NEC PD78P214 Manuale Utente

384

µ

PD78214 Sub-Series

Check the following items to ensure that current consumption is appropriately reduced in STOP mode:

The appropriate output level of each pin depends on the circuit of the next stage.  Select an output level that
minimizes current consumption.

• When the circuit of the next stage has a low input impedance, high output causes current to flow from the

power supply to the port, thus increasing current consumption.  A CMOS IC is an example of such a circuit.
CMOS ICs having low impedance when the power is turned off.  Apply a low voltage to pins used for output
to CMOS ICs, to reduce the current consumption and minimize any degradation in the reliability of the CMOS
ICs.  A high output may result in latch-up when the power is next turned on.

• Low output may increase the current consumption, depending on the circuit of the next stage.  In such a case,

apply a high voltage to the relevant pins or place the pins in the high-impedance state to reduce current
consumption.

The procedure for setting the output level for each port depends on the state of the port.

• When a port is set to control mode, the output level depends on the state of the internal hardware.  The state

of the internal hardware must be considered when setting the output level.

• When a port is set to port mode, the output level can be set by writing appropriate data to the output latch

for that port, as well as the port mode register, by means of a program.

When a port is set to control mode, setting of the output level is facilitated by changing the setting of the port
to port mode.

The voltage input to each pin must be maintained at a level between V

SS

 and V

DD

.  A voltage that falls outside

this range will result in increased current consumption as well as adversely affecting the reliability of the
microcomputer.  Any intermediate voltage must also be avoided.

Any unnecessary pull-up resistors may increase current consumption and result in the latch-up of other
devices.  Specify the use of only necessary pull-up resistors.

If a port consists both of pins that require pull-up resistors and those that do not require pull-up resistors,
connect external pull-up resistors to those pins that require them and specify internal pull-up resistors for a
port that is not to be used.

The outputs of the address bus pins are unpredictable (high or low) in STOP mode.  If the external circuit has
a low input impedance, current flows from the address bus pins, thus increasing the current consumption.
The input impedance of the external circuits connected to the address bus pins must therefore be high, even
in STOP mode.

In particular, CMOS ICs have a low input impedance when their power is turned off.  Therefore, the following
arrangements are necessary to prevent current consumption from increasing or the reliability of the ICs from
being degraded:

• Do not turn off the CMOS IC (always supply power, even in STOP mode).

• Connect a diode with a small V

F

 to prevent current from flowing, as shown in Fig. 14-6.