AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 42

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

Application Note
Input Pulldowns
42
NOTE:
The diode voltage drop and change with temperature are not accurate
enough to measure any closer than 10 C, unless the device is
calibrated with a table of values versus temperature that is stored in the
personality EPROM or user EPROM found on the MC68HC705JJ7 or
MC68HC705JP7.
External pulldown resistors are normally needed for inputs to switches
or other high impedance sources. To reduce cost, it is advantageous to
supply these pulldown currents internally in the MCU. The JJ/JP series
has pulldowns, which can be controlled by software, on all pins on ports
A, B, and C.
As the default condition following a reset or power-on condition, these
pulldowns come up activated so that all I/O port inputs will be
immediately defined. If the user does not want these pulldowns to be
present during power on, they can be disabled by the SWPDI bit in the
mask option register (MOR) or by a mask option on ROM-based
devices. These pulldowns also can be individually disabled by software
which writes to the PDRA and PDRB registers at $0010 and $0011,
respectively.
These pulldowns are intended for digital functions to eliminate external
resistors and are actually weak N-channel MOS devices connected to
V
behavior as shown in
Be sure to turn off any pulldowns on the PB0:PB7 pins when these are
used for analog signals. Leaving these pulldowns on, especially on PB0,
can lead to unwanted analog parasitics and unexpected measurement
results. When the pulldown for PB0 is on when charging an external
capacitor, the ramp will look like an RC charge curve rather than a linear
ramp.
SS
Freescale Semiconductor, Inc.
. Because they are MOS devices, they have non-linear pulldown
For More Information On This Product,
Go to: www.freescale.com
Figure
19.
MOTOROLA
AN1740