Technical information

AN2317/D
MON08 Connections on a Target Board
Low-Cost Programming and Debugging Options for M68HC08 MCUs 17
OSC/OSC1 The OSC signal is always applied to the OSC1 pin on every M68HC08 for
monitor mode entry. This pin may have a crystal or RC attached to it in normal
operation. The external oscillator can easily overdrive the crystal or RC to
provide the 9.8 MHz monitor mode frequency. On device families like the
M68HC908GT, M68HC908KX, and M68HC908QYQT, this pin may be a
general-purpose I/O pin in addition to an optional oscillator input in normal
operation.
Unless the appropriate clock source is available in the application, the OSC
signal should be supplied from the external MON08 interface.
RST
The RST pin is an input and a low-driving output on most M68HC08s. As an
input it has an internal or external pullup resistor. As an output it drives the
pulled-up reset line low to signal an internal reset condition. Depending on the
level of debugging necessary in the target, this pin may not need to be brought
out to a MON08 interface. That is, if the reset input is not a critical part of the
application, the reset function can be accomplished by cycling power to the
board, thereby providing a power-on-reset (POR) which is sufficient for
programming and most debugging.
In the cases where the IRQ
is an active part of the application, applying V
TST
to the RST
pin during monitor mode releases the IRQ pin to be operated as the
interrupt input, but only after successful monitor mode entry with V
TST
on IRQ.
This sequencing of IRQ
and RST is generally provided by sophisticated (and
more expensive) debug tools.
On some low-pin count devices, such as the M68HC908KX and
M68HC908QY/QT Families, the RST
pin is shared with general-purpose I/O. In
these cases, the RST
pin must be pulled to V
DD
with a resistor to enter normal
monitor mode. V
TST
cannot be applied to the pin on these parts.
In general, the RST signal does not have to be supplied by the external MON08
interface.
Ground/V
SS
Ground must be provided as the signal reference in the external MON08
interface.
Power/V
DD
This connection can be used to power the MON08 interface if the pod does not
have its own power supply. The current requirements of the MON08 interface
must be considered in the application’s power supply design when deciding to
use a target-powered MON08 interface.
In summary, the V
TST
/IRQ, COM/PTA0, OSC/OSC1, and Ground/V
SS
pins
must be provided in the external MON08 interface. The Power/V
DD
pin can be
provided if the MON08 circuit needs to be powered from the target. The RST
pin and the four mode select signals may not need to be provided if the
application can bias them in normal operation.
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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