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Chapter 15 Background Debug Module (S12XBDMV2)

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

593

stop mode has been reached. Hence after a system stop mode the handshake feature must be

enabled again by sending the ACK_ENABLE command.

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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