LM80 datasheet(12/29 Pages) NSC

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Functional Description (Continued)

3.0 USING THE LM80

3.1 Power On

When power is first applied, the LM80 performs a “power on

reset” on several of its registers. The power on condition of

registers is shown in

Table 1. Registers whose power on val-

ues are not shown have power on conditions that are inde-

terminate (this includes the value RAM and WATCHDOG

limits). The ADC is inactive. In most applications, usually the

first action after power on would be to write WATCHDOG lim-

its into the Value RAM.

3.2 Resets

Configuration Register INITIALIZATION accomplishes the

same function as power on reset. The Value RAM conver-

sion results, and Value RAM WATCHDOG limits are not Re-

set and will be indeterminate immediately after power on. If

the Value RAM contains valid conversion results and/or

Value RAM WATCHDOG limits have been previously set,

they will not be affected by a Configuration Register INITIAL-

IZATION. Power on reset, or Configuration Register INITIAL-

IZATION, clear or initialize the following registers (the initial-

ized values are shown in

Table 1):

Configuration Register

Interrupt Status Register 1

Interrupt Status Register 2

Interrupt Mask Register 1

Interrupt Mask Register 2

Fan Divisor/RST_OUT/OS Register

OS Configuration/Temperature Resolution Register

Value Ram (Registers at Address 20h - 3Fh, which include:

Temperature reading, IN0-IN6 readings, FAN1 and FAN2

readings, and WATCHDOG limits)

Configuration Register INITIALIZATION is accomplished by

setting Bit 7 of the Configuration Register high. This Bit auto-

matically clears after being set.

The LM80 can be reset to it’s “power on state” by taking

NTEST_IN/Reset_IN pin low for at least 50 ns.

3.3 Using the Configuration Register

The Configuration Register provides all control over the

LM80. At power on, the ADC is stopped and INT_Clear is as-

serted, clearing the INT and RST_OUT/OS hardwire out-

puts. The Configuration Register starts and stops the LM80,

enables and disables INT outputs, clears and sets CI and

GPO I/O pins, initiates reset pulse on RST_OUT/OS pin, and

provides the Reset function described in

Section 3.2.

Bit 0 of the Configuration Register controls the monitoring

loop of the LM80. Setting Bit 0 low stops the LM80 monitor-

ing loop and puts the LM80 in shutdown mode, reducing

power consumption. Serial Bus communication is possible

with any register in the LM80 although activity on these lines

will increase shutdown current, up to as much as maximum

rated supply current, while the activity takes place. Taking Bit

0 high starts the monitoring loop, described in more detail

subsequently.

Bit 1 of the Configuration Register enables the INT Interrupt

hardwire output when this bit is taken high.

Bit 2 of the Configuration Register defines whether the INT

pin is open source or open drain.

Bit 3 clears the INT output when taken high. The LM80 moni-

toring function will stop until bit 3 is taken low. The content of

the Interrupt (INT) Status Registers will not be affected.

Bit4, when taken high, will initiate a 10 ms RESET signal on

the RST_OUT/OS output (when this pin is in the RST mode).

When bit 5 is taken high the CI (Chassis Intrusion) pin is re-

set.

Bit 6 of the configuration register sets or clears the GPO out-

put. This pin can be used in software power control by acti-

vating an external power control MOSFET.

3.4 Starting Conversions

3.4 STARTING CONVERSION The monitoring function

(Analog inputs, temperature, and fan speeds) in the LM80 is

started by writing to the Configuration Register and setting

INT_Clear (Bit 3), low, and Start (Bit 0), high. The LM80 then

performs a round-robin monitoring of all analog inputs, tem-

perature, and fan speed inputs approximately once a sec-

ond. If the temperature resolution is set to 12 bits one com-

plete monitoring function will take approximately 2 seconds.

The sequence of items being monitored corresponds to loca-

tions in the Value RAM (except for the Temperature reading)

and is:

Temperature

IN0

IN1

IN2

IN3

IN4

IN5

IN6

Fan 1

10. Fan 2

3.5 Reading Conversion Results

The conversion results are available in the Value RAM. Con-

versions can be read at any time and will provide the result

of the last conversion. Because the ADC stops, and starts a

new conversion whenever the conversion is read, reads of

any single value should not be done more often than once

every 120 ms. When reading all values with the temperature

resolution set to 9-bits, allow at least 1.5 seconds between

reading groups of values. Reading more frequently than

once every 1.5 seconds can also prevent complete updates

of Interrupt Status Registers and Interrupt Outputs. If the

temperature resolution is set to 12-bit, allow at least 2.0 sec-

onds between reading groups of values.

A typical sequence of events upon power on of the LM80

would consist of:

Set WATCHDOG Limits

Set Interrupt Masks

Start the LM80 monitoring process

4.0 ANALOG INPUTS

The 8-bit ADC has a 10 mV LSB, yielding a 0V to 2.55V

(2.56 - 1LSB) input range. This is true for all analog inputs. In

PC monitoring applications these inputs would most often be

connected to power supplies. The 2.5, 3.3, ± 5 and ± 12 volt

inputs should be attenuated with external resistors to any de-

sired value within the input range. Care should be taken not

to exceed the power supply voltage (V

+) at any time.

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