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

28-Port I/O Expander

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7

Serial Interface

Serial Addressing

The MAX7300 operates as a slave that sends and

receives data through an I

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 MAX7300, and generates the SCL clock that

synchronizes the data transfer (Figure 2).

The MAX7300 SDA line operates as both an input and

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

Ω,

is required on SDA. The MAX7300 SCL line operates

only as an input. A pullup resistor, typically 4.7k

Ω, 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 MAX7300

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

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 MAX7300, the

MAX7300 generates the acknowledge bit since the

Table 1. Port Configuration Map

REGISTER DATA

REGISTER

ADDRESS

CODE (HEX)

D7

D6

D5

D4

D3

D2

D1

D0

Port Configuration for P7, P6, P5, P4

0x09

P7

P6

P5

P4

Port Configuration for P11, P10, P9, P8

0x0A

P11

P10

P9

P8

Port Configuration for P15, P14, P13, P12

0x0B

P15

P14

P13

P12

Port Configuration for P19, P18, P17, P16

0x0C

P19

P18

P17

P16

Port Configuration for P23, P22, P21, P20

0x0D

P23

P22

P21

P20

Port Configuration for P27, P26, P25, P24

0x0E

P27

P26

P25

P24

Port Configuration for P31, P30, P29, P28

0x0F

P31

P30

P29

P28

Table 2. Port Configuration Matrix

PORT

CONFIGURATION

BIT PAIR

MODE

FUNCTION

PORT

REGISTER

PIN BEHAVIOR

ADDRESS

CODE (HEX)

UPPER

LOWER

DO NOT USE THIS SETTING

0x09 to 0x0F

0

0

Written Low

Active-low logic output

Output

GPIO Output

Written High

Active-high logic output

0x09 to 0x0F

0

1

Input

GPIO Input

without Pullup

Reading Port

Schmitt logic output

0x09 to 0x0F

1

0

Input

GPIO Input with Pullup

Reading Port

Schmitt logic input with pullup

0x09 to 0x0F

1

1