MC68HC705JJ7 Datasheet, PDF (89/164 Pages) Freescale Semiconductor, Inc – Microcontrollers

Voltage Measurement Methods
8.6 Voltage Measurement Methods
The methods for obtaining a voltage measurement can use software techniques to express these
voltages as absolute or ratiometric readings.
In most applications the external capacitor, the clock source, the reference voltage, and the charging
current may vary between devices and with changes in supply voltage or ambient temperature. All of
these variations must be considered when determining the desired resolution of the measurement. The
maximum and minimum extremes for the full scale count will be:
N_FSMIN = C_EXTMIN x V_FSMIN x f_OSCMIN / (P x I_CHGMAX)
N_FSMAX = C_EXTMAX x V_FSMAX x f_OSCMAX / (P x I_CHGMIN)
The minimum count should be the desired resolution, and the counting mechanism must be capable of
counting to the maximum. The final scaling of the count will be by a math routine which calculates:
V_X = V_REF x (N_X – N_OFF) / (N_REF – N_OFF)
Where:
V_REF = Known reference voltage
V_X = Unknown voltage between V_SS and V_REF
N_X = Conversion count for unknown voltage
N_REF = Conversion count for known reference voltage (V_REF)
N_OFF = Conversion count for minimum reference voltage (V_SS)
When V_REF is a stable voltage source such as a zener or other reference source, then the unknown
voltage will be determined as an absolute reading. If V_REF is the supply source to the device (V_DD), then
the unknown voltage will be determined as a ratio of V_DD, or a ratiometric reading.
If the unknown voltage applied to the comparator is greater than its common-mode range (V_DD –1.5 volts),
then the external capacitor will try to charge to the same level. This will cause both comparator inputs to
be above the common-mode range and the output of the comparator will be indeterminate. In this case
the comparator output flags may also be set even if the actual voltage on the positive input (+) is less than
the voltage on the negative input (–). All A/D conversion methods should have a maximum time check to
determine if this case is occurring.
Once the maximum timeout detection has been made, the state of the comparator outputs can be tested
to determine the situation. However, such tests should be carefully designed when using modes 1, 2, or
3 as these modes cause the immediate automatic discharge of the external ramping capacitor before any
software check can be made of the output state of comparator 2.
NOTE
All A/D conversion methods should include a test for a maximum elapsed
time to detect error cases where the inputs may be outside of the design
specification.
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
Freescale Semiconductor
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