LM81 datasheet(15/33 Pages) NSC

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Functional Description (Continued)

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 0.82 seconds

between reading groups of values. Reading more frequently

than once every 0.82 seconds can also prevent complete

updates of Interrupt Status Registers and Interrupt Output’s.

A typical sequence of events upon power on of the LM81

would consist of:

Set WATCHDOG Limits

Set Interrupt Masks

Start the LM81 monitoring process

3.6 Digital Communication Noise Considerations

The SMBData and SMBCLK logic input levels were changed

in the SMBus 1.1 specification. SMBus 1.0 levels were set to

1.4V for a logic high and 0.6V for a logic low. In SMBus 1.1

they were changed to 2.1V for a logic high and 0.8V for a

logic low. Devices that meet the SMBus 1.0 specification

have issues in that the logic levels did not allow for enough

noise immunity for some pcb layouts. This has changed with

the SMBus 1.1 specification because the higher logic levels

allow for more hysteresis in the schmitt trigger inputs stages

and thus more noise immunity. It may be required in some

cases to add a series 5.1k

Ω resistor connected at the SM-

BCLK pin of the LM81C to improve its noise immunity. In

addition to meeting the SMBus 1.1 logic levels, the LM81B

has a built-in glitch filter that rejects 100MHz or greater to

make it impervious to noise.

4.0 ANALOG INPUTS

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

All analog inputs, except for Vccp1 and Vccp2, include inter-

nal 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

2.5 Vin

13 mV

2.5V

3.320V

3.3 Vin

17.2 mV

3.3V

4.383V

5 Vin

26 mV

6.641V

12 Vin

62.5 mV

12V

15.93V

Vccp1, Vccp2

14.1 mV

2.7V

3.586V

Thus monitoring power supplies within a system can be

easily accomplished by tying the Vccp, +2.5 Vin, +3.3 Vin,

+5 Vin and +12 Vin analog inputs to the corresponding

system supply. A digital reading can be converted to a volt-

age 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. Vccp inputs do not have resistor attenuators and

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

larger input impedance.

A negative power supply voltage can be applied to a Vccp

input through a resistor divider referenced to a known posi-

tive DC voltage as show in

Figure 6. The resistor values

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

provide approximately 1.25V at the Vccp analog inputs of the

LM81 for a nominal reading of 89.

Voltage

Measure-

ments

Voltage

Analog In-

puts

(ADC code

89)

−12V

40 k

Ω

141 k

Ω

+5V

+1.25V

−5V

40 k

Ω

66.7

Ω

+5V

+1.25V

The resistors were selected by setting R2 = 40 k

Ω and then

calculating R2 using the following equation:

R1 = [(1.25V − V

S)÷(V

+ − 1.25V)] x 40 k

Ω

The maximum R1 can be is restricted by the DC input

current of a Vccp input.

Inputs with internal resistor dividers (+2.5 Vin, +3.3 Vin or

+5 Vin, +12 Vin) can have voltage applied that exceeds the

power supply up to: 3.6V for +2.5 Vin, 4.6V for +3.3 Vin, 6.8V

for +5 Vin, and 15V for +12 Vin. The Vccp inputs have a

parasitic diode to the positive supply, so care should be

taken not to forward bias this diode. All analog inputs have

internal diodes that clamp the input voltage when going

below ground thus limiting the negative analog input voltage

range to −50 mV. Violating the analog input voltage range of

any analog input has no detrimental effect on the other

analog inputs. External resistors should be included to limit

input currents to the values given in the ABSOLUTE MAXI-

MUM RATINGS for Input Current At Any Pin whenever ex-

ceeding the analog input voltage range, even on an

un-powered LM81. Inputs with external attenuator networks

will usually meet these requirements. If it is possible for

inputs without attenuators (such as Vccp1 and Vccp2) to be

turned on while LM81 is powered off, additional resistors of

about 10 k

Ω should be added in series with the inputs to limit

the input current.

4.1 Analog Input Interrupts

A WATCHDOG window comparison on the analog inputs

can activate the INT interrupt output. A converted input volt-

age that is above its respective HIGH limit or less than or

DS100072-11

FIGURE 6. Input Examples. Resistor values shown in

table provide approximately 1.25V at the Vccp inputs.

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