SC312XDP512F0MPVR datasheet(589/1348 Pages) FREESCALE

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Chapter 15 Background Debug Module (S12XBDMV2)

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

589

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 15.4.9, “SYNC — Request

Timed Reference Pulse”.

Figure 15-13 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 15-13. ACK Abort Procedure at the Command Level

NOTE

Figure 15-13 does not represent the signals in a true timing scale

READ_BYTE

READ_STATUS

BKGD Pin

Memory Address

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)

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