MC9S08SH8CTJ Datasheet, PDF (126/341 Pages) Freescale Semiconductor, Inc – MC9S08SH8 Datasheet

Chapter 9 Analog-to-Digital Converter (S08ADCV1)
9.1.2 Alternate Clock
The ADC module is capable of performing conversions using the MCU bus clock, the bus clock divided
by two, the local asynchronous clock (ADACK) within the module, or the alternate clock, ALTCLK. The
alternate clock for the MC9S08SH8 MCU devices is the external reference clock (ICSERCLK).
The selected clock source must run at a frequency such that the ADC conversion clock (ADCK) runs at a
frequency within its specified range (f_ADCK) after being divided down from the ALTCLK input as
determined by the ADIV bits.
ALTCLK is active while the MCU is in wait mode provided the conditions described above are met. This
allows ALTCLK to be used as the conversion clock source for the ADC while the MCU is in wait mode.
ALTCLK cannot be used as the ADC conversion clock source while the MCU is in either stop2 or stop3.
9.1.3 Hardware Trigger
The ADC hardware trigger, ADHWT, is the output from the real time counter (RTC). The RTC counter
can be clocked by either ICSERCLK, ICSIRCLK or a nominal 1 kHz clock source.
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
overflow. The RTIE does not have to be set for RTC to cause a hardware trigger.
The RTC can be configured to cause a hardware trigger in MCU run, wait, and stop3.
9.1.4 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
V_BG the user can determine V_DD. For value of bandgap voltage, see Section A.6, “DC
Characteristics”.
• 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 9-1 provides an approximate transfer function of the on-chip temperature sensor for V_DD = 5.0V,
Temp = 25°C, using the ADC1 at f_ADCK = 1.0MHz and configured for long sample.
Temp_C = 25 - ( (V_TEMP -V_TEMP25) / m)
Eqn. 9-1
where:
— V_TEMP is the voltage of the temperature sensor channel at the ambient temperature.
— V_TEMP25 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 V_TEMP25 and m values in the data sheet.
MC9S08SH8 MCU Series Data Sheet, Rev. 3
122
Freescale Semiconductor