MCP9805T-BESLASHST datasheet(17/34 Pages) MICROCHIP

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DS21977B-page 17

MCP9805

4.2

Sensor Feature Description

4.2.1

SHUTDOWN MODE

Shutdown

mode

disables

all

power-consuming

activities (including temperature sampling operations)

while leaving the serial interface active. This mode is

selected by setting bit 8 of CONFIG to ‘1’. In this mode,

the device consumes ISHDN. It remains in this mode

until bit 8 is cleared ‘0’ to enable Continuous

Conversion mode, or until power is recycled.

The Shutdown bit (bit 8) cannot be set to ‘1’ while bits

6 and 7 of CONFIG (Lock bits) are set to ‘1’. However,

it can be cleared ‘0’ or returned to Continuous

Conversion while locked.

In Shutdown mode, all registers can be read or written.

However, the serial bus activity increases the shutdown

current. In addition, if the device is shutdown while the

Event pin is asserted as active-low or deasserted

active-low (see Section 4.2.3.1 “Comparator Mode”

and Section 4.2.3.2 “Interrupt Mode”), the device will

retain

the

active-low

state.

This

increases

the

shutdown current due to the additional Event output

pull-down current.

4.2.2

TEMPERATURE HYSTERESIS

(THYST)

A hysteresis of 0°C, 1.5°C, 3°C or 6°C can be selected

for the TUPPER, TLOWER and TCRIT temperate bound-

aries using bits 10 and 9 of CONFIG. The hysteresis

applies for decreasing temperature only (hot to cold), or

as temperature drifts below the specified limit.

The TUPPER, TLOWER and TCRIT boundary conditions

are described graphically in Figure 4-2.

4.2.3

EVENT OUTPUT CONFIGURATION

The Event output can be enabled using bit 3 of

CONFIG (Event output control bit) and can be

configured as either a comparator output or as Interrupt

Output mode using bit 0 of CONFIG (Event mode). The

polarity can also be specified as an active-high or

active-low using bit 1 of CONFIG (Event polarity).

When the ambient temperature increases above the

critical temperature limit, the Event output is forced to a

comparator output (regardless of bit 0 of CONFIG).

When the temperature drifts below the critical temper-

ature limit, the Event output automatically returns to the

state specified by bit 0 of CONFIG.

The status of the Event output can be read using bit 4

of CONFIG (Event status).

Bit 7 and 6 of the CONFIG register can be used to lock

the TUPPER, TLOWER and TCRIT registers. The bits

prevent false triggers at the Event output due to an

accidental rewrite to these registers.

The Event output can also be used as a critical temper-

ature output using bit 2 of CONFIG (critical output

only). When this feature is selected, the Event output

becomes a comparator output. In this mode, the

interrupt output configuration (bit 0 of CONFIG) is

ignored.

4.2.3.1

Comparator Mode

Comparator mode is selected using bit 0 of CONFIG. In

this mode, the Event output is asserted as active-high

or active-low using bit 1 of CONFIG. Figure 4-2 shows

the conditions that toggle the Event output.

If the device enters Shutdown mode with asserted

Event output, the output remains asserted during

Shutdown. The device must be operating in Continu-

ous Conversion mode for tCONV; the TA vs. TUPPER,

TLOWER and TCRIT boundary conditions need to be

satisfied in order for the Event output to deassert.

Comparator mode is useful for thermostat-type applica-

tions, such as turning on a cooling fan or triggering a

system shutdown when the temperature exceeds a

safe operating range.

4.2.3.2

Interrupt Mode

Interrupt mode is selected using bit 0 of CONFIG. In

this mode, the Event output is asserted as active-high

or active-low using bit 1 of CONFIG. The output is

deasserted by setting ‘1’ to bit 5 of CONFIG (interrupt

clear). Shutting down the device will not reset or deas-

sert the Event output. However, clearing the interrupt

using bit 5 of CONFIG while in Shutdown mode will

deassert the Event output.

This mode is ignored when the Event output is used as

critical temperature output only (bit 2 of CONFIG).

Interrupt mode applies to interrupt-driven, microcon-

troller-based systems. The microcontroller receiving

the interrupt will have to acknowledge the interrupt by

setting ‘1’ to bit 5 of CONFIG.

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