AN2321 Freescale Semiconductor / Motorola, AN2321 Datasheet - Page 17

• Manufacturer Part Number: AN2321
• Description: Designing for Board Level Electromagnetic Compatibility
• Manufacturer: Freescale Semiconductor / Motorola

I/O port pins
IRQ pin
Reset pin
MOTOROLA
With most MCUs, pins are usually high impedance inputs or mixed
inputs/outputs. High impedance input pins are susceptible to noise and can
register false levels if not properly terminated. Pins which are inputs and not
internally terminated need some high resistance (e.g. 4.7k Ω or 10k Ω resistor)
attached to each pin to ground or supply to ensure a known logic state.
Unconnected input pins often float to the mid-point of the supply rail or to an
undefined voltage due to internal leakage paths.
For the IRQ or reset pins (input pins) the termination is more important than the
general I/O ports. If noise causes these two pins to mis-trigger it will have a
catastrophic effect on circuit behavior. A high current consumption is often
observed, particularly in CMOS devices, when the input pins are unconnected,
as the input latch is half open, half closed, resulting in a leakage current internal
to the IC. Terminating high impedance input pins can therefore lead to a
reduction in supply current, as well as other EMC benefits.
Owing to the effects that interrupts have on MCU operation this is one of the
most sensitive pins on the device. The IRQ could be polled from devices at a
distance to the MCU on the PCB, or even on a plug-in adapter or subsystem
cards. Consequently, it is important to ensure that any line connecting to an
interrupt request is protected against ESD transients. Bidirectional diodes,
transorbs or metal oxide varistor terminations on the IRQ line are usually
adequate for ESD and will help reduce overshoot and ringing without producing
a significant line load. For cost sensitive applications, a resistive termination will
also maintain the IRQ line in a fixed state.
Improper resets can cause many problems since different applications impose
very different conditions on the start up and power down of the MCU. Reset in
its most basic function ensures that the MCU starts executing code in a
controlled manner once power is applied.
At power on, the supply rises to the MCU operating voltage, and it will be some
time before the oscillator becomes stable. Therefore, it is necessary to have
some time delay on the reset pin. The simplest delay is a resistor-capacitor
(RC) network, where the capacitor is charged up by the current flowing through
the resistor until it reaches a level detected by the MCU reset pin circuitry as a
logic 1 state.
Ideally there are no restrictions on the values of the resistors and capacitors,
but there are other considerations. Internal leakage currents for the reset pin
are normally specified at 1 µ A (for Motorola HC08 MCU) maximum, which
means that the resistor has a maximum value of 100k Ω and the capacitor
should not be an electrolytic type, keeping the stop current to a minimum.
Ceramic capacitors are recommended as a compromise between low cost, low
leakage, and good high-frequency response. The reset pin capacitance is very
low (less than 5pF for Motorola HC08 MCU). There are also limits for the
minimum resistor impedance since the maximum pull-down current is
Designing for Board Level Electromagnetic Compatibility
Freescale Semiconductor, Inc.
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PART 2: COMPONENT SELECTION AND CIRCUIT DESIGN TECHNIQUES
AN2321/D
17