Motorola MC68HC908JB8, Справочное руководство

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Designer Reference Manual USB08 Evaluation Board
32 Hardware Description MOTOROLA
Hardware Description
2.4.2 Input/Output Functions
For demonstration purposes, the board has:
• Three push buttons
• Three light-emitting diodes (LED)
• Sensor resistors
The push buttons are connected to the three port pins PTA4, PTA5, and
PTA6. By pushing the buttons, a low level is produced on the appropriate
input. Since the port pins have internal pullup resistors, no external
resistors are required. The buttons are bridged with capacitors, to
support correct reading of the inputs by the software and to avoid noise.
The occurrence of the high-low edge at the respective pin of port A is an
input event which results in the generation of a keyboard interrupt by the
MC68HC08JB8. This interrupt is then used by the program (see
Section 3. Software Module Descriptions ).
For optical signalling, three LEDs are attached to port D. These port pins
have a high drive capability of up to 25 mA. Therefore, it is not necessary
to use a driver. On the board, PTD0, PTD1, and PTD2 are used for LED
control. All outputs generated by the port D pins have an open-drain
characteristic and are 5-V tolerant.
The remaining port D pins (PTD3–PTD6) are used for controlling the
software analog-to-digital converter (ADC). The ADC implementation is
described in detail in
Section 3. Software Module Descriptions as well
as in the application note entitled Simple A/D for MCUs without Built-in
A/D Converters, Motorola document order number AN477/D. This
application note can be found on the World Wide Web at:
http://www.motorola.com/semiconductors/
The software ADC senses the resistance of:
• R1 (photo resistor)
• R2 (thermistor)
• R3 (potentiometer)
To determine capacitor load times, the MCU pins PTE0–PTE2 serve as
trigger inputs for the software ADC.