Renesas R8C/25 Manual descargar pdf (Pagina 5)

Rev. 1.1

R8C25 QuickDesign Guide.doc Page 5 of 10 12/23/2007

• Port 0 direction register is protected from accidental modification.

Before modifying this register Bit 2 in the Protect Register (PRCR) must

be Set. The protection bit is automatically Reset by the next internal

write operation of the MCU; therefore the setting of the Protect bit and

the modification of the direction register should be consecutive

instructions with interrupts disabled during this time (see the Protection

Section for more information)

• When setting a pin as an output it is recommended that the desired output

value of the port be written to the data latch first, then the direction register is

set to an output. Though not important in all systems, this prevents an

unintended output glitch on the port being setup.

• P4_2 can be used as an input if AD is not used. If AD is used this pin must

have a Vref voltage applied to it (see ADC section for more information)

5.2. Internal Pull-Up Resistors

• Most GPIO can have internal pull-up resistors enabled when they are used as

an input. The pullup resistors are typically applied as groups of pins, see the

HW manual for the grouping.

• If any pin is set to an output by setting the associated DDR to 1 the internal

pullup for that pin will be disabled.

5.3. Unassigned pins–

See section 7.5 of the HW manual for specific recommendations. For most

GPIO the designer has two options with regard to handling unused pins. If a

pin is just left floating it can consume extra power and is leaves the system

more susceptible to noise problems.

The first option is to set the pin to an input (the default state after Reset) and

connect the pin to Vcc or Vss using a resistor. There is no difference from an

MCU standpoint to one connection or another, however, there may be an

advantage from a system noise perspective. Vss is probably the most typical

choice. Connecting directly to Vcc or Vss is not recommended since a

direction register getting accidentally set to an output could create a shorted

output.

The second method sets the pin to an output. It does not really matter whether

the pin level is set high or low, however, selecting the pin as an output and

low connects the pin internally to the ground plane. This may help with

overall system noise concerns. One disadvantage the method of setting the

port pin to an output is that the configuration of the port must be done via

software control. While the MCU is held in Reset and until the direction

current can be tolerated during this time this does eliminate the pull-up

resistors required in previous method.

A variation on leaving the pins as inputs uses the internal pull-up resistors of

the MCU. This has the same limitation as setting the pins to outputs (requires

the program to set the port up) but it does limit the effect of accidental pin

shorts to ground, adjacent pins or Vcc since the device will not be driving the

pin.

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