MIC74_06 Datasheet, PDF (13/18 Pages) Micrel Semiconductor – 2-Wire Serial I/O Expander and Fan Controller

Micrel, Inc.
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:
MIC74
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.
Initialize for
interrupts
Initialize for
polling
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)
Write DEV_CFG to
turn on fan (if using)
Read STATUS
to clear it
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)
Initialization
complete
Figure 9a. Initializing the MIC74 for Polled Operation
October 2006
13
Initialization
complete
Figure 9b. Initializing the MIC74 for Interrupts
M9999-101006