LM87 datasheet(14/32 Pages) NSC

14 / 32 page

Functional Description (Continued)

Bit 2 of the Channel Mode Register, when set to 0, config-

ures pins 18 and 19 as +2.5V and V

CCP2 voltage inputs.

When set to 1, pins 18 and 19 are configured as a second

remote temperature sensing channel.

Bit 3 of the Channel Mode Register, when set to 0, sets the

nominal voltage for internal V

measurement to 3.3V.

When set to 1, the nominal V

CC range is 5V.

Bit 4 of the Channel Mode Register, when set to 1, enables

pin 24 as an active low (IRQ0) interrupt input. When set to 0,

this input is disabled as an IRQ interrupt.

Bit 5 of the Channel Mode Register, when set to 1, enables

pin 23 as an active low (IRQ1) interrupt input. When set to 0,

this input is disabled as an IRQ interrupt.

Bit 6 of the Channel Mode Register, when set to 1, enables

pin 22 as an active low (IRQ2) interrupt input. When set to 0,

this input is disabled as an IRQ interrupt.

Bit 7 of the Channel Mode Register, when set to 1, config-

ures pins 20 to 24 as interrupt inputs. When set to 0, pins 20

to 24 are configured as processor voltage ID pins.

3.4 Starting Conversions

The monitoring function (Analog inputs, temperature, and

fan speeds) in the LM87 is started by writing to Configuration

high. The LM87 then performs a “round-robin” monitoring of

all analog inputs, temperature, and fan speed inputs approxi-

mately once every 0.3 s. The sequence of items being

monitored is:

Check D1 connections

Check D2 connections

Internal Temperature

External D1 Temperature

External D2 Temperature

+2.5V

+Vccp1

Vcc 3.3V

Vcc 5.0V

10. +5Vin

11. +12Vin

12. +Vccp2

13. AIN1

14. AIN2

15. Fan 1

16. Fan 2

DACOut immediately changes after the DAC Data Register

in the Value RAM has been updated. For a zero to full scale

transition DACOut will typically settle within 100 µsec of the

stop by master in the write to the DAC Data Register Serial

Bus transaction. The DAC Data Register is not reset by the

INITIALIZATION bit found in the Configuration Register.

3.5 Reading Conversion Results

The conversion results are available in the Value RAM.

Conversions 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 it is read, reads of any

single value should not be done more often than once every

56 ms. When reading all values, allow at least 0.6 seconds

between reading groups of values. Reading more frequently

than once every 0.6 seconds can also prevent complete

updates of Interrupt Status Registers and Interrupt Output’s.

A typical sequence of events upon power on of the LM87

would consist of:

Set WATCHDOG Limits

Set Interrupt Masks

Start the LM87 monitoring process

4.0 ANALOG INPUTS

All analog input voltages are digitized to 8-bits of resolution.

For safety purposes, and to provide maximum accuracy, a

510

Ω resistor should be placed in series with all analog

voltage inputs. The resistors will limit the possible current

drawn from the power supplies in the event that circuit board

traces are bridged, or accidentally shorted during test. All

analog inputs, except for AIN1 and AIN2, include internal

resistor attenuators. The theoretical LSB size, theoretical

voltage input required for an ADC reading of 192 (3/4 scale)

and 255 (full scale) for each analog input is detailed in the

table below:

Input

LSB size

Vin for 192 Vin for 255

2.5Vin

13 mV

2.5 V

3.320 V

3.3Vcc

17.2 mV

3.3 V

4.383 V

5Vin/Vcc

26 mV

5 V

6.641 V

12Vin

62.5 mV

12 V

15.93 V

Vccp1, Vccp2

14.1 mV

2.7 V

3.586 V

AIN1/AIN2

9.8 mV

1.875 V

2.49 V

Thus monitoring power supplies within a system can be

easily accomplished by tying the Vccp, +2.5Vin, +5Vin and

+12Vin analog inputs to the corresponding system supply.

Vcc of the LM87 will also be monitored. A digital reading can

be converted to a voltage by simply multiplying the decimal

value of the reading by the LSB size.

For inputs with attenuators the input impedance is greater

than 90 k

Ω. AIN inputs do not have resistor attenuators and

are directly tied to the ADC, therefore having a much larger

input impedance.

A negative power supply voltage can be applied to a AIN

input through a resistor divider referenced to a known posi-

tive DC voltage as shown in Figure 6. The resistor values

shown in the table below for the circuit of Figure 6 will

provide approximately 1.25 V at the AIN analog inputs of the

LM87 for a nominal reading of 128.

Voltage

Measurements

Voltage

Analog

Inputs

( ADC code

128)

−12V

20 k

Ω

130 k

Ω

+3.3 V

+1.25 V

−5V

20 k

Ω 61.0 kΩ +3.3 V

+1.25 V

LM87 Arkusz danych(PDF) 14 Page - National Semiconductor (TI)

LM87 Arkusz danych(HTML) 14 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Arkusz danych Pobierz

Link URL