MC68HC705JP7 Datasheet, PDF (134/242 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Analog Subsystem
8.7.1.2 External Absolute Reference
If a stable external source is provided, the reference measurement point
can be any one of the channel selected pins from PB1–PB4. In this case
the reference reading can be taken by setting the MUX bit in the AMUX
which connects channel selection bus to the pin connected to the
external reference source. If the external reference is greater than
V_DD –1.5 volts, then the DHOLD bit should be used to select the 1/2
divided input.
Table 8-6. Absolute Voltage Reading Errors
Error Source
Change in reference
voltage
Change in magnitude of
ramp current source
Non-linearity of ramp
current source vs.
voltage
Frequency shift in
internal low-power
oscillator
Sampling capacitor
leakage
Internal voltage divider
ratio
Input offset voltage of
comparator 2
Noise internal to MCU
Accuracy Improvements Possible
In Hardware
In Software
Provide closer tolerance reference
Calibration and storage of reference source
over temperature and supply voltage
Not adjustable
Compare unknown with recent
measurement from reference
Not adjustable
Calibration and storage of voltages at 1/4,
1/2, 3/4, and FS
Use external oscillator with crystal
Compare unknown with recent
measurement from reference
Use faster conversion times
Not adjustable
Not adjustable
Close decoupling at V_DD and V_SS pins
and reduce supply source impedance
Compare unknown with recent
measurement from reference
Compare unknown with recent
measurement from reference OR avoid use
of divided input
Sum two readings on reference or
unknown using INV and INV control bit and
divide by 2 (average of both)
Average multiple readings on both the
reference and the unknown voltage
8.7.2 Ratiometric Voltage Readings
The ratiometric value of a voltage measurement can be calculated in
software by first taking a reference reading from a reference source and
then comparing subsequent unknown voltages to that reading as a
percentage of the reference value. The accuracy of ratiometric readings
Advance Information
134
MC68HC705JJ7 • MC68HC705JP7 — REV 4
Analog Subsystem
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA