Engineering Bulletin

EB413

6

MOTOROLA

Care should be taken not to force excessive current into the MCU during

this time in case a latch-up condition arises. It is likely that some external

good clean and preferably noise free power source. A simple method is

to place in-line resistors on port pins that could see voltage levels in

good value for the resistor is normally about 50

Ω since there is normally

a maximum allowed current per pin of 20 mA and, it is assumed that the

in most applications) must be current limited to avoid damage to the

MCU.

Another phenomenon is the ability of low-power HC05 devices to be

powered up by an oscillating signal attached to the port pin of the HC05,

while the HC05 device is powered off. This occurs due to current being

diodes to both power supply rails within the MCU. As long as the current

is limited to 20 mA total for all pins in this state, no harm will come to the

MCU. However, it is important to keep the MCU in the reset condition

while powered off, at least so that during power up the MCU does not

drive data onto the port and draw a large current. While such current is

flowing, functionality of the MCU is not guaranteed due to the possibility

of substrate currents changing the state of the internal logic gates.

Often the reset signal is used by other sections of the application to start

up after the power-on sequence is complete and so it is possible that the

reset line is distributed throughout the board. Some devices will be

released from reset after different voltages on the reset signal and so it

(normally 4064 cycles) to avoid the MCU coming out of reset before

other parts of the board have done the same. This is particularly

important when there are several printed circuit boards (PCBs) and

multiple MCU devices talking to one another.

Freescale Semiconductor, Inc.

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