User`s manual

MOTOROLA MC68332TUT/D
14
2.7 Power Supply
Always connect all power and ground pins to power sources. Since internal power buses only serve about
8 - 10 pins each, the power and ground pins are usually not connected together within the device. If any
power pin is left floating, the pins served by the floating power pin can receive power from internal circuitry
such as internal protection diodes. However, the current path will usually have several diode drops, resulting
in a low output high voltage (about 3 volts) on associated output pins.
2.7.1 Low Voltage Inhibit Devices
A low voltage inhibit (LVI) device (also referred to as a reset supervisor circuit) protects the MCU by keeping
it in reset until full voltage is applied and by forcing an external reset as soon as power starts to fall. This
prevents the MCU from going into an indeterminate state due to a power supply failure or slow power supply
ramp-up time.
A number of manufacturers make LVI devices that can be used with the MCU.
Analog Devices — [phone]
LVI part numbers are ADM698 and ADM699. These devices require pull-up resistors.
Dallas Semiconductor— [phone]
Various reset supervisor circuits. Part numbers DS1233A, D, and M do not require pull-up resistors.
Linear Technologies— [phone]
LVI part numbers are LTC692 and LTC693. These devices require a pull-up resistor.
Maxim — [phone] or [phone]
Various reset supervisor circuits. MAX 690 and MAX 700 series devices require pull-up resistors, but
MAX 809 devices do not.
Motorola, Inc.— [phone]
Reset supervisor circuit part number is MC34064. It requires an external pull-up resistor.
2.7.1.1 Using LVI Devices with External Oscillators
An LVI device provides an extra degree of protection when an external oscillator that has an independent
power supply is used to generate the system clock. In this case, the LVI device ensures that the oscillator
does not power up before the MCU.
2.7.1.2 Using LVI Devices with Multiple Power Supplies
Take special precautions when system components that are connected to each other have separate power
supplies. Generally, one power supply will reach operating voltage more quickly than another. A device con-
nected to this fast supply can begin to operate before devices connected to a slower supply have reached
operating voltage. If a device connected to a fast supply drives logic one levels to a device connected to a
slow supply, the input protection diodes of the slow -starting device can be momentarily forward biased, and
significant current can be injected into the device substrate. In the case of an MCU, the injected current can
cause internal nodes to be improperly charged or discharged. Since this action is random, it is impossible
to predict what will happen when injection occurs. Usually, the processor will fail to fetch opcodes. Figure
10 shows how to use LVI devices to prevent this problem. Each power supply is monitored by a separate
LVI device. Signals from other boards are inhibited until correct operating voltage is applied.