Technical information
AN2317/D
MON08 Connections on a Target Board
Low-Cost Programming and Debugging Options for M68HC08 MCUs 15
MON08 Connections on a Target Board
A target-powered monitor mode interface gets its power and ground
connections from the target printed circuit board. Depending on the usage of
the general-purpose I/O and control pins, as few as three other signals (COM,
V
TST
, and OSC) need to be brought onto the board.
With proper planning, an application can have the appropriate mode and
control pins preconfigured to reduce the number of pins required to enter
monitor mode. Engineering Bulletins EB366 through EB370 show examples of
user circuits that have the mode select pins pulled high or low, and therefore,
demonstrate that as few as five signals, including power and ground, are
needed to communicate with the application. The following figure from EB366
shows this concept.
Mode Select Signals
(MOD1, MOD0, DIV4,
SSEL)
Let’s consider the mode select signals first. These four signals can be biased
by the default state of the application’s I/O so that the MON08 connector does
not have to connect to them. They can be either inputs or outputs in the target
circuit. During monitor mode entry they just have to be at the appropriate logic
state to provide mode selection. For instance, in Figure 9, pins PTC0 and
PTC3 can be configured as inputs that are active low in the application. The
external pullup resistors would be a necessary expense to provide the proper
biasing since the internal pullup resistors are not enabled during reset. PTC0
and PTC3 configured as active low outputs would also be effective. As LED
drive ports, these pins can be pulled up to V
DD
through the LED and current
limiting resistor and provide the high logic level without additional components.
Figure 9. Target-Powered Monitor Mode Interface.
RS-232
HC125
V
TST
PTC0 PTC3
PTC1 PTA7
V
DD
V
DD
IRQ
PTA0
GND
OSC1
9.8304 MHz
OPTIONAL
9600 BAUD
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
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