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What are the reasons behind handling unused pins?

Latest Updated:04/10/2002

Question:

(I/O Port , Pin) Please tell me more about the handling of unused pins and the reasons behind it. [2002/04/10]

Answer:

The handling of unused CMOS 3 state I/O ports is explained below.
(1) Example of Process Type 1

Set ports to input mode and connect Vcc or Vss to each pin through 1k-ohm to 10k-ohm resistor.

[Reason]
When a port is set to input mode, both "H" and "L"of the output transistor are turned off, and the port is in a floating state.
To prevent through current from flowing to the pin when middle potential is applied in this state, connect the pin to Vcc or Vss.
Since both output transistors are off even if the pin is connected to Vcc and Vss, current will not flow.

Moreover, although a resistor is not required when only input is considered, when a direction register is reversed and an output transistor is turned on due to noise or program run-away, current flows to the MCU from Vcc or from the MCU to Vss. A resistor is required to prevent port destruction and operation malfunction due to the flow of especially large current.

In addition, overwriting the direction register periodically (refresh) is recommended to prevent the direction register from reversing due to noise or program run-away.

(2) Example of Process Type 2: port with built-in pull-up resistor

If a port that has a built-in resistor, it can be used instead of connecting an external resistor.
Set a port to input mode and select "pull-up resistor is available".

[Reason]
When a port is set to input mode, both "H" and "L" of the output transistor are turned off, and the port is in a floating state.
To prevent through current from flowing to the pin when middle potential is applied in this state, connect the pin to Vcc or Vss.
Since both output transistors are off even if the pin is connected to Vcc and Vss, current will not flow.

Moreover, although a resistor is not required when only input is considered, when a direction register is reversed and an output transistor is turned on due to noise or program run-away, current flows to the MCU from Vcc or from the MCU to Vss. A resistor is required to prevent port destruction and operation malfunction due to the flow of especially large current.

If the built-in pull-up resistor is turned on, it will produce the same results as if the pin were connected to Vcc through an externally connected resistor.
However, the voltage level may become unstable and power supply current may increase until the pull-up resistor is selected.
We recommend selecting "pull-up resistor is available" as early as possible after reset.
In addition, overwriting the direction register periodically (refresh) is recommended to prevent the direction register from reversing due to noise or program run-away.

(3) Example of Process Type 3

Set a port to output mode and set to open.

[Reason]
When a port is set to output mode, based on the contents of a port latch, one of the output transistors is turned on and "H" or "L" is output from the pin.
At this time, since the input side is in the off state, current does not flow.

In addition, the voltage level may become unstable and the power supply current may increase until an output is selected by the direction register.
We recommend setting the direction register to "1" as early as possible after reset.
In addition, overwriting the direction register periodically (refresh) is recommended to prevent the direction register from reversing due to noise or program run-away.

(#105176)

Suitable Products
3803H
3803L
3823
3850A
3858
38D2
38D5
7544
7546
7547
7548
7549
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