GENERAL RELEASE SPECIFICATION
August 27, 1998
Table 15-6. Absolute Voltage Reading Errors
Accuracy Improvements Possible
Error Source
In Hardware
In Software
Close decoupling of power supply, low
source impedances, good board layout,
use of multi-layer board
Average multiple readings on both the
reference and the unknown voltage
Noise external to MCU
Internal Absolute Reference
If a stable source of V is provided, the reference measurement point can be
DD
internally selected. In this case the reference reading can be taken by setting the
VREF bit and clearing the MUX7:0 bits in the AMUX register. This connects the
channel selection bus to the V pin.
DD
Alternatively, the internal bandgap voltage can be used as the reference
measurement point, by setting the IBREF bit in AMUX2 Register and TSEN bit in
the Miscellaneous Control Register.
External Absolute Reference
If a stable external source is provided, the reference measurement point can be
any one of the channel selected pins from PB4 through PB7. In this case the refer-
ence 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.
15.5.2 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 sub-
sequent unknown voltages to that reading as a percentage of the reference value.
The accuracy of ratiometric readings will depend on 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 15-7. 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.
Internal Ratiometric Reference
If readings are to be ratiometric to V , the reference measurement point can be
DD
internally selected. In this case the reference reading can be taken by setting the
VREF bit and clearing the MUX7:0 bits in the AMUX register which connects the
channel selection bus to the V pin.
DD
Alternatively, the internal bandgap voltage can be used as the reference
measurement point, by setting the IBREF bit in AMUX2 Register and TSEN bit in
the Miscellaneous Control Register.
MOTOROLA
15-28
ANALOG SUBSYSTEM
MC68HC05SB7
REV 2.1
FREESCALE [ Freescale ]
