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
Host
Memory Address
Target
READ_STATUS
New BDM Command
Host Target
Host
Target
BDM Decode
New BDM Command
and Starts to Execute
the READ_BYTE 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.
At Least 128 Cycles
BDM Clock
(Target MCU)
ACK Pulse
Target MCU
High-Impedance
Electrical Conflict
Drives to
BKGD Pin
Speedup Pulse
Host and
Host
Drives SYNC
To BKGD Pin
Target Drive
to BKGD Pin
Host SYNC Request Pulse
BKGD Pin
16 Cycles
Figure 5-13. ACK Pulse and SYNC Request Conflict
S12P-Family Reference Manual, Rev. 1.13
Freescale Semiconductor
149
FREESCALE [ Freescale ]
