August 27, 1998
GENERAL RELEASE SPECIFICATION
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 CPFR bit to clear
an extraneous CPF flag that may have occurred.
VREF
This is a read/write bit that connects the channel select bus to V for purposes
DD
of making 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 15-2. This bit is cleared by a reset of the device.
1 = Channel select bus connected to V
cleared.
if MUX7:0 and IBREF are
DD
0 = Channel select bus cannot be connected to V .
DD
IBREF
This is a read/write bit that connects the channel select bus to V for purposes
IB
of making 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 15-2. This bit is cleared by a reset of the device.
1 = Channel select bus connected to V if MUX7:0 and VREF are
IB
cleared.
0 = Channel select bus cannot be connected to V .
IB
MUX7:0
These are read/write bits that connect the analog subsystem pins to the chan-
nel select bus and voltage comparator for purposes of making a voltage mea-
surement. They can be selected individually or combined with any of the other
input sources to the channel select bus as shown in Table 15-2.
NOTE
The V
voltage source shown in Figure 15-1 depicts a small offset voltage
AOFF
generated by the total chip current passing through the package bond wires and
lead frame that are attached to the single V pin. The offset raises the internal
SS
V
reference (AV ) in the analog subsystem with respect to the external V
SS
SS SS
pin. Turning on the V MUX to the channel select bus connects it to this internal
SS
AVSS reference line.
When making A/D conversions this AV
offset gets placed on the external
SS
ramping capacitor since the discharge device on the CAP pin discharges the
external capacitor to the internal AV line. Under these circumstances the
SS
positive input (+) to the comparator will always be higher than the negative input (–
) until the negative input reaches the AV offset voltage plus any offset in the
SS
comparator.
Therefore, input voltages cannot be resolved if they are less than the sum of the
AV offset and the comparator offset, because they will always yield a low output
SS
from the comparator
MC68HC05SB7
REV 2.1
ANALOG SUBSYSTEM
MOTOROLA
15-5
FREESCALE [ Freescale ]
