MCP9804-ESLASHMS datasheet(25/54 Pages) MICROCHIP

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DS22203A-page 25

MCP9804

5.1.3.1

TA bits to Temperature Conversion

To convert the TA bits to decimal temperature, the

upper three boundary bits (bits 15, 14 and 13) must be

masked out. Then determine the sign bit (bit 12) to

check positive or negative temperature, shift the bits

accordingly and combine the upper and lower bytes of

the 16-bit register. The upper byte contains data for

temperatures greater than 32°C while the lower byte

contains data for temperature less than 32°C, including

fractional data. When combining the upper and lower

bytes, the upper byte must be Right-shifted by 4 bits (or

multiply by 24) and the lower byte must be Left-shifted

by 4 bits (or multiply by 2-4). Adding the results of the

shifted values provides the temperature data in decimal

format, see Equation 5-1.

The temperature bits are in two’s compliment format,

therefore, positive temperature data and negative

temperature data are computed differently. Equation 5-

1 shows the temperature computation. The example

instruction code outlined in Figure 5-5 shows the

communication flow, also see Figure 5-6 for timing

diagram.

EQUATION 5-1:

BYTES TO

TEMPERATURE

CONVERSION

FIGURE 5-5:

Example Instruction Code.

Where:

TA = Ambient Temperature (°C)

UpperByte = TA bit 15 to bit 8

LowerByte = TA bit 7 to bit 0

Temperature

ΤΑ ≥ 0°C

Temperature

< 0°C

UpperByte

LowerByte

–

()

256

UpperByte

LowerByte

–

()

–

i2c_start();

// send START command

i2c_write (AddressByte & 0xFE);

//WRITE Command (see

Section 4.1.4)

//also, make sure bit 0 is cleared ‘0’

i2c_write(0x05);

// Write TA Register Address

i2c_start();

//Repeat START

i2c_write(AddressByte | 0x01);

// READ Command (see

Section 4.1.4)

//also, make sure bit 0 is Set ‘1’

UpperByte = i2c_read(ACK);

// READ 8 bits

//and Send ACK bit

LowerByte = i2c_read(NAK);

// READ 8 bits

//and Send NAK bit

i2c_stop();

// send STOP command

//Convert the temperature data

//First Check flag bits

if ((UpperByte & 0x80) == 0x80){

//TA ≥ TCRIT

}

if ((UpperByte & 0x40) == 0x40){

//TA > TUPPER

}

if ((UpperByte & 0x20) == 0x20){

//TA < TLOWER

}

UpperByte = UpperByte & 0x1F;

//Clear flag bits

if ((UpperByte & 0x10) == 0x10){

//TA < 0°C

UpperByte = UpperByte & 0x0F;

//Clear SIGN

Temperature = 256 - (UpperByte x 16 + LowerByte / 16);

}else

//TA ≥ 0°C

Temperature = (UpperByte x 16 + LowerByte / 16);

//Temperature = Ambient Temperature (°C)

This example routine assumes the variables and i2c communication subroutines are predefined:

(See

Appendix A: “Source Code”)

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