Holtek HT46R22, Handbook

Page: 39 / 162

Pull-high Resistors
Many product applications require pull-high resistors for their switch inputs usually requiring the
use of an external resistor. To eliminate the need for these external resistors, all I/O pins, when con-
figured as an input have the capability of being connected to an internal pull-high resistor. These
pull-high resistors are selectable via configuration option and are implemented using a weak
PMOS transistor. Note that on some ports, individual pins can be selected to have pull-high resis-
tors, while on other ports all pins or no pins must be selected to have pull-high resistors.
Port A Wake-up
Each device has a HALT feature enabling the microcontroller to enter a power down mode and pre-
serve power, a feature that is important for battery and other low power applications. Various meth-
ods exist to wake-up the microcontroller, one of which is to change the logic condition on one of
the Port A pins from high to low. After a
²
HALT
²
instruction forces the microcontroller into entering
a HALT condition, the processor will remain idle or in a low-power state until the logic condition of
the selected wake-up pin on Port A changes from high to low. This function is especially suitable
for applications that can be woken up via external switches. Note that each pin on Port A can be se-
lected individually to have this wake-up feature.
I/O Port Control Registers
Each I/O line has its own control register (PAC, PBC, PCC, etc.) to control the input/output configu-
ration. With this control register, each CMOS output or Schmitt Trigger input with or without
pull-high resistor structures can be reconfigured dynamically under software control. Each pin of
the I/O ports is directly mapped to a bit in its associated port control register. For the I/O pin to func-
tion as an input, the corresponding bit of the control register must be written as a
²
1
²
. This will then
allow the logic state of the input pin to be directly read by instructions. When the corresponding bit
of the control register is written as a
²
0
²
, the I/O pin will be setup as a CMOS output. If the pin is cur-
rently setup as an output, instructions can still be used to read the output register. However, it
should be noted that the program will in fact only read the status of the output data latch and not
the actual logic status of the output pin.
Pin-shared Functions
The flexibility of the microcontroller range is greatly enhanced by the use of pins that have more
than one function. Limited numbers of pins can force serious design constraints on designers but
by supplying pins with multi-functions, many of these difficulties can be overcome. For some pins,
the chosen function of the multi-function I/O pins is set by configuration options while for others the
function is set by application program control.
®
External Interrupt Input
The external interrupt pin INT is pin-shared with the I/O pin PA5. For applications not requiring an
external interrupt input, the pin can be used as a normal I/O pin, however, to do this, the external
interrupt enable bits in the INTC register must be disabled.
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A/D Type MCU