AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 24

• Manufacturer Part Number: AN1740
• Description: Applications Using the Analog Subsystem on MC68HC05JJ/JP Series Microcontrollers
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Current Source/Discharge
24
NOTE:
Avoid using both the input capture and the analog interrupt at the same
time for the comparator trip detection. In this case, the ICF always wins
priority and the analog interrupt essentially will be ignored.
The analog system’s other significant feature is the constant current
source with discharge device that is connected directly to the PB0 pin.
As given in
operated in three modes which are dependent on the states of the ISEN
and CHG bits in the ACR located at $001E. ISEN controls whether the
function is active and the CHG bit controls whether the PB0 pin sources
100 A or sinks at least 1 mA. When the ISEN bit is clear, the PB0 pin
behaves the same as a standard I/O port pin.
As shown in the block diagram in
device, and the positive input of comparator 2 all share PB0. Therefore,
this current source can be used to supply current to external devices
which the comparator can then test against some other voltage
connected to its negative input through the channel select bus.
If PB0 is left unconnected, it appears like a 20-pF capacitor which
requires about 800 nanoseconds to charge from V
ACR control bits
ISEN
Freescale Semiconductor, Inc.
0
1
1
For More Information On This Product,
CHG
Table
Go to: www.freescale.com
X
0
1
Table 4. Current Source/Discharge Control
4, the current source and the discharge device can be
Current
source
OFF
OFF
ON
Discharge
Figure
device
OFF
OFF
ON
1, the current source, discharge
Current source and discharge
Discharge current into PB0
Source current from PB0
device both disabled
SS
to 4 volts.
Action
MOTOROLA
AN1740