MC68HC705JJ7 Datasheet, PDF (91/164 Pages) Freescale Semiconductor, Inc – Microcontrollers

Voltage Measurement Methods
8.6.2 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 will depend on the variety of
sources, but will generally be better than for absolute readings. Many of these error sources can be
taken into account using the features of the analog subsystem and appropriate software as described in
Table 8-7. As with absolute measurements, most of the errors can be reduced by frequent comparisons
to the reference voltage, use of the inverted comparator inputs, and averaging of multiple samples.
Table 8-7. Ratiometric 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
Not required for ratiometric
Compare unknown with recent measurement
from reference
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
Not required for ratiometric
Compare unknown with recent measurement
from reference
Use faster conversion times
Compare unknown with recent measurement
from reference
Not adjustable
Compare unknown with recent measurement
from reference
Not adjustable
Sum two readings on reference or unknown
using INV and INV control bit and divide by 2
(average of both)
Close decoupling at V_DD and V_SS pins and
reduce supply source impedance
Average multiple readings on both the
reference and the unknown voltage
8.6.2.1 Internal Ratiometric Reference
If readings are to be ratiometric to V_DD, 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 which connects the channel selection bus to the V_DD pin. In order to stay within the V_MAX
range, the DHOLD bit should be used to select the 1/2 divided input.
8.6.2.2 External Ratiometric Reference
If readings are to be ratiometric to some external source, the reference measurement point can be
connected to 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.
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
Freescale Semiconductor
91