Freescale Semiconductor, Inc.
Analog Subsystem
Analog Multiplex Register
NOTE: Either comparator may generate an output flag when the inputs are
exchanged due to a change in the state of the INV bit. It is therefore
recommended that the INV bit not be changed while waiting for a
comparator flag. Further, any changes to the state of the INV bit should
be followed by writing a logical one to both the CPFR1 and CPFR2 bits
to clear any extraneous CPF1 or CPF2 flags that may have occurred.
VREF
This read/write bit connects the channel select bus to V for making
DD
a reference voltage measurement. It cannot be selected if any of the
other input sources to the channel select bus are selected as shown
in Table 8-2. This bit is cleared by a reset of the device.
1 = Channel select bus connected to V if all MUX1:4 are cleared.
DD
0 = Channel select bus cannot be connected to V .
DD
MUX1:4
These are read/write bits that connect the analog subsystem pins to
the channel select bus and voltage comparator 2 for purposes of
making a voltage measurement. They can be selected individually or
combined with any of the other input sources to the channel select
bus as shown in Table 8-2.
NOTE: The V
voltage source shown in Figure 8-1 depicts a small offset
AOFF
voltage generated by the total chip current passing through the package
bond wires and lead frame that are attached to the single V pin. This
SS
offset raises the internal V reference (AV ) in the analog subsystem
SS
SS
with respect to the external V pin. Turning on the V MUX to the
SS
SS
channel select bus connects it to this internal AV reference line.
SS
When making A/D conversions this AV offset gets placed on the
SS
external ramping capacitor since the discharge device on the PB0/AN0
pin discharges the external capacitor to the internal AV line. Under
SS
these circumstances the positive input (+) to comparator 2 will always be
higher than the negative input (–) until the negative input reaches the
AV offset voltage plus any offset in comparator 2.
SS
Therefore, input voltages cannot be resolved if they are less than the
sum of the AV offset and the comparator offset, because they will
SS
always yield a low output from the comparator.
MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0
General Release Specification
Analog Subsystem
For More Information On This Product,
Go to: www.freescale.com
FREESCALE [ Freescale ]
