Freescale Semiconductor, Inc.
Ana log Sub syste m
The above two equations for the charge time, t
, can be combined to
CHG
form the following expression for the full scale count (N ) of the
FS
measured time versus the full scale unknown voltage (V ):
FS
N
= C
x V x f
/ (P x I
)
FS
EXT
FS
OSC
CHG
Since a given timing method has a fixed charge current and prescaler,
then the variation in the resultant count for a given unknown voltage is
mainly dependent on the operating frequency and the capacitance value
used. The desired external capacitance for a given voltage range, f
conversion method, and resolution is defined as:
,
OSC
C
= N x P x I
/ (V x f
)
EXT
FS
CHG
FS
OSC
NOTE: The value of any capacitor connected directly to the PB0/AN0 pin should
be limited to less than 2 microfarads. Larger capacitances will create
high discharge currents which may damage the device or create signal
noise.
The full scale voltage range for a given capacitance, f
method, and resolution is defined as:
, conversion
OSC
V
= N x P x I
/ (C
x f
)
FS
FS
CHG
EXT
OSC
Once charged to a given voltage a finite amount of time will be required
to discharge the capacitor back to its start voltage at V . This discharge
SS
time will be solely based on the value of capacitance used and the
sinking current of the internal discharge device. To allow a reasonable
time for the capacitor to return to V levels, the discharge time should
SS
last about 10 milliseconds per microfarad of capacitance attached to the
PB0 pin. If the total charge/discharge cycle time is critical, then the
discharge time should be at least 1/10 of the most recent charge time.
Shorter discharge times may be used if lesser accuracy in the voltage
measurement is acceptable.
NOTE: Sufficient time should be allowed to discharge the external capacitor or
subsequent charge times will be shortened with resultant errors in timing
conversion.
General Release Specification
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
Analog Subsystem
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