HCS08 datasheet(120/316 Pages) NXP

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Chapter 9 Analog-to-Digital Converter (S08ADCV1)

MC9S08SG8 MCU Series Data Sheet, Rev. 8

120

Freescale Semiconductor

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 MC9S08SG8 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 (fADCK) 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

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

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 9-1 provides an approximate transfer function of the on-chip temperature sensor.

TempC = 25 - ((VTEMP - VTEMP25)  m

Eqn. 9-1

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.

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