MAX6956ATL+T Datasheet MAX6956AAX+, MAX6956ATL+, MAX6956AAI+ | P7-P9

Shutdown mode is temporarily overridden by the dis-

play test function.

Serial Interface

Serial Addressing

The MAX6956 operates as a slave that sends and

receives data through an I

C-compatible 2-wire inter-

face. The interface uses a serial data line (SDA) and a

serial clock line (SCL) to achieve bidirectional commu-

nication between master(s) and slave(s). A master (typ-

ically a microcontroller) initiates all data transfers to and

from the MAX6956, and generates the SCL clock that

synchronizes the data transfer (Figure 2).

The MAX6956 SDA line operates as both an input and

an open-drain output. A pullup resistor, typically 4.7kΩ,

is required on SDA. The MAX6956 SCL line operates

only as an input. A pullup resistor, typically 4.7kΩ, is

required on SCL if there are multiple masters on the 2-

wire interface, or if the master in a single-master system

has an open-drain SCL output.

Each transmission consists of a START condition

(Figure 3) sent by a master, followed by the MAX6956

7-bit slave address plus R/W bit (Figure 6), a register

address byte, one or more data bytes, and finally a

STOP condition (Figure 3).

Start and Stop Conditions

Both SCL and SDA remain high when the interface is

not busy. A master signals the beginning of a transmis-

sion with a START (S) condition by transitioning SDA

from high to low while SCL is high. When the master

MAX6956

2-Wire-Interfaced, 2.5V to 5.5V, 20-Port or

28-Port LED Display Driver and I/O Expander

_______________________________________________________________________________________________________ 7

SLAVE ADDRESS BYTE

D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15

R/W

CEDATA

SDA

SCL

TEST REGISTER

INTENSITY REGISTERS

PORT REGISTERS

LED DRIVERS AND GPIO

INTENSITY

CONFIGURATION

TEST

P4 TO P31

LED DRIVERS

OR GPIO

ADDRESS

MATCHER

AD0

AD1

COMMAND BYTEDATA BYTE

R/W7-BIT DEVICE ADDRESS

TO COMMAND REGISTERS

TO/FROM DATA REGISTERS

SEGMENT OR

GPIO DATA

R/W

CONFIGURATION

REGISTERS

PORT CHANGE

DETECTOR

MASK REGISTER

COMMAND

DATA BYTE COMMAND BYTE

MAX6956

Figure 1. MAX6956 Functional Diagram

MAX6956

has finished communicating with the slave, it issues a

STOP (P) condition by transitioning SDA from low to

high while SCL is high. The bus is then free for another

transmission (Figure 3).

Bit Transfer

One data bit is transferred during each clock pulse.

The data on SDA must remain stable while SCL is high

(Figure 4).

Acknowledge

The acknowledge bit is a clocked 9th bit, which the

recipient uses to handshake receipt of each byte of

data (Figure 5). Thus, each byte transferred effectively

requires 9 bits. The master generates the 9th clock

pulse, and the recipient pulls down SDA during the

acknowledge clock pulse, such that the SDA line is sta-

ble low during the high period of the clock pulse. When

the master is transmitting to the MAX6956, the

MAX6956 generates the acknowledge bit because the

2-Wire-Interfaced, 2.5V to 5.5V, 20-Port or

28-Port LED Display Driver and I/O Expander

8 _______________________________________________________________________________________

Figure 2. 2-Wire Serial Interface Timing Details

SCL

SDA

START CONDITIONSTOP CONDITION

REPEATED START CONDITION

START CONDITION

SU, DAT

HD, DAT

LOW

HD, STA

HIGH

SU, STA

HD, STA

SU, STO

BUF

Figure 3. Standard Stop Conditions

SDA

SCL

START

CONDITION

STOP

CONDITION

SDA

SCL

DATA LINE STABLE;

DATA VALID

CHANGE OF DATA ALLOWED

Figure 4. Bit Transfer

MAX6956 is the recipient. When the MAX6956 is trans-

mitting to the master, the master generates the

acknowledge bit because the master is the recipient.

Slave Address

The MAX6956 has a 7-bit-long slave address (Figure 6).

The eighth bit following the 7-bit slave address is the

R/W bit. It is low for a write command, high for a read

command.

The first 3 bits (MSBs) of the MAX6956 slave address

are always 100. Slave address bits A3, A2, A1, and A0

are selected by address inputs, AD1 and AD0. These

two input pins may be connected to GND, V+, SDA, or

SCL. The MAX6956 has 16 possible slave addresses

(Table 3) and therefore, a maximum of 16 MAX6956

devices may share the same interface.

Message Format for Writing

the MAX6956

A write to the MAX6956 comprises the transmission of

the MAX6956’s slave address with the R/W bit set to

zero, followed by at least 1 byte of information. The first

byte of information is the command byte. The com-

mand byte determines which register of the MAX6956

is to be written by the next byte, if received. If a STOP

condition is detected after the command byte is

received, then the MAX6956 takes no further action

(Figure 8) beyond storing the command byte.

Any bytes received after the command byte are data

bytes. The first data byte goes into the internal register of

the MAX6956 selected by the command byte (Figure 9). If

multiple data bytes are transmitted before a STOP condi-

tion is detected, these bytes are generally stored in subse-

quent MAX6956 internal registers because the command

byte address generally autoincrements (Table 4).

Message Format for Reading

The MAX6956 is read using the MAX6956’s internally

stored command byte as address pointer, the same

way the stored command byte is used as address

pointer for a write. The pointer generally autoincre-

ments after each data byte is read using the same rules

as for a write (Table 4). Thus, a read is initiated by first

configuring the MAX6956’s command byte by perform-

MAX6956

2-Wire-Interfaced, 2.5V to 5.5V, 20-Port or

28-Port LED Display Driver and I/O Expander

_______________________________________________________________________________________ 9

SCL

SDA

BY TRANSMITTER

CLOCK PULSE FOR ACKNOWLEDGMENT

START CONDITION

SDA

BY RECEIVER

12 89

Figure 5. Acknowledge

SDA

SCL

1 0 A3 A2 A1 A00

MSB

LSB

R/W

ACK

Figure 6. Slave Address

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24

MAX6956ATL+T

Mfr. #:

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LED Display Drivers 2.5-5.5V 20/28Port LED Display Driver

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MAX6956ATL+T

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