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Is it possible to handle unused pins in ways other those recommended ?

Latest Updated:06/16/2009

Question:

Is it possible to handle unused pins in ways other those recommended ?

Answer:

It is the responsibility of the user to keep the entire system in mind when considering the treatment of unused pins and countermeasures against malfunction. Handling of unused pins will be explained here.

In general, when terminating unused I/O ports, the following must be applied (see (9) below):
"Set the I/O ports for the input mode, connect low-impedance Vcc or Vss to the port through each resistor at the shortest possible distance from the port."
"Shortest possible wiring distance" and "low impedance" greatly influence the performance of the actual circuit. Longer wiring used to connect the Vcc and Vss may work adversely as an antenna for noise. When long wiring and high Vcc or Vss impedence cannot be avoided, the more effective method would be [open port in the output mode].
Always examine the termination of unused pins within your actual system from both hardware and software aspects.
Please use the information provided in the table below and the following explanations concerning each unused port.

Termination Methods of Unused Ports by their Attributes

Attribute Termination Method
Open
Termination Method
Direct connection to
Vss or Vcc
Termination Method
Connection to Vss or Vcc
through resistor
SAFETY
Input-only port
×
See (1)
OK
See (2)
OK
See (3)
SAFETY
Output-only port
OK
See (4)
×
See (5)
×
See (6)
SAFETY
I/O port
OK(Output Mode)
See (7)
×
See (8)
OK(Input Mode)
See (9)

Number in parenthesis indicates corresponding reference below.

(1) Input-only Port: Open
This method is not recommended due to possible intermediate potential affecting the MCU. The interval potential may cause problems such as through current flow from Vcc to Vss in internal circuits, increased consumption current, etc. However, the risk of destruction of the port or MCU due to the large current flow is fairly low.

(2) Input-only Port: Direct connection to Vss, Vcc
Input voltage is stabilized and influence of noise caused by power supply is increased.

(3) Input-only Port: Connection to Vss, Vcc through Resistor
This method is recommended.
Input voltage is stabilized and influence of noise caused by power supply is decreased. Place the resistor as close as possible to the pin. When the resistor and pin are too far apart, the long wiring acts adversely as an antenna for noise.
(When using internal pull-up/down resistors, please refer to the section concerning [Using Internal Pull-up/down Resistors] below.)

(4) Output-only Port: Open
This method is recommended.
Frequent changes to the level of the opened port will become a source of noise. We recommend fixing the output level by program.

(5) Output-only Port: Direct Connection to Vss, Vcc
This method is not recommended for the following reason: Current will flow when a potential difference occurs between the output voltage and the connected Vss or Vcc. Because there is nothing to restrict the current, over current may occur, which may destroy the port or MCU.

(6) Output-only Port: Connection to Vss, Vcc through Resistor
This method is not recommended, as current will flow when a potential difference occurs between the output voltage and the connected Vss or Vcc. If the resistor can be used to restrict the current, the risk of over current will be reduced.

(7) Input/Output Port: Open
In the output mode, this method is recommended as with the output-only port. However, frequent changes to the level of the open port will become a source of noise. We recommend fixing the output level by program.
In the input mode, this method is not recommended, as with the input-only port. The intermediate potential may cause problems such as through current flow from Vcc to Vss in internal circuits, increased consumption current, etc. However, the risk of destruction of the port or MCU due to the large current flow is fairly low.
This port is in the input mode after reset until it is switched to the output mode by program. Switch the port to the output mode as early as possible within the program.
In addition, noise can affect this method in the input mode as well. Because the port may be switched to the input mode due to noise, etc., we recommend periodically overwriting the port direction register as a countermeasure to noise interference.

(8) I/O Port: Direct Connection to Vss, Vcc
In the input mode, as with the input-only port, the input voltage is stabilized and influence of noise caused by power supply is increased.
In the output mode, this method is not recommended, as with the output-only port. The reason is as follows: Current will flow when a potential difference occurs between the output voltage and the connected Vss or Vcc. Because there is nothing to restrict the current, over current may occur, which may destroy the port or MCU.
Because the port may be switched to the output mode due to noise, etc., we recommend periodically overwriting the port direction register as a countermeasure to noise interference.

(9) I/O Port: Connection to Vss, Vcc through Resistor
In the input mode, as with the input-only port, the input voltage is stabilized, and influence of noise caused by power supply is decreased. Place the resistor as close as possible to the pin. When the resistor and pin are too far apart, the long wiring acts adversely as an antenna for noise.
In the output mode, this method is not recommended, as with the output-only port, as current will flow when a potential difference occurs between the output voltage and the connected Vss or Vcc. If the resistor can be used to restrict the current, the risk of over current will be reduced.
Because the port may be switched to the output mode due to noise, etc., we recommend periodically overwriting the port direction register as a countermeasure to noise interference.
(When using the internal pull-up/down resistor, please refer to the [Using Internal Pull-up/down Resistors] section below.)

[Using Internal Pull-up/down Resistors]
When using internal pull-up/down resistors rather than connecting external resistors, note that the internal pull-up resistors are OFF after reset. Therefore, the port will be in the OPEN state until the pull-up resistors are switched to ON. Switch the pull-up/down resistor to ON as early as possible within the program. Also, note that the internal pull-up/down is only valid in the input mode.

Suitable Products
R8C/1x
R8C/2x
R8C/3x
R8C/Lx
R8C/Mx