Datasheet

© 2009 Microchip Technology Inc. DS22153C-page 27
MCP9843/98243
5.1.4.1 T
A
bits to Temperature Conversion
To convert the T
A
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 combinding the upper and lower
bytes, the upper byte must be Right-shifted by 4bits (or
multiply by 2
4
) 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, postive temperature data and negative tem-
perature data are computed differently. Equation 5-1
shows the temperature computation. The example
instruction code outlined in Figure 5-6 shows the
communication flow, also see Figure 5-7 for timing
diagram.
EQUATION 5-1: BYTES TO
TEMPERATURE
CONVERSION
FIGURE 5-6: Example Instruction Code.
Where:
T
A
= Ambient Temperature (°C)
UpperByte = T
A
bit 15 to bit 8
LowerByte = T
A
bit 7 to bit 0
Temperature ≥ 0°C
Temperature < 0°C
T
A
UpperByte 2
4
LowerByte 2
4–
× + × ()=
T
A
256 UpperByte 2
4
LowerByte 2
4–
× + × ()–=
i2c_start(); // send START command
i2c_write(AddressByte & 0xFE); //WRITE Command
//also, make sure bit 0 is cleared ‘0’
i2c_write(0x05); // Write T
A
Register Address
i2c_start(); //Repeat START
i2c_write(AddressByte | 0x01); // READ Command
//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){ //T
A
≥ T
CRIT
}
if ((UpperByte & 0x40) == 0x40){ //T
A
> T
UPPER
}
if ((UpperByte & 0x20) == 0x20){ //T
A
< T
LOWER
}
UpperByte = UpperByte & 0x1F; //Clear flag bits
if ((UpperByte & 0x10) == 0x10){ //T
A
< 0°C
UpperByte = UpperByte & 0x0F; //Clear SIGN
Temperature = 256 - (UpperByte x 16 + LowerByte / 16);
}else //T
A
≥ 0°C
Temperature = (UpperByte x 16 + LowerByte / 16);
//Temperature = Ambient Temperature (°C)
This example routine assumes the variables and I
2
C communication subroutines are predefined: