MM912F634CV2AP datasheet(206/335 Pages) FREESCALE

206 / 335 page

Functional Description and Application Information

Background Debug Module (S12SBDMV1)

MM912F634

Freescale Semiconductor

206

NOTE

The ACK pulse does not provide a timeout. This means for the GO_UNTIL(169) command

that it can not be distinguished if a stop or wait has been executed (command discarded and

ACK not issued) or if the “UNTIL” condition (BDM active) is just not reached yet. Hence in

any case where the ACK pulse of a command is not issued the possible pending command

should be aborted before issuing a new command. See the handshake abort procedure

described in Section 4.30.4.8, “Hardware Handshake Abort Procedure"”.

The ACK handshake protocol does not support nested ACK pulses. If a BDM command is not acknowledge by an ACK pulse,

the host needs to abort the pending command first in order to be able to issue a new BDM command. When the CPU enters wait

or stop while the host issues a hardware command (e.g., WRITE_BYTE), the target discards the incoming command due to the

wait or stop being detected. Therefore, the command is not acknowledged by the target, which means that the ACK pulse will

not be issued in this case. After a certain time the host (not aware of stop or wait) should decide to abort any possible pending

ACK pulse in order to be sure a new command can be issued. Therefore, the protocol provides a mechanism in which a

command, and its corresponding ACK, can be aborted.

4.30.4.8

Hardware Handshake Abort Procedure

The abort procedure is based on the SYNC command. In order to abort a command, which has not issued the corresponding

ACK pulse, the host controller should generate a low pulse in the BKGD pin by driving it low for at least 128 serial clock cycles

and then driving it high for one serial clock cycle, providing a speedup pulse. By detecting this long low pulse in the BKGD pin,

the target executes the SYNC protocol, see Section 4.30.4.9, “SYNC — Request Timed Reference Pulse"”, and assumes that

the pending command and therefore the related ACK pulse, are being aborted. Therefore, after the SYNC protocol has been

completed the host is free to issue new BDM commands. For Firmware READ or WRITE commands it can not be guaranteed

that the pending command is aborted when issuing a SYNC before the corresponding ACK pulse. There is a short latency time

from the time the READ or WRITE access begins until it is finished and the corresponding ACK pulse is issued. The latency time

depends on the firmware READ or WRITE command that is issued and on the selected bus clock rate. When the SYNC command

starts during this latency time the READ or WRITE command will not be aborted, but the corresponding ACK pulse will be

aborted. A pending GO, TRACE1 or GO_UNTIL(169) command can not be aborted. Only the corresponding ACK pulse can be

aborted by the SYNC 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.

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.

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 4.30.4.9, “SYNC — Request Timed Reference Pulse"”.

MM912F634CV2AP Datasheet(PDF) 206 Page - Freescale Semiconductor, Inc

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