
MOTOROLA
5-50
SINGLE-CHIP INTEGRATION MODULE 2
MC68HC16R1/916R1
USER’S MANUAL
5.7.3.6 Clock Mode Selection
The state of the clock mode (MODCLK) pin during reset determines what clock source
the MCU uses. When MODCLK is held high during reset, the clock signal is generated
from a reference frequency using the clock synthesizer. When MODCLK is held low
during reset, the clock synthesizer is disabled, and an external system clock signal
must be applied. Refer to
5.3 System Clock
for more information.
NOTE
The MODCLK pin can also be used as parallel I/O pin PF0. To pre-
vent inadvertent clock mode selection by logic connected to port F,
use an active device to drive MODCLK during reset.
5.7.3.7 Breakpoint Mode Selection
Background debug mode (BDM) is enabled when the breakpoint (BKPT) pin is
sampled at a logic level zero at the release of RESET. Subsequent assertion of the
BKPT pin or the internal breakpoint signal (for instance, the execution of the CPU16
BKPT instruction) will place the CPU16 in BDM.
If BKPT is sampled at a logic level one at the rising edge of RESET, BDM is disabled.
Assertion of the BKPT pin or execution of the BKPT instruction will result in normal
breakpoint exception processing.
BDM remains enabled until the next system reset. BKPT is relatched on each rising
transition of RESET. BKPT is internally synchronized and must be held low for at least
two clock cycles prior to RESET negation for BDM to be enabled. BKPT assertion logic
must be designed with special care. If BKPT assertion extends into the first bus cycle
following the release of RESET, the bus cycle could inadvertently be tagged with a
breakpoint.
Refer to
4.14.4 Background Debug Mode
and the CPU16 Reference Manual
(CPU16RM/AD) for more information on background debug mode. Refer to the SCIM
Reference Manual(SCIMRM/AD) and
APPENDIX A ELECTRICAL CHARACTERIS-
TICS
for more information concerning BKPT signal timing.
5.7.3.8 Emulation Mode Selection
The SCIM2 contains logic that can be used to replace on-chip ports externally. The
SCIM2 also contains special support logic that allows external emulation of internal
ROM. This emulation support feature enables the development of a single-chip appli-
cation in expanded mode.
NOTE
The masked ROM is available only on the MC68HC16R1.