Technical information

AN2317/D

16 Low-Cost Programming and Debugging Options for M68HC08 MCUs

Here are some suggestions on how to configure I/O pins for monitor mode

entry:

High level needed:

• Use as inputs, active low. Need weak pullup resistors (~10 kΩ).

• Use as outputs, active low. External circuitry could be LEDs with pullup

resistors.

Low level needed:

• Use as inputs, active high. Need weak pulldown resistors (~10 kΩ).

• Use as outputs, active high. External circuitry could be an NPN transistor

or N-channel FET with a pulldown resistor.

As discussed here, the mode select signals do not have to be supplied from the

external MON08 interface if they can be “pre-biased” by the application.

TST

/IRQ The IRQ pin is generally an input on most M68HC08s. As an input it can

withstand the higher-than-V

voltage that V

TST

needs to be. In most

applications this pin is held high by the internal pullup resistor in normal mode.

If an active driver like a latch or NOR gate is used to drive this input low during

an actual application, then proper isolation should be used to prevent the

applied V

TST

voltage from damaging the external driver. A series resistor may

be all that is needed in most cases.

The V

TST

signal should be supplied from the external MON08 interface to

prevent accidental monitor mode entry while the application is in normal

operation.

COM/PTA0 The PTA0 signal is best when configured as an input pin to receive the

bidirectional COM serial signal. On most M68HC08 MCUs, this pin is a

keyboard interrupt input. It also is an analog-to-digital input and

general-purpose I/O on other devices. If configured as an input pin in normal

modes, there will be no conflict in monitor mode when the COM signal is

connected to this pin. If this pin is configured as an output, the circuitry

connected to the pin must be isolated from the bidirectional serial data to avoid

inadvertent triggering of the application’s output function.

The COM signal must be supplied from the external MON08 interface.

Freescale Semiconductor, I

Freescale Semiconductor, Inc.

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