MCP9808 datasheet(30/52 Pages) MICROCHIP

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MCP9808

DS25095A-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 to ‘0’ to enable Continuous

Conversion mode or until power is recycled.

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

CONFIG<7:6> bits (Lock bits) are set to ‘1’. However, it

can be cleared to ‘0’ or returned to Continuous

Conversion mode 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 (CONFIG<7:6) are set to ‘1’.

The TUPPER, TLOWER and TCRIT boundary conditions

are described graphically in Figure 5-10.

5.2.3

ALERT OUTPUT CONFIGURATION

The Alert output can be enabled by using bit 3 of the

CONFIG register (Alert Output Control bit) and can be

configured as either a comparator output or as an

Interrupt Output mode using bit 0 of CONFIG (Alert

Output Mode bit). The polarity can also be specified as

active-high or active-low using bit 1 of CONFIG (Alert

Polarity bit). 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 CONFIG<0>). When

the temperature drifts below the critical temperature

limit minus hysteresis, the Alert output automatically

returns to the state specified by CONFIG<0> bit.

FIGURE 5-9:

Active-Low Alert Output

Configuration.

The status of the Alert output can be read using

CONFIG<4> (Alert Output Status bit). This bit can not

be set to ‘1’ in Shutdown mode.

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

the TUPPER, TLOWER and TCRIT registers. These 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 temper-

ature output using bit 2 of CONFIG (Alert Output Select

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

becomes a comparator output. In this mode, the

interrupt output configuration (Alert Output Mode bit,

CONFIG<0>) 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 5-10 shows

the conditions that toggle the Alert output.

If the device enters Shutdown mode with asserted Alert

output, the output remains asserted during Shutdown

mode. The device must be operating in 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 is 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.

MCP9808

Alert Output

RPU

VDD

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