CHAPTER 11: DSTC
526 FM4 Peripheral Manual, Doc. No. 002-04856 Rev. *E
3.2.6 Read Skip Buffer Function
The transfer engine of the DSTC refers to the transfer information of the DES on the memory while
executing a transfer. If all transfers do not end in one Start Trigger, the DSTC writes back to each DES the
number of executed transfers of a DES and transfer addresses. If a transfer address is fixed or does not
need to be updated, the DSTC skips the write-back process. At the next Start Trigger, the DSTC
continues executing transfers according to updated DES information.
Since it takes time for the DSTC to refer to the DES at every Start Trigger, the DSTC has the read skip
buffer function as shown in Figure 3-5. The DSTC stores in its internal read skip buffer the transfer
information in the DES that the DSTC has read. If the next Start Trigger refers to the same DESP as the
current Start Trigger, the DSTC does not refer to the DES on the memory, but uses the values in the read
skip buffer to execute the transfer to increase the processing speed.
The read skip buffer function can be enabled and disabled by using the RBDIS bit in the CFG Register. In
practice, to increase the transfer speed, enable the read skip buffer function. If "1" is written to the RBDIS
bit in the CFG Register to disable the read skip buffer function, always refer to the DES directly on the
memory.
In the case of stopping the transfer operation of the DSTC (infinite loop out) by modifying the value of DV
in DES0 via the CPU to "10", after modifying the value of DV in DES0, invalidate the DES information
stored in the read skip buffer by writing "1" to the RBDIS bit in the CFG Register. As long as the read skip
buffer function remains enabled, the DSTC may skip referring to the DES and not be able to recognize
any change in the value of DV by the CPU. For its details, see "5.5 CFG Register".
3.2.7 Operation of the Transfer End
If the DSTC ends a transfer normally, according to details of the DES, it executes the Chain Start, sets the
interrupt flag (SWTR:SWST or HWINT[n]) and the DES close process.
If a transfer error occurs, a transfer is interrupted immediately. This interruption of a transfer is called error
end. In an error end, the DSTC does not execute the Chain Start. In addition, the DSTC does not set the
interrupt flag (SWTR:SWST or HWINT[n]), but records details related to the occurrence of the error in the
MONERS Register instead. Details of the error determine whether the DSTC executes the DES close
process.
In an HW Transfer, if a transfer error occurs, the DSTC sets the DQMSK[n] Register corresponding to the
channel on which that transfer error has occurred, and suppresses future HW transfer requests on that
channel.
Since details of the DES in which an error has occurred remain the same as they were during the transfer,
before starting a new transfer, rebuild the DES area with the CPU.
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