MC9S08LC60 Datasheet, PDF (268/358 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 15 Analog-to-Digital Converter (S08ADC12V1)
15.1.1.2 Alternate Clock
The ADC is capable of performing conversions using the MCU bus clock, the bus clock divided by two,
or the local asynchronous clock (ADACK) within the module. The alternate clock, ALTCLK, input for the
MC9S08LC60/36/20 MCU devices is not implemented.
15.1.1.3 Hardware Trigger
The ADC hardware trigger, ADHWT, is output from the real-time interrupt (RTI) counter. The RTI counter
can be clocked by either ICGERCLK or a nominal 1-kHz clock source within the RTI block.
The period of the RTI is determined by the input clock frequency and the RTIS bits. The RTI counter is a
free running counter that generates an overflow at the RTI rate determined by the RTIS bits. When the ADC
hardware trigger is enabled, a conversion is initiated upon a RTI counter overflow.
The RTI can be configured to cause a hardware trigger in MCU run, wait, and stop3.
15.1.1.4 Analog Pin Enables
The ADC on MC9S08LC60/36/20 MCU devices contains only one analog pin enable registers, APCTL1.
15.1.1.5 Temperature Sensor
To use the on-chip temperature sensor, the user must perform the following:
1. Configure ADC for long sample with a maximum of 1-MHz clock.
2. 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 V_DD. For value of bandgap voltage, see Section A.5, “DC Characteristics”.
3. Convert the temperature sensor channel (AD26).
By using the calculated value of V_DD, convert the digital value of AD26 into a voltage, V_temp
Equation 15-1 provides an approximate transfer function of the on-chip temperature sensor for:
V_DD = 3.0 V, Temp = 25˚C, using the ADC at f_ADCK = 1.0 MHz, and configured for long sample.
TempC = (V_temp – 0.7013) ÷ (0.0017)
Eqn. 15-1
0.0017 is the uncalibrated voltage versus temperature slope in V/˚C. Uncalibrated accuracy of the
temperature sensor is approximately ± 12˚C, using Equation 15-1.
4. 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 temperature points (–40˚C, 25˚C, and 125˚C) will improve accuracy to
± 2.5˚C. After calibration has been completed, the user must calculate the slope for both hot
and cold. In application code, the user would then calculate the temperature using
Equation 15-1 and then determine whether the temperature is above or below 25˚C. After
determining whether the temperature is above or below 25˚C, the user can recalculate the
temperature using either the hot or cold slope value obtained during calibration.
MC9S08LC60 Series Advance Information Data Sheet, Rev. 2
268
PRELIMINARY
Freescale Semiconductor