DS2438

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OPERATION-MEASURING TEMPERATURE

The DS2438 measures temperatures through the use of an on-board temperature measurement technique.

The temperature reading is provided in a 13-bit, two’s complement format, which provides 0.03125

°C of

resolution. Table 1 describes the exact relationship of output data to measured temperature. The data is

transmitted serially over the 1-Wire interface. The DS2438 can measure temperature over the range of

-55

°C to +125°C in 0.03125°C increments. For Fahrenheit usage, a lookup table or conversion factor

must be used.

Note that temperature is represented in the DS2438 in terms of a 0.03125

°C LSb, yielding the following

13-bit format. The 3 least significant bits of the Temperature Register will always be 0. The remaining

13 bits contain the two’s complement representation of the temperature in

°C, with the MSb holding the

sign (S) bit. See “Memory Map” section for the Temperature Register address location.

Temperature Register Format Table 1

0

0

0

LSB

MSb

(unit = °C)

LSb

S

MSB

TEMPERATURE

DIGITAL OUTPUT (Binary)

DIGITAL OUTPUT (Hex)

+125°C

01111101 00000000

7D00h

+25.0625°C

00011001 00010000

1910h

+0.5°C

00000000 10000000

0080h

0°C

00000000 00000000

0000h

-0.5°C

11111111 10000000

FF80h

-25.0625°C

11100110 11110000

E6F0h

-55°C

11001001 00000000

C900h

OPERATION-MEASURING BATTERY VOLTAGE

The on-board analog-to-digital converter (ADC) has 10 bits of resolution and will perform a conversion

when the DS2438 receives a command protocol (Convert V) instructing it to do so. The result of this

measurement is placed in the 2-byte Voltage Register. The range for the DS2438 ADC is 0 volt to

10 volt; this range is suitable for NiCd or NiMH battery packs up to six cells, and for lithium ion battery

packs of two cells. The full-scale range of the ADC is scaled to 10.23 volt, resulting in a resolution of

10 mV.

While the ADC has a range that extends down to 0 volt, it is important to note that the battery voltage can

also be the supply voltage to the DS2438. As such, the accuracy of the ADC begins to degrade below

battery voltages of 2.4 volt, and the ability to make conversions is limited by the operating voltage range

of the DS2438.

Voltage is expressed in this register in scaled binary format, as outlined in Table 2. Note that while codes

exist for values below 2.4 volt, accuracy of the ADC and the limitation on the DS2438’s supply voltage

make it unlikely that these values would be used in actual practice. See “Memory Map” section for the

Voltage Register address location.