MC68HC05JJ6 Datasheet, PDF (119/216 Pages) Freescale Semiconductor, Inc – General Release Specification Microcontrollers

Analog Subsystem
cleared when the device is reset, the MC68HC05JJ6/MC68HC05JP6
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_CHG = C_EXT x V_X / I_CHG
Where:
t_CHG = Charge time (seconds)
C_EXT = Capacitance (µF)
V_X = Unknown voltage (volts)
I_CHG = Charge current (µA)
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_OSC). Therefore, the
charge time also equals:
NOTE:
t_CHG = P x N / f_OSC
Where:
P
N
f_OSC
= Prescaler value (÷ 2, ÷ 4, ÷ 8, etc.)
= Number of counts during charge time
= Oscillator clock frequency (Hz)
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).
MC68HC05JJ6/MC68HC05JP6 — Rev. 3.2
Freescale Semiconductor
Analog Subsystem
General Release Specification
119