MCP9844 datasheet(29/44 Pages) MICROCHIP

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 2013 Microchip Technology Inc.

DS20005192A-page 29

MCP9844

5.2.3.1

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 5-9 shows

the conditions that toggle the event output.

If the device enters Shutdown mode with asserted

event output, the output will de-assert. It will remain de-

asserted until the device enters Continuous Conver-

sion mode and after the first temperature conversion is

completed, tCONV. After the initial temperature conver-

sion, TA must satisfy the TUPPER or TLOWER boundary

conditions in order for event output to be asserted.

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.

5.2.3.2

Interrupt Mode

In Interrupt mode, the event 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 de-asserted by setting

bit 5 (Interrupt Clear) of CONFIG. If the device enters

Shutdown mode with asserted event output, the output

will de-assert. It will remain de-asserted until the device

enters Continuous Conversion mode and after the first

temperature conversion is completed, tCONV. If the inter-

rupt clear bit (Bit 5) is never set, then the event output will

re-assert after the first temperature conversion.

In addition, if TA >= TCRIT, the event output is forced as

Comparator mode and asserts until TA < TCRIT - THYST.

While the event output is asserted, the user must send a

Clear Interrupt command (bit 5 of CONFIG) for the event

output to de-assert when the temperature drops below

the critical limit, TA < TCRIT - THYST. Otherwise, the event

output remains asserted (see Figure 5-9 for a graphical

description). Switching from Interrupt mode to Compara-

tor mode also de-asserts event output.

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 the CONFIG register from the MCP9844.

5.2.4

TEMPERATURE RESOLUTION

The MCP9844 device is capable of providing tempera-

ture data with 0.5°C to 0.0625°C resolution. The

Resolution can selected using the Resolution register

(Register 5-8)

which

located

address

‘00001001’b. This address location is not specified in

JEDEC Standard JC42.4. However, it provides

additional flexibility while being functionally compatible

with JC42.4 and provides a 0.25°C resolution at

125 ms (max.). In order to prevent accidentally chang-

ing the resolution and exceeding the 125 ms conver-

sion time, the device must be in Shutdown mode to

change this register. The selected resolution can be

read by the user using bit 4 and bit 3 of the Capability

POR default by the factory.

TABLE 5-2:

TEMPERATURE

CONVERSION TIME

Resolution

tCONV

(ms)

Samples/sec

(typical)

0.5°C

0.25°C

(Power-up default)

0.125°C

130

0.0625°C

260

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