mc9s12xd256 Freescale Semiconductor, Inc, mc9s12xd256 Datasheet - Page 588

• Manufacturer Part Number: mc9s12xd256
• Description: Hcs12x Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Chapter 15 Background Debug Module (S12XBDMV2)
Differently from the normal bit transfer (where the host initiates the transmission), the serial interface ACK
handshake pulse is initiated by the target MCU by issuing a negative edge in the BKGD pin. The hardware
handshake protocol in
should follow this timing constraint in order to avoid the risk of an electrical conflict in the BKGD pin.
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.
15.4.8
The abort procedure is based on the SYNC command. In order to abort a command, which had 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
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 if the serial interface is running on a different clock rate
than the bus. 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
588
Section 15.4.9, “SYNC — Request Timed Reference
Hardware Handshake Abort Procedure
The only place the BKGD pin can have an electrical conflict is when one
side is driving low and the other side is issuing a speedup pulse (high). Other
“highs” are pulled rather than driven. However, at low rates the time of the
speedup pulse can become lengthy and so the potential conflict time
becomes longer as well.
The ACK pulse does not provide a time out. This means for the GO_UNTIL
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
Figure 15-11
Section 15.4.8, “Hardware Handshake Abort
MC9S12XDP512 Data Sheet, Rev. 2.17
specifies the timing when the BKGD pin is being driven, so the host
NOTE
NOTE
Pulse”, and assumes that the pending command
Procedure”.
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