anyone know how to terminate unused pins for 68HC11A1P?

Thread Starter


Joined Nov 22, 2006
i am using 68HC11A1P (MCU) to control the LED to turn on and off when received ASCII code from PC through serial port.

my problem now is i don't know how to terminated the unused pin of the MCU, so anyone know how to terminate the pins state below:

1)PA0-PA2 (pin6-pin8) and PA7(pin1)
2)PE0- PE3 (pin17-pin20)
3)PD1-PD5 (pin43-pin47)
5)E (pin27)
6)STRA (pin26)

hope to hear from u shortly. thanks all expert!


Joined Feb 24, 2006
The infix "HC" in the part number is a giveaway. This is a CMOS part. It is standard operating procedure to tie unused CMOS input pins to Vdd or Vss(GND) through a resistor. This is required by test engineers if the board will ever be tested on a bed-of nails tester. Most port pins on single chip processors are defined as inputs at RESET. Even if you use a data direction register to define them as outputs they will be inputs for a while. The other possibility is internal pullups or pulldowns. Careful reading of the datasheet is required to discover which if any of your ports have this capability.

As to values. 4.7K to 47K would be typical for a pin used as an input. The actual choice is a compromise between current consumption and rise or fall time. If in doubt about what you require, use 10K.

On an output, that will be an input for a short time any high value pullup will be in parallel with an active driver and will have a slight effect on the external circuit.


Joined Nov 28, 2006
thanks papabravo.

the thread is me (c_en ) borrow Kevy account with acquire his permission to post bcos at that instant i stil haven register yet.

just for further information, i am using the MCU in single chip mode for my project.

description of the project:
-A PC with GUI design by using VB6 which have the 'on' and 'off' button.

-when the user click the 'on' or 'off' button,the VB6 will send 1 byte ASCII code from serial comm. port to RS232 interface (MAX232 IC) and MAX232 will send code to MCU (68HC11A1P) PD0 (pin 42).

-The MCU will then send the signal to that particular output pin to turn 'on' or 'off' of that particular LED.

-my output pins is
PA3-PA6 ( pin2 to pin 5)
PB0-PB7 (pin9 to pin 16)
PC0-PC7 (pin31 to pin 38)

besides the output pins connect to LED circuit,which connect the output pins to a inverter then to resistor (330 ohm),then to LED and lastly LED to 5V, other pins i just connect it according to the circuit that i attach here.

so there are some unused pins that i was stated on this thread under Kevy account, so kindly please provide more clear picture point about what should i do for those unused pin.

your response is highly appreciated.thanks!!!