MC9S12P128 Datasheet, PDF (149/564 Pages) Freescale Semiconductor, Inc – S12 Microcontrollers

Background Debug Module (S12SBDMV1)
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 5.4.9, “SYNC — Request
Timed Reference Pulse”.
Figure 5-12 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.
READ_BYTE CMD is Aborted
by the SYNC Request
(Out of Scale)
SYNC Response
From the Target
(Out of Scale)
BKGD Pin READ_BYTE Memory Address
Host Target
READ_STATUS
Host Target
New BDM Command
Host Target
BDM Decode
and Starts to Execute
the READ_BYTE Command
New BDM Command
Figure 5-12. ACK Abort Procedure at the Command Level
NOTE
Figure 5-12 does not represent the signals in a true timing scale
Figure 5-13 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.
BDM Clock
(Target MCU)
Target MCU
Drives to
BKGD Pin
Host
Drives SYNC
To BKGD Pin
BKGD Pin
At Least 128 Cycles
ACK Pulse
Host and
Target Drive
to BKGD Pin
Host SYNC Request Pulse
High-Impedance
Electrical Conflict
Speedup Pulse
16 Cycles
Figure 5-13. ACK Pulse and SYNC Request Conflict
S12P-Family Reference Manual, Rev. 1.12
Freescale Semiconductor
149