August 1, 2000

1

MIC74

MIC74

Micrel

MIC74

2-Wire Serial I/O Expander and Fan Controller

Advance Information

General Description

The MIC74 is a fully programmable serial-to-parallel I/O

expander compatible with the SMBus™ (system manage-

ment bus) protocol. It acts as a “slave” on the bus, providing

eight independent I/O lines.

Each I/O bit can be individually programmed as an input or

output. If programmed as an output, each I/O bit can be

programmed as an open-drain or complementary push-pull

output. If desired, the four most significant I/O bits can be

programmed to implement fan speed control. An internal

clock generator and state machine eliminate the overhead

generally associated with “bit-banging” fan speed control.

Programming the device and reading/writing the I/O bits is

accomplished using seven internal registers. All registers can

be read by the host. Output bits are capable of directly driving

high-current loads such as LEDs. A separate interrupt output

can notify the host of state changes on the input bits without

requiring the MIC74 to perform a transaction on the serial bus

or be polled by the host. Three address selection inputs are

provided, allowing up to eight devices to share the same bus

and provide a total of 64 bits of I/O.

The MIC74 is available in an ultra-small-footprint 16-lead

QSOP. Low quiescent current, small footprint, and low pack-

age height make the MIC74 ideal for portable and desktop

applications.

Typical Application

R1

LED1

LED8

R9

R2

R3

R4

R5

R6

R7

R8

3.0V

3.0V

VDD

/ALERT

DATA

CLK

ALERT

DATA

CLK

A0

A1

A2

GND

P0

P1

P2

P3

P4

P5

P6

P7

MIC74

Serial-Bus-Controlled LED Annunciator

Features

• Provides eight bits of general purpose I/O

• Built in fan speed control logic (optional)

plus interrupt output

• 2.7V to 3.6V operating voltage range

• 5V-tolerant I/O

• Low quiescent current: 2

µA (typical)

• Bit-programmable I/O options:

input or output

push-pull or open-drain output

interrupt on input changes

• Outputs can directly drive LEDs (10mA I

• Up to 8 devices per bus

Applications

• General purpose I/O expansion via serial bus

• Personal computer system management

• Distributed sensing and control

• Microcontroller I/O expansion

• Fan Control

Ordering Information

Part Number

Temperature Range

Package

MIC74BQS

–40

°C to +85°C

16-lead QSOP

Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com