

1

0

1

0

1

0

Write Digit 3a Planes P0 and P1 with Same

Data (7 Segment Only), Reads as 0x00

X

1

1

0

1

0

1

1

Write Digit 4a Planes P0 and P1 with Same

Data (7 Segment Only), Reads as 0x00

X

1

1

0

1

1

0

0

Write Digit 5a Planes P0 and P1 with Same

Data (7 Segment Only), Reads as 0x00

X

1

1

0

1

1

0

1

Write Digit 6a Planes P0 and P1 with Same

Data (7 Segment Only), Reads as 0x00

X

1

1

0

1

1

1

0

Write Digit 7a Planes P0 and P1 with Same

X

1

1

0

1

1

1

1

Data (7 Segment Only), Reads as 0x00

Note: Unused register bits read as zero.

HEX CODE

0x6A

0x6B

0x6C

0x6D

0x6E

0x6F

SDA

SCL

DATA LINE STABLE, CHANGE OF DATA

DATA VALID

ALLOWED

START CONDITION

SCL

CLOCK PULSE FOR ACKNOWLEDGMENT

1

2

8

9

SDA

BY TRANSMITTER

S

SDA

BY RECEIVER

Figure 5. Bit Transfer

One diode is required per key switch. Note that the for-

ward voltages of the diode and LED must exceed VIH

of P0–P3. If this condition is not met, the voltage input

to the port might be lower than the logic threshold and

keys will not be detected properly.

The MAX6955 can only scan the maximum 32 keys if

the scan-limit register is set to scan the maximum eight

digits. If the MAX6955 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 MAX6955 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 orig-

Figure 6. Acknowledge

inal GPIO functionality. If key scanning is enabled,

regardless of the number of keys being scanned,

P4/IRQ is always configured as IRQ (Table 32).

The key-scanning circuit utilizes the LEDs’ common-

cathode driver outputs as the key-scan drivers. O0 to

O7 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 appropri-

ate digit multiplex period when the key is pressed. The

timing diagram of Figure 12 shows the normal situation

where all eight LED cathode drivers are used.

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