Some important pins are Vdd and Vss, Reset, XTAL and EXTAL, E-clock Output,Vrl and Vrh, and Port Signals. The XTAL and EXTAL provide the interface for either a crystal or a CMOS compatible clock to control the internal clock generator circuitry. The frequency applied to these pins is four times higher than the desired E-clock rate. The board uses a 8-MHz crystal and the E (system clock) is 2-MHz. The XTAL pin is normally left unterminated when an external CMOS compatible clock input is connected to the EXTAL pin.
The Vrl and Vrh inputs provide the reference voltages for the analog-to digital converter circuitry. Vrl is the low reference, and the board connects this to 0 Vdc. Vrh is the high reference. For proper A/D converter operation, Vrh should be at least 3 Vdc greater than Vrl, and both Vrl and Vrh should be between Vss and Vdd. The board connects Vrh to Vdd through a 1K resistor, with a 0.1 micro Farad Capacitor connected to Vss (ground).
Port pins have different functions in different operating modes. Pin functions for ports A,D, and E are independent of operating modes. Ports B and C, however depend on the mode for their operation. Port B provides 8 general-purpose output signals in single-chip operating modes. When the microcontroller is in expanded multiplexed operating mode, port B pins are the eight high-order address lines. Port C provides eight general-purpose input/output signals when the MCU is in the single-chip operating mode, but in expanded multiplexed operating mode, port C pins are a multiplexed address/data bus. Details of various functions of the 40 port signals are shown in page 2-8 in the Technical Data book.
Pins PD[5:0] can be used for general-purpose I/O signals. These pins alternately serve as the serial communication interface(SCI) and serial peripheral interface(SPI) signals when those subsystems are enabled. Pin PD0 is the receive data input(RxD) signal for the SCI. Pin PD1 is the transmit data output(TxD) signal for the SCI. I used the 8 pins of port B as output to write data to the D/As and 3 pins of port C to control the 3 D/As, so that we write the correct data to the appropriate D/A at a certain time. Since the SCI is always on for our requirement, pins PD0 and PD1 are used for RxD and TxD.