Datasheet

Analog to Digital Converter
MC9S12DP256 — Revision 1.1
Analog to Digital Converter
the input pins at the digital sampling time that don’t meet the V
IL
or V
IH
specification will return unknown digital values. A read of the PORTAD1
may affect the accuracy of an in progress sample period but will not
affect an in progress A/D conversion.
There is a digital, 8-bit, input-only port associated with the ATD. It is
accessed through the 8-bit Port Data Register (PORTAD1). The number
of bits utilized in the port register depends on the number of analog
channels implemented with the 8 channel ATD module.
Clock Prescale
Function
To keep the ATD module’s conversion clock within the specified
frequency range, a clock prescale function is available. This function
divides the ATD module clock by a program selectable binary constant
in order to generate the ATD module’s conversion clock. The ATD
conversion clock is used to clock both the sample machine and the A/D
machine.
Another important benefit of the prescaled clock feature is that it allows
the user control over the sample period. Note that if the prescale function
is used this way, the conversion time is also affected.
The prescale feature is based on a 5 bit modulus counter and will divide
the clock by an integer value between 1 and 32. The final clock
frequency is obtained with a further division by 2. Therefore, the highest
possible ATD conversion clock frequency is one half of the ATD module
clock frequency, but cannot be faster than the maximum ATD operating
frequency as defined in the electrical specification. The maximum speed
of ATD conversion frequency is 2 MHz. Table 102 lists the prescale
value for various setting of the control register bits. Figure 109 is a block
diagram of the clock prescale logic.
The ATD conversion clock and the ATD module clock have a direct
phase relationship. However, the ATD module operates as if it is
effectively asynchronous to IP bus cycles.
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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