MC9S12HY64 Datasheet, PDF (182/792 Pages) Freescale Semiconductor, Inc – S12 Microcontrollers

Background Debug Module (S12SBDMV1)
5.4.11 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. 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.
MC9S12HY/HA-Family Reference Manual, Rev. 1.04
182
Freescale Semiconductor