MC68HC705JP7 Datasheet, PDF (124/242 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Analog Subsystem
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).
The previous two equations for the charge time, t_CHG, can be combined
to form the following expression for the full scale count (N_FS) of the
measured time versus the full scale unknown voltage (V_FS):
N_FS = C_EXT x V_FS x f_OSC / (P x I_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_OSC,
conversion method, and resolution is defined as:
NOTE:
C_EXT = N_FS x P x I_CHG / (V_FS x f_OSC)
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_OSC, conversion
method, and resolution is defined as:
V_FS = N_FS x P x I_CHG / (C_EXT x f_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_SS. This discharge
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_SS levels, the discharge time should
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.
Advance Information
124
MC68HC705JJ7 • MC68HC705JP7 — REV 4
Analog Subsystem
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