MCP9808 datasheet(25/52 Pages) MICROCHIP

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DS25095A-page 25

MCP9808

5.1.3.1

TA Bits to Temperature Conversion

To convert the TA bits to decimal temperature, the

upper three boundary bits (TA<15:13>) 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, includ-

ing 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 tem-

perature data are computed differently. Equation 5-1

shows the temperature computation. The example

instruction code, outlined in Example 5-1, shows the

communication flow; also see Figure 5-5 for the timing

diagram.

EQUATION 5-1:

BYTES TO

TEMPERATURE

CONVERSION

EXAMPLE 5-1:

SAMPLE INSTRUCTION CODE

Where:

TA = Ambient Temperature (°C)

UpperByte = TA bit 15 to bit 8

LowerByte = TA bit 7 to bit 0

Temperature TA ≥ 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 “Address Byte”)

//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 “Address Byte”)

//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 ³ 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

³ 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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