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11.4
Usage Notes
Keep the following points in mind when using the I/O ports.
When the MIS MIS2 bit is set to 1, the R0
2/SO pin will be an NMOS open drain output
regardless of whether it is selected for use as the R0
2 pin or as the SO pin by the PMRA
PMRA0 bit.
I/O pins that are unused in user systems must be tied to a fixed potential, since floating I/O
pins can cause noise that can interfere with LSI operation. The following are examples of
techniques that can prevent noise problems.
High voltage pin:
Select “no pull-down MOS transistor (PMOS open drain)” as the mask
option and connect the pin to V
CC on the user system printed circuit
board.
Standard voltage pin:
Either use the built-in pull-up MOS transistor to pull the pin up to V
CC,
or pull up the pin to V
CC externally with a pull-up resistor of about
100 k
.
Application programs should maintain the PDR and DCR contents for unused pins at their
reset state values. Also note that unused pins must not be selected for use as peripheral
function I/O pins.
When the MIS MIS3 bit is set to 1 (pull-up MOS transistors active) and the PDR for an R
port/analog input shared function pin has the value 1, the MOS transistor for the corresponding
pin will not be turned off by selecting the analog input function with the AMR1 or AMR2
register.
To use an R port/analog input shared function pin as an analog input when the pull-up MOS
transistors are active, always clear the PDR for the corresponding pin to 0 first and then turn
off the pull-up MOS transistor. (Note that the PDR registers are set to 1 immediately following
a reset.)
Figure 11-3 shows the circuit for the R port/analog input shared function pins. AMR1 and
AMR2 are used to set the port outputs to high impedance. ACR is used to switch the analog
input channel.
The states of the R port/analog input shared function pins are set, as shown in table 11-9, by
the combination of the AMR1 (or AMR2) register, the MIS3 bit, the DCR, and the PDR
settings.