Technical information

AN2317/D
8 Low-Cost Programming and Debugging Options for M68HC08 MCUs
Custom MON08
Interfaces
As mentioned above, monitor mode entry conditions can be introduced to a
circuit board in many ways. The method of mode entry and communications
can be as simple as including a “standard” MON08 interface (16-pin ribbon
cable to a 2 x 8 dual row header) to program or debug. Since dual row headers
can add unnecessary expense and area to a production board, other
connection methods should be explored in some applications.
Successful in-circuit programming schemes implemented by customers
include a bed-of-nails test environment, a board edge connector and socket
arrangement, and custom cabling that makes alternate use of cable
connections already present on the circuit board. These schemes can route the
necessary monitor mode signals to the board in a test or calibration
environment and not interfere with the end application. Mating the
MON08-Cyclone to any of these schemes makes a programming or
reprogramming session easy to operate, and does not require a PC.
Let’s explore some methods to communicate with a target in monitor mode
without the benefit of an ICS, MON08-Cyclone, MON08-Multilink, or other
production tool. These methods all can be made into target-powered MON08
interfaces (power for the communications circuit comes from the target board)
and become the “pod” for the monitor mode interface scheme.
Normal Monitor Mode
Circuit
The normal monitor mode circuit in most M68HC08 data books looks
something like that shown in Figure 4. The circuit to the left of the MCU in
Figure 4 shows the MAX232 serial level shifter and the 74HC125 buffers that
provide the bidirectional data switching for the serial communications on port
PTA0. This portion can be used a stand-alone pod for monitor mode
communication with many M68HC08 variants. Notice that we use the V+
supply from the MAX232 to provide the V
TST
voltage for the IRQ pin. The signal
generator in the middle of the figure is usually a canned oscillator which can
also be placed in the stand-alone pod.
The RST
pin circuit is optional. The reset function can be accomplished by
simply toggling power to the target board since a power on reset (POR —
specified as V
DD
less than 0.1 V) is adequate for most programming and debug
sessions. The requirement to have a high level on RST
is addressed by the
internal pullup resistor that’s present on most M68HC08 RST
pins or an
external pullup for devices like the M68HC908A and M68HC908MR Families.
More information on the RST
pin is given below.
The mode select signals on the right of the MCU are only necessary during the
rising edge of reset to enter monitor mode. The biasing of these signals is
described in more detail below.
Also note that a 9.8304 MHz clock source is used. This requires the DIV4 signal
to be pulled high to provide the 9600 baud rate.
Freescale Semiconductor, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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