Datasheet

Background Debug Module (S12SBDMV1)
MC9S12G Family Reference Manual, Rev.1.23
Freescale Semiconductor 307
Since the host knows the target serial clock frequency, the SYNC command (used to abort a command)
does not need to consider the lower possible target frequency. In this case, the host could issue a SYNC
very close to the 128 serial clock cycles length. Providing a small overhead on the pulse length in order to
assure the SYNC pulse will not be misinterpreted by the target. See Section 7.4.9, “SYNC — Request
Timed Reference Pulse”.
Figure 7-12 shows a SYNC command being issued after a READ_BYTE, which aborts the READ_BYTE
command. Note that, after the command is aborted a new command could be issued by the host computer.
Figure 7-12. ACK Abort Procedure at the Command Level
NOTE
Figure 7-12 does not represent the signals in a true timing scale
Figure 7-13 shows a conflict between the ACK pulse and the SYNC request pulse. This conflict could
occur if a POD device is connected to the target BKGD pin and the target is already in debug active mode.
Consider that the target CPU is executing a pending BDM command at the exact moment the POD is being
connected to the BKGD pin. In this case, an ACK pulse is issued along with the SYNC command. In this
case, there is an electrical conflict between the ACK speedup pulse and the SYNC pulse. Since this is not
a probable situation, the protocol does not prevent this conflict from happening.
Figure 7-13. ACK Pulse and SYNC Request Conflict
READ_BYTE READ_STATUSBKGD Pin Memory Address New BDM Command
New BDM Command
Host Target
Host Target
Host Target
SYNC Response
From the Target
(Out of Scale)
BDM Decode
and Starts to Execute
the READ_BYTE Command
READ_BYTE CMD is Aborted
by the SYNC Request
(Out of Scale)
BDM Clock
(Target MCU)
Target MCU
Drives to
BKGD Pin
BKGD Pin
16 Cycles
Speedup Pulse
High-Impedance
Host
Drives SYNC
To BKGD Pin
ACK Pulse
Host SYNC Request Pulse
At Least 128 Cycles
Electrical Conflict
Host and
Target Drive
to BKGD Pin