CHAPTER 11: DSTC
FM4 Peripheral Manual, Doc. No. 002-04856 Rev. *E 519
3.1.5 Other DES Settings
DV[1:0]
The DSTC refers to the DES area and updates it while executing a transfer operation. If the CPU updates
a DES area that the DSTC is using, the DSTC may executes an illegal transfer operation, which is not set
in a program. To prevent any illegal transfer operation, the DES area uses a mutually exclusive memory
management system for the CPU and the DSTC. In DES0, there are the DV[1:0] bits (Descriptor Valid),
indicating that the DES write update right ownership belongs to the CPU or the DSTC. Table 3-9 shows
details of the DV and the related operations of the DSTC.
Table 3-9 Details of DV
DV specifies which of the CPU and the DSTC the ownership of the DES belongs to.
DV specifies whether a transfer is executed after the DES open process
DV specifies whether the DES close process is to be executed after transfer ended.
00: The owner is the CPU. No transfer is executed. No DES close process is executed.
( If the DSTC read this value, the DSTC notifies a DES open error.)
01: The owner is the DSTC. A transfer is executed. The DES close process is executed.
10: The owner is the DSTC. No transfer is executed. The DES close process is executed.
11: The owner is the DSTC. A transfer is executed. No DES close process is executed.
That DV is 00 indicates that the CPU has the ownership of the DES area. That DV is 01, 10 or 11
indicates that the DSTC has the ownership of the DES area. After setting the initial values of a DES, the
CPU sets DV to 01, 10 or 11 to notify that the ownership of that DES belongs to the DSTC. After receiving
a Start Trigger, the DSTC checks the DV value of DES0 to determine subsequent operations. (The DSTC
reading a DES upon a Start Trigger is called DES open process.)
In the case of DV set to 01, the DSTC executes transfers for the number of times (ORM×IIN) specified.
While the transfers are in progress, the value of DV remains 01 and the DSTC keeps the ownership of the
DES. When all specified times of transfer end, the DSTV updates the value of DV to 00 and the DSTC
returns the ownership of the DES to the CPU. (The process that the DSTC updates the value of DV to 00
and the DSTC returns the ownership of the DES to the CPU is called DES close process.)
In the case of DV set to 11, the DSTC executes transfers for the number of times (ORM×IIN) specified.
While the transfers are in progress, the value of DV remains 11 and the DSTC keeps the ownership of the
DES. Even after all specified times of transfer have ended, the DSTC does not execute the DES close
process and keeps having the ownership of the DES.
In the case of DV set to 10, the DSTC does not execute a transfer, but executes only the DES close
process.
In the case of DV set to 00, the DSTC recognizes that the DES area is being updated by the CPU. It does
not execute any transfer or update the DES. The DSTC notifies the CPU of a DES open error.
The program determines the value of DV as explained below based on the way of using a DES.
If details of a transfer defined in a DES are subject to change, and the CPU has to update details of the
DES at every transfer, setting DV to 01 grants the ownership of the DES to the DSTC. In this situation,
after checking that the value of DV is 00 and that the ownership of the DES has been returned to the CPU,
the CPU can safely update the DES. The CPU transfers the ownership of the DES to the DSTC again
after completing the initialization of the DES.
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