GENERAL RELEASE SPECIFICATION
August 27, 1998
Table 15-8. Voltage Comparator Setup Conditions
Current
Source
Enable
Discharge
Device
Disable
Prog. Timer
Input Capture
Source
Port B Pin
as Inputs
DDRB4 = 0
DDRB5 = 0
ISEN = 0
ISEN = 0
ICEN = 0
15.7 CURRENT SOURCE FEATURES
The internal current source connected to the CAP pin supplies about 100 µA of
current when the ramp discharge device is disabled and the current source is
active. Therefore this current source can be used in an application if the ISEN
enable bit is set to power up the current source is enabled by setting the A/D con-
version method to manual Mode 0 (ATD1 and ATD2 cleared) and the charge cur-
rent enabled (CHG set).
15.8 SAMPLE AND HOLD
When using the internal sample capacitor to capture a voltage for later conver-
sion, the HOLD and DHOLD bit must be cleared first before changing any channel
selection. If both the HOLD (or DHOLD) bit and the channel selection are
changed on the same write cycle, the sample may be corrupted during the switch-
ing transitions.
NOTE
The sample capacitor can be affected by excessive noise created with respect to
the device’s V pin such that it may appear to leak down or charge up depending
SS
on the voltage level stored on the sample capacitor. It is recommended to avoid
switching large currents through the port pins while a voltage is to remain stored
on the same capacitor.
The additional option of adding an offset voltage to the bottom of the sample
capacitor allows unknown voltages near VSS to be sampled and then shifted up
past the comparator offset and the device offset caused by a single V return pin.
SS
The offset also provides a means to measure the internal V level regardless of
SS
the comparator offset in order to determine N
as described in Section 15.5.
OFF
15.9 PORT B INTERACTION WITH ANALOG INPUTS
The analog subsystem is connected directly to the Port B I/O pins without any
intervening gates. It is therefore possible to measure the voltages on Port B pins
set as inputs; or to have the analog voltage measurements corrupted by Port B
pins set as outputs.
MOTOROLA
15-30
ANALOG SUBSYSTEM
MC68HC05SB7
REV 2.1
FREESCALE [ Freescale ]
