MCP9804-ESLASHMS datasheet(30/54 Pages) MICROCHIP

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MCP9804

DS22203A-page 30

5.2

SENSOR FEATURE DESCRIPTION

5.2.1

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 in shutdown while

the Alert pin is asserted, the device will retain the active

state

during shutdown. This increases the shutdown

current due to the additional Alert output current.

5.2.2

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

boundaries 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 hysteresis bits can not be changed if either of the

lock bits, bits 6 and 7 of CONFIG, are set to ‘1’.

The TUPPER, TLOWER and TCRIT boundary conditions

are described graphically in Figure 5-11.

5.2.3

ALERT OUTPUT CONFIGURATION

The Alert output can be enabled using bit 3 of CONFIG

(Alert output control bit) and can be configured as

either a comparator output or as Interrupt Output mode

using bit 0 of CONFIG (Alert mode). The polarity can

also be specified as an active-high or active-low using

bit 1 of CONFIG (Alert polarity). This is an open drain

output and requires a pull-up resistor.

When the ambient temperature increases above the

critical temperature limit, the Alert output is forced to a

comparator output (regardless of bit 0 of CONFIG).

When

the

temperature

drifts

below

the

critical

temperature limit minus hysteresis, the Alert output

automatically returns to the state specified by bit 0 of

CONFIG.

FIGURE 5-10:

Active-Low Alert Output

Configuration.

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

CONFIG (Alert status). This bit can not be set to ‘1’ in

shutdown mode.

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 Alert output due to an

accidental rewrite to these registers.

The Alert output can also be used as a critical

temperature output using bit 2 of CONFIG (critical

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

output becomes a comparator output. In this mode, the

interrupt output configuration (bit 0 of CONFIG) is

ignored.

5.2.3.1

Comparator Mode

Comparator mode is selected using bit 0 of CONFIG. In

this mode, the Alert output is asserted as active-high or

active-low using bit 1 of CONFIG. Figure 5-11 shows

the conditions that toggle the Alert output.

If the device enters Shutdown mode with asserted Alert

output, the output remains asserted during Shutdown.

The

device

must

operating

Continuous

Conversion mode for tCONV; the TA vs. TUPPER,

TLOWER and TCRIT boundary conditions need to be

satisfied in order for the Alert output to deassert.

Comparator

mode

useful

for

thermostat-type

applications, such as turning on a cooling fan or

triggering a system shutdown when the temperature

exceeds a safe operating range.

5.2.3.2

Interrupt Mode

In the Interrupt mode, the Alert output is asserted as

active-high or active-low (depending on the polarity

configuration) when TA drifts above or below TUPPER

and TLOWER limits. The output is deasserted by setting

bit 5 (Interrupt Clear) of CONFIG. Shutting down the

device will not reset or deassert the Alert output. This

mode can not be selected when the Alert output is used

as critical temperature output only, using bit 2 of CONFIG.

This mode is designed for interrupt driven microcontroller

based systems. The microcontroller receiving the

interrupt will have to acknowledge the interrupt by setting

bit 5 of CONFIG register from the MCP9804.

MCP9804

Alert Output

RPU

VDD

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