MC68HC05J3 Datasheet, PDF (22/92 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit

2.2.3.4 External clock
2
An external clock should be applied to the OSC1 input, with the OSC2 pin left unconnected, as
shown in Figure 2-3(d). The t_OXOV specification (see Table 9-5 and Table 9-6) does not apply
when using an external clock input. The equivalent specification of the external clock source
should be used in lieu of t_OXOV.
2.2.4
RESET
This active low input-only 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 stabilise. This input has an internal Schmitt trigger to improve noise
immunity.
2.2.5 PA0–PA7
These 8 I/O lines comprise ports A. The state of any pin is software programmable, and all the
pins are configured as inputs during power-on or reset.
2.2.6 PB0–PB5
These six pins comprise port B. The state of any pin is software programmable, and all the pins
are configured as inputs during power-on or reset. In addition to their normal I/O functions, pins
PB0–PB3 can be used to generate a keyboard interrupt when configured as an input while PB4
is shared with the output compare function (TCMP) of the programmable timer and PB5 with the
input capture function (TCAP) (see Section 4.3). PB0–PB5 contain an internal Schmitt trigger as
part of their input to improve noise immunity.
MOTOROLA
2-6
MODES OF OPERATION AND PIN DESCRIPTIONS
TPG
MC68HC05J3