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Chapter 19 Background Debug Module (S12XBDMV2)
MC9S12XDP512 Data Sheet, Rev. 2.11
Freescale Semiconductor 785
GO_UNTIL command can not be aborted. Only the corresponding ACK pulse can be aborted by the
SYNC command.
Although it is not recommended, the host could abort a pending BDM command by issuing a low pulse in
the BKGD pin shorter than 128 serial clock cycles, which will not be interpreted as the SYNC command.
The ACK is actually aborted when a negative edge is perceived by the target in the BKGD pin. The short
abort pulse should have at least 4 clock cycles keeping the BKGD pin low, in order to allow the negative
edge to be detected by the target. In this case, the target will not execute the SYNC protocol but the pending
command will be aborted along with the ACK pulse. The potential problem with this abort procedure is
when there is a conflict between the ACK pulse and the short abort pulse. In this case, the target may not
perceive the abort pulse. The worst case is when the pending command is a read command (i.e.,
READ_BYTE). If the abort pulse is not perceived by the target the host will attempt to send a new
command after the abort pulse was issued, while the target expects the host to retrieve the accessed
memory byte. In this case, host and target will run out of synchronism. However, if the command to be
aborted is not a read command the short abort pulse could be used. After a command is aborted the target
assumes the next negative edge, after the abort pulse, is the first bit of a new BDM command.
NOTE
The details about the short abort pulse are being provided only as a reference
for the reader to better understand the BDM internal behavior. It is not
recommended that this procedure be used in a real application.
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 19.4.9, “SYNC — Request
Timed Reference Pulse”.
Figure 19-13 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 19-13. ACK Abort Procedure at the Command Level
NOTE
Figure 19-13 does not represent the signals in a true timing scale
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)