MIC74_06 datasheet(13/18 Pages) MICREL

13 / 18 page

Micrel, Inc.

MIC74

October 2006

M9999-101006

Initializing the MIC74

The MIC74’s internal registers are reset to their default

state at power-on. The MIC74’s default state can be

summarized as follows:

•

All I/O’s configured as inputs (DIR = 00h)

•

Output configuration set to open-

drain(OUT_CFG = 00h)

•

All outputs high/floating (DATA = FFh)

•

Fan functions disabled (FAN_SPEED =

00h,FAN bit of DEV_CFG = 0)

•

All interrupts masked (IE bit of DEV_CFG = 0)

The result of this configuration is that all I/O pins will

essentially float unless driven by external circuitry. Any

system using the MIC74 will need to initialize the internal

registers to the state required for proper system

operation. The recommended order for initializing the

MIC74’s registers is as follows:

Write desired output

values to DATA

Set output configuration

in OUT_CFG

Set desired I/O's as

outputs by writing DIR

Set initial fan speed in

FAN_SPEED

(if using)

Read STATUS

to clear it

Write DEV_CFG to

turn on fan (if using)

Initialize for

polling

Initialization

complete

Figure 9a. Initializing the MIC74 for Polled Operation

1. Write DATA

2. Write OUT_CFG

3. Write DIR

4. Write FAN_SPEED (if using fan mode)

5. Write INT_MASK (if using interrupts)

6. Read STATUS to clear it.

7. Write DEV_CFG to enable fan mode and/or

interrupts, if using

At the conclusion of step three, any I/O’s configured as

outputs in step two will be driven to the levels

programmed into the data register in step one. The order

of step 1 through step 3 is important to insure that

spurious data does not appear at the I/O’s during

configuration. Following step 7, programming the device

configuration register, the MIC74 will begin generating

interrupts if they are enabled, and the fan will be started

if FAN_SPEED contains a nonzero value. The corresp-

onddding interrupt service routines (if any) must be

initialized and enabled prior to step seven. STATUS

should be cleared (step 6) in both polled and interrupt

driven systems.

Write desired ouput

values to DATA

Set output configuration

in OUT_CFG

Set desired I/O's as

outputs by writing DIR

Set initial fan speed in

FAN_SPEED

(if using)

Write INT_MASK to

enable interrupts

(if using)

Read STATUS to

clear it

Write DEV_CFG to

turn on interrupts

and fan (if using)

Initialize for

interrupts

Initialization

complete

Figure 9b. Initializing the MIC74 for Interrupts

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