MIC74 datasheet(7/20 Pages) MICREL

7 / 20 page

August 1, 2000

MIC74

Micrel

Functional Description

Pin Descriptions

VDD

Power supply input connection. See “Operating Ratings.”

GND

Ground or return connection for all MIC74 functions.

CLK

An CLK signal is provided by the host (master) and is

common to all devices on the bus. The CLK signal controls all

transactions in both directions on the bus and is applied to

each MIC74 at the CLK pin.

DATA

Serial data is bidirectional and is common to all devices on the

bus. The MIC74’s DATA output is open-drain.

The DATA line requires one external pull-up resistor or

current source per system that can be located anywhere

along the line.

A2, A1, A0

An MIC74 responds to its own unique address which is

assigned using the A0–A2 pins. A0–A2 set the three LSBs

(least significant bits) of the MIC74’s 7-bit slave address. The

three address pins allow eight unique MIC74 addresses in a

system. When the MIC74’s address matches an address

received in the serial bit stream, communication is initiated.

A2, A1 and A0 should be connected to GND or V

DD. The state

of these pins is sampled only once at device power-on. New

slave addresses are not accepted unless the MIC74 is

powered off then on.

Functional Diagram

GND

VDD

n (typical I/O port)

EDGE

DETECT

INT

STATUS

STATUS_READ

DATA

(INPUT)

OUT_CFG

DIR

DATA

(OUTPUT)

Typical I/O Port (Fan Speed Control Logic Not Shown)

000

001

010

011

100

101

110

111

Table 1. MIC74 Address Configuration

Alert Response Address

The MIC74 also responds to the ARA (Alert Response

Address). The ARA is used by the master (host) to request the

address of a slave that has provided an interrupt to the master

via the /ALERT line.

The ARA is a single address (0001 100) common to all slaves

and is described in more detail under “Interrupt Generation”

with related information under “/ALERT.” Also see Figure 7.

Pn, /SHDN, and /FS0–/FS2

P0 through P7 are general-purpose input/output bits. Each bit

is independently programmable as an input or an output. If

programmed as an output, each bit is further programmable

as either a complementary push-pull or open-drain output.

If properly enabled, any P

n programmed as an input will

generate an interrupt to the host using the /ALERT output

when the input changes state. In this way, the MIC74 can

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