TMP421 datasheet(12/36 Pages) TI2

12 / 36 page

One-ShotStartRegister

ConfigurationRegisters

StatusRegister

IdentificationRegisters

N-FactorCorrectionRegisters

ConversionRateRegister

LocalandRemoteTemperatureRegisters

SDA

SCL

PointerRegister

I/O

Control

Interface

SoftwareReset

TMP421

TMP422

TMP423

SBOS398C – JULY 2007 – REVISED MAY 2012

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Standard Decimal to Binary Temperature Data

Both local and remote temperature data use two

Calculation Example

bytes for data storage. The high byte stores the

temperature with 1°C resolution. The second or low

For positive temperatures (for example, +20°C):

byte

stores

the

decimal

fraction

value

the

(+20°C)/(+1°C/count) = 20

→ 14h → 0001 0100

temperature

and

allows

higher

measurement

Convert the number to binary code with 8-bit,

resolution, as shown in Table 2. The measurement

right-justified format, and MSB = '0' to denote a

resolution for the both the local and remote channels

positive sign.

is 0.0625°C, and is not adjustable.

+20°C is stored as 0001 0100

→ 14h.

Table 2. Decimal Fraction Temperature Data

For negative temperatures (for example, –20°C):

Format (Local and Remote Temperature Low

(|–20|)/(+1°C/count) = 20

→ 14h → 0001 0100

Bytes)

Generate the two's complement of a negative

TEMPERATURE REGISTER LOW BYTE VALUE

(0.0625°C RESOLUTION)(1)

number by complementing the absolute value

TEMP

binary number and adding 1.

(°C)

STANDARD AND EXTENDED BINARY

HEX

0000 0000

–20°C is stored as 1110 1100

→ ECh.

0.0625

0001 0000

0.1250

0010 0000

0.1875

0011 0000

The TMP421/22/23 contain multiple registers for

0.2500

0100 0000

holding

configuration

information,

temperature

0.3125

0101 0000

measurement results, and status information. These

0.3750

0110 0000

registers are described in Figure 14 and Table 3.

0.4375

0111 0000

POINTER REGISTER

0.5000

1000 0000

0.5625

1001 0000

Figure 14 shows the internal register structure of the

0.6250

1010 0000

TMP421/22/23. The 8-bit Pointer Register is used to

0.6875

1011 0000

address a given data register. The Pointer Register

identifies which of the data registers should respond

0.7500

1100 0000

to a read or write command on the two-wire bus. This

0.8125

1101 0000

0.8750

1110 0000

command must be issued to set the proper value in

0.9385

1111 0000

the

Pointer

before

executing

read

(1) Resolution is 0.0625°C/count. All possible values are shown.

command. Table 3 describes the pointer address of

the TMP421/22/23 registers. The power-on reset

Standard Binary to Decimal Temperature Data

(POR) value of the Pointer Register is 00h (0000

Calculation Example

0000b).

High byte conversion (for example, 0111 0011):

Convert the right-justified binary high byte to

hexadecimal.

From hexadecimal, multiply the first number by

160 = 1 and the second number by 161 = 16.

The sum equals the decimal equivalent.

0111 0011b

→ 73h → (3 × 160) + (7 × 161) = 115

Low byte conversion (for example, 0111 0000):

To convert the left-justified binary low-byte to

decimal, use bits 7 through 4 and ignore bits 3

through 0 because they do not affect the value of

the number.

0111b

→ (0 × 1/2)1 + (1 × 1/2)2 + (1 × 1/2)3 + (1

× 1/2)4 = 0.4375

Figure 14. Internal Register Structure

Note that the final numerical result is the sum of the

high byte and low byte. In negative temperatures, the

unsigned low byte adds to the negative high byte to

result in a value less than the high byte (for instance,

–15 + 0.75 = –14.25, not –15.75).

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