MC68HC05B4 Datasheet, PDF (38/298 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit

2.5
2
2.5.1
Pin descriptions
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 by-passing at
the MCU. By-pass 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.
2.5.2
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 9-3.
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 9.2.3.1.
The IRQ pin contains an internal Schmitt trigger as part of its input to improve noise immunity.
2.5.3 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 to allow
the oscillator circuit to stabilize. 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.
2.5.4 TCAP1
The TCAP1 input controls the input capture 1 function of the on-chip programmable timer system.
MOTOROLA
2-10
MODES OF OPERATION AND PIN DESCRIPTIONS
TPG
MC68HC05B6
Rev. 4