MC68HC05X16 Datasheet, PDF (35/232 Pages) Freescale Semiconductor, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit

Freescale Semiconductor, Inc.
2.3
Pin descriptions
2
2.3.1 VDD and VSS
Power is supplied to the microcontroller using these two pins. VDD is the positive supply and VSS
is ground.
It is in the nature of CMOS designs that very fast signal transitions occur on the MCU pins. These
short rise and fall times place very high short-duration current demands on the power supply. To
prevent noise problems, special care must be taken to provide good power supply bypassing at
the MCU. Bypass capacitors should have good high-frequency characteristics and be as close to
the MCU as possible. Bypassing requirements vary, depending on how heavily the MCU pins are
loaded.
2.3.2
IRQ
This is an input-only pin for external interrupt sources. Interrupt triggering is selected using the
INTP and INTN bits in the miscellaneous register, to be one of four options detailed in Table 10-3.
In addition, the external interrupt facility (IRQ) can be disabled using the INTE bit in the
Miscellaneous register (see Section 3.8). It is only possible to change the interrupt option bits in
the miscellaneous register while the I-bit is set. Selecting a different interrupt option will
automatically clear any pending interrupts. Further details of the external interrupt procedure can
be found in Section 10.2.3.2.
The IRQ pin contains an internal Schmitt trigger as part of its input to improve noise immunity. A
high voltage detector is provided on this pin to select modes of operation other than single-chip
mode. See Section 2.1.
2.3.3 RESET
This active low I/O pin is used to reset the MCU. Applying a logic zero to this pin forces the device
to a known start-up state. An external RC-circuit can be connected to this pin to generate a
power-on reset (POR) if required. In this case, the time constant must be great enough (at least
100ms) to allow the oscillator circuit to stabilise. This input has an internal Schmitt trigger to
improve noise immunity. When a reset condition occurs internally, i.e. from the COP watchdog, the
RESET pin provides an active-low open drain output signal that may be used to reset external
hardware.
MC68HC05X16 MODES OF OPERATION AND PIN DESCRIPTIONS
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