Chapter 10 Analog-to-Digital Converter (S08ADC12V1)
MC9S08DV60 Series Data Sheet, Rev 2
Freescale Semiconductor
171
The period of the RTC is determined by the input clock frequency, the RTCPS bits, and the RTCMOD
register. When the ADC hardware trigger is enabled, a conversion is initiated upon an RTC counter
overow.
The RTC can be congured to cause a hardware trigger in MCU run, wait, and stop3.
10.1.5
Temperature Sensor
To use the on-chip temperature sensor, the user must perform the following:
Congure 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 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.