mc9s12xd256 Freescale Semiconductor, Inc, mc9s12xd256 Datasheet - Page 593

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

15.4.11 Serial Communication Time Out
The host initiates a host-to-target serial transmission by generating a falling edge on the BKGD pin. If
BKGD is kept low for more than 128 target clock cycles, the target understands that a SYNC command
was issued. In this case, the target will keep waiting for a rising edge on BKGD in order to answer the
SYNC request pulse. If the rising edge is not detected, the target will keep waiting forever without any
time-out limit.
Consider now the case where the host returns BKGD to logic one before 128 cycles. This is interpreted as
a valid bit transmission, and not as a SYNC request. The target will keep waiting for another falling edge
marking the start of a new bit. If, however, a new falling edge is not detected by the target within 512 clock
cycles since the last falling edge, a time-out occurs and the current command is discarded without affecting
memory or the operating mode of the MCU. This is referred to as a soft-reset.
If a read command is issued but the data is not retrieved within 512 serial clock cycles, a soft-reset will
occur causing the command to be disregarded. The data is not available for retrieval after the time-out has
occurred. This is the expected behavior if the handshake protocol is not enabled. However, consider the
behavior where the BDM is running in a frequency much greater than the CPU frequency. In this case, the
command could time out before the data is ready to be retrieved. In order to allow the data to be retrieved
even with a large clock frequency mismatch (between BDM and CPU) when the hardware handshake
protocol is enabled, the time out between a read command and the data retrieval is disabled. Therefore, the
host could wait for more then 512 serial clock cycles and still be able to retrieve the data from an issued
read command. However, once the handshake pulse (ACK pulse) is issued, the time-out feature is
re-activated, meaning that the target will time out after 512 clock cycles. Therefore, the host needs to
retrieve the data within a 512 serial clock cycles time frame after the ACK pulse had been issued. After
that period, the read command is discarded and the data is no longer available for retrieval. Any negative
edge in the BKGD pin after the time-out period is considered to be a new command or a SYNC request.
Note that whenever a partially issued command, or partially retrieved data, has occurred the time out in the
serial communication is active. This means that if a time frame higher than 512 serial clock cycles is
observed between two consecutive negative edges and the command being issued or data being retrieved
is not complete, a soft-reset will occur causing the partially received command or data retrieved to be
disregarded. The next negative edge in the BKGD pin, after a soft-reset has occurred, is considered by the
target as the start of a new BDM command, or the start of a SYNC request pulse.
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stop mode has been reached. Hence after a system stop mode the handshake feature must be
enabled again by sending the ACK_ENABLE command.
MC9S12XDP512 Data Sheet, Rev. 2.17
Chapter 15 Background Debug Module (S12XBDMV2)
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