NXP Semiconductors MC9S08LG16, Справочное руководство

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Chapter 9 Analog-to-Digital Converter (S08ADC12V1)
MC9S08LG32 MCU Series, Rev. 5
Freescale Semiconductor 215
10.4.1 Clock Select and Divide Control
One of four clock sources can be selected as the clock source for the ADC module. This clock source is
then divided by a configurable value to generate the input clock to the converter (ADCK). The clock is
selected from one of the following sources by means of the ADICLK bits.
• The bus clock, which is equal to the frequency at which software is executed. This is the default
selection following reset.
• The bus clock divided by two. For higher bus clock rates, this allows a maximum divide by 16 of
the bus clock.
• ALTCLK, as defined for this MCU (See module section introduction).
• The asynchronous clock (ADACK). This clock is generated from a clock source within the ADC
module. When selected as the clock source, this clock remains active while the MCU is in wait or
stop3 mode and allows conversions in these modes for lower noise operation.
Whichever clock is selected, its frequency must fall within the specified frequency range for ADCK. If the
available clocks are too slow, the ADC do not perform according to specifications. If the available clocks
are too fast, the clock must be divided to the appropriate frequency. This divider is specified by the ADIV
bits and can be divide-by 1, 2, 4, or 8.
10.4.2 Input Select and Pin Control
The pin control registers (APCTL3, APCTL2, and APCTL1) disable the I/O port control of the pins used
as analog inputs.When a pin control register bit is set, the following conditions are forced for the associated
MCU pin:
• The output buffer is forced to its high impedance state.
• The input buffer is disabled. A read of the I/O port returns a zero for any pin with its input buffer
disabled.
• The pullup is disabled.
10.4.3 Hardware Trigger
The ADC module has a selectable asynchronous hardware conversion trigger, ADHWT, that is enabled
when the ADTRG bit is set. This source is not available on all MCUs. Consult the module introduction for
information on the ADHWT source specific to this MCU.
When ADHWT source is available and hardware trigger is enabled (ADTRG=1), a conversion is initiated
on the rising edge of ADHWT. If a conversion is in progress when a rising edge occurs, the rising edge is
ignored. In continuous convert configuration, only the initial rising edge to launch continuous conversions
is observed. The hardware trigger function operates in conjunction with any of the conversion modes and
configurations.
10.4.4 Conversion Control
Conversions can be performed in 12-bit mode, 10-bit mode, or 8-bit mode as determined by the MODE
bits. Conversions can be initiated by a software or hardware trigger. In addition, the ADC module can be