HCS08 datasheet(263/316 Pages) NXP

263 / 316 page

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

263

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Non-intrusive commands can be executed at any time even while the user’s program is running.

Non-intrusive commands allow a user to read or write MCU memory locations or access status and

control registers within the background debug controller.

Typically, a relatively simple interface pod is used to translate commands from a host computer into

commands for the custom serial interface to the single-wire background debug system. Depending on the

development tool vendor, this interface pod may use a standard RS-232 serial port, a parallel printer port,

or some other type of communications such as a universal serial bus (USB) to communicate between the

host PC and the pod. The pod typically connects to the target system with ground, the BKGD pin, RESET,

and sometimes VDD. An open-drain connection to reset allows the host to force a target system reset,

which is useful to regain control of a lost target system or to control startup of a target system before the

on-chip nonvolatile memory has been programmed. Sometimes VDD can be used to allow the pod to use

power from the target system to avoid the need for a separate power supply. However, if the pod is powered

separately, it can be connected to a running target system without forcing a target system reset or otherwise

disturbing the running application program.

Figure 17-1. BDM Tool Connector

17.2.1

BKGD Pin Description

BKGD is the single-wire background debug interface pin. The primary function of this pin is for

bidirectional serial communication of active background mode commands and data. During reset, this pin

is used to select between starting in active background mode or starting the user’s application program.

This pin is also used to request a timed sync response pulse to allow a host development tool to determine

the correct clock frequency for background debug serial communications.

BDC serial communications use a custom serial protocol first introduced on the M68HC12 Family of

microcontrollers. This protocol assumes the host knows the communication clock rate that is determined

by the target BDC clock rate. All communication is initiated and controlled by the host that drives a

high-to-low edge to signal the beginning of each bit time. Commands and data are sent most significant

bit first (MSB first). For a detailed description of the communications protocol, refer to Section 17.2.2,

“Communication Details.”

If a host is attempting to communicate with a target MCU that has an unknown BDC clock rate, a SYNC

command may be sent to the target MCU to request a timed sync response signal from which the host can

determine the correct communication speed.

BKGD is a pseudo-open-drain pin and there is an on-chip pullup so no external pullup resistor is required.

Unlike typical open-drain pins, the external RC time constant on this pin, which is influenced by external

capacitance, plays almost no role in signal rise time. The custom protocol provides for brief, actively

driven speedup pulses to force rapid rise times on this pin without risking harmful drive level conflicts.

Refer to Section 17.2.2, “Communication Details,” for more detail.

NO CONNECT 5

NO CONNECT 3

RESET

BKGD

GND

VDD

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