MC68HC705JJ7 Datasheet, PDF (90/164 Pages) Freescale Semiconductor, Inc – Microcontrollers

Analog Subsystem
8.6.1 Absolute Voltage Readings
The absolute value of a voltage measurement can be calculated in software by first taking a reference
reading from a fixed source and then comparing subsequent unknown voltages to that reading as a
percentage of the reference voltage multiplied times the known reference value.
The accuracy of absolute readings will depend on the error sources taken into account using the features
of the analog subsystem and appropriate software as described in Table 8-6. As can be seen from this
table, most of the errors can be reduced by frequent comparisons to a known voltage, use of the inverted
comparator inputs, and averaging of multiple samples.
8.6.1.1 Internal Absolute Reference
If a stable source of V_DD is provided, the reference measurement point can be internally selected. In this
case, the reference reading can be taken by setting the V_REF bit and clearing the MUX1:4 bits in the
AMUX register. This connects the channel selection bus to the V_DD pin. To stay within the V_MAX range,
the DHOLD bit should be used to select the 1/2 divided input.
8.6.1.2 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
Accuracy Improvements Possible
In Hardware
In Software
Change in reference voltage Provide closer tolerance reference
Calibration and storage of reference source over
temperature and supply voltage
Change in magnitude of
ramp current source
Not adjustable
Compare unknown with recent measurement
from reference
Non-linearity of ramp
current source vs. voltage
Not adjustable
Calibration and storage of voltages at 1/4, 1/2,
3/4, and FS
Frequency shift in internal
low-power oscillator
Use external oscillator with crystal
Compare unknown with recent measurement
from reference
Sampling capacitor leakage Use faster conversion times
Compare unknown with recent measurement
from reference
Internal voltage divider ratio Not adjustable
Compare unknown with recent measurement
from reference OR avoid use of divided input
Input offset voltage of
comparator 2
Not adjustable
Sum two readings on reference or unknown
using INV and INV control bit and divide by 2
(average of both)
Noise internal to MCU
Close decoupling at V_DD and V_SS pins and Average multiple readings on both the reference
reduce supply source impedance
and the unknown voltage
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
90
Freescale Semiconductor