Bkgd Pin Description - NXP Semiconductors MC9S08SU16 Reference Manual

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 V . An open-drain connection to reset allows the host to force a target
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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 V can be used to allow the pod to use power from the target system to
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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.

27.2.1 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
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.
NXP Semiconductors
BKGD 1
NO CONNECT 3
NO CONNECT 5
Figure 27-1. BDM tool connector
MC9S08SU16 Reference Manual, Rev. 5, 4/2017
Chapter 27 Development support
2 GND
4
RESET
6 V
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