XC68HC705JJ7 Motorola, XC68HC705JJ7 Datasheet - Page 123

• Manufacturer Part Number: XC68HC705JJ7
• Description: MICROCONTROLLER
• Manufacturer: Motorola

MC68HC705JJ7 • MC68HC705JP7 — REV 4
MOTOROLA
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:
The charging behavior is described by the general equation:
Where:
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:
Where:
•
•
•
•
•
t
C
V
I
P
N
f
CHG
CHG
OSC
X
EXT
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
= Charge time (seconds)
= Capacitance ( F)
= Unknown voltage (volts)
= Charge current ( A)
= Prescaler value ( 2,
= Number of counts during charge time
= Oscillator clock frequency (Hz)
Analog Subsystem
t
CHG
t
CHG
= C
= P x N / f
EXT
4,
x V
X
8, etc.)
OSC
OSC
/ I
CHG
). Therefore, the charge
A/D Conversion Methods
Advance Information
Analog Subsystem
123