Datasheet

Chapter 10 Analog-to-Digital Converter (S08ADCV1)
MC9S08EL32 Series and MC9S08SL16 Series Data Sheet, Rev. 3
138 Freescale Semiconductor
10.1.2 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 MC9S08EL32 Series and MC9S08SL16 Series 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.
10.1.3 Hardware Trigger
The ADC hardware trigger, ADHWT, is the output from the real time counter (RTC) overflow. The RTC
can be configured to cause a hardware trigger in MCU run, wait, and stop3 modes.
10.1.4 Temperature Sensor
The ADC module includes a temperature sensor whose output is connected to AD26. Equation 10-1
provides an approximate transfer function of the temperature sensor.
Temp = 25 - ((V
TEMP
-V
TEMP25
) ÷ m) Eqn. 10-1
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 from the ADC Electricals table.
In application code, the user reads the temperature sensor channel, calculates V
TEMP
, and compares to
V
TEMP25
. If V
TEMP
is greater than V
TEMP25
the cold slope value is applied in Equation 10-1. If V
TEMP
is
less than V
TEMP25
the hot slope value is applied in Equation 10-1.