Block Guide — S12BDM V4

33

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 BDC 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 BDC 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 negedge

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 negedge in the BKGD pin, after a soft-reset has occurred, is considered by the target

as the start of a new BDC command, or the start of a SYNC request pulse.