Freescale Semiconductor, Inc.
Analog Subsystem
A/D Conversion Methods
cleared when the device is reset, the MC68HC705JJ7/MC68HC705JP7
starts with the charge and discharge function disabled.
The length of time required to reach the maximum voltage to be
measured and the speed of the time counting mechanism will determine
the resolution of the reading. The time to ramp the external capacitor
voltage to match the maximum voltage is dependent on:
• Charging current to external capacitor
• Value of the external capacitor
• Clock rate for timing function
• Any prescaling of the clock to the timing function
• Desired resolution
The charging behavior is described by the general equation:
t
= C
x V / I
EXT X CHG
CHG
Where:
t
= Charge time (seconds)
CHG
C
V
= Capacitance (µF)
= Unknown voltage (volts)
EXT
X
I
= Charge current (µA)
CHG
Since the MCU can measure time in a variety of ways, the resolution of
the conversion will depend on the length of the time keeping function and
its prescaling to the oscillator frequency (f
time also equals:
). Therefore the charge
OSC
t
= P x N / f
OSC
CHG
Where:
P
N
= Prescaler value (÷ 2, ÷ 4, ÷ 8, etc.)
= Number of counts during charge time
= Oscillator clock frequency (Hz)
f
OSC
NOTE: Noise on the system ground or the external ramping capacitor can cause
the comparator to trip prematurely. Therefore in any given application it
is best to use the fastest possible ramp rate (shortest charge time).
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
General Release Specification
Analog Subsystem
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