MC9S08DZ96F2CLL datasheet(209/458 Pages) FREESCALE

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Chapter 10 Analog-to-Digital Converter (S08ADC12V1)

MC9S08DZ128 Series Data Sheet, Rev. 1

Freescale Semiconductor

209

ADHTS, in the SOPT2 register. The RTC or IRQ can be conﬁgured to cause a hardware trigger in run,

wait, and stop3 modes.

10.1.5

Temperature Sensor

To use the on-chip temperature sensor, the user must perform the following:

•

Conﬁgure ADC for long sample with a maximum of 1 MHz clock

•

Convert the bandgap voltage reference channel (AD27)

— By converting the digital value of the bandgap voltage reference channel using the value of

VBG the user can determine VDD. For value of bandgap voltage, see Section A.6, “DC

Characteristics”.

•

Convert the temperature sensor channel (AD26)

— By using the calculated value of VDD, convert the digital value of AD26 into a voltage, VTEMP

Equation 10-1 provides an approximate transfer function of the temperature sensor.

Temp = 25 - ((VTEMP -VTEMP25) ÷ m)

Eqn. 10-1

where:

—VTEMP is the voltage of the temperature sensor channel at the ambient temperature.

—VTEMP25 is the voltage of the temperature sensor channel at 25°C.

— m is the hot or cold voltage versus temperature slope in V/

°C.

For temperature calculations, use the VTEMP25 and m values from the ADC Electricals table.

In application code, the user reads the temperature sensor channel, calculates VTEMP, and compares to

VTEMP25.IfVTEMP is greater than VTEMP25 the cold slope value is applied in Equation 10-1.IfVTEMP is

less than VTEMP25 the hot slope value is applied in Equation 10-1.

To improve accuracy the user should calibrate the bandgap voltage reference and temperature sensor.

Calibrating at 25

°C will improve accuracy to ± 4.5°C.

Calibration at three points, -40

°C, 25°C, and 125°C will improve accuracy to ± 2.5°C. Once calibration

has been completed, the user will need to calculate the slope for both hot and cold. In application code, the

user would then calculate the temperature using Equation 10-1 as detailed above and then determine if the

temperature is above or below 25

°C. Once determined if the temperature is above or below 25°C, the user

can recalculate the temperature using the hot or cold slope value obtained during calibration.

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