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Enhanced Direct Memory Access (eDMA)
MPC5566 Microcontroller Reference Manual, Rev. 2
Freescale Semiconductor
9-49
9.4.5.3
Modulo Feature
The modulo feature of the eDMA provides the ability to easily implement a circular data queue in which
the size of the queue is a power of 2. MOD is a 5-bit field for the source and destination in the TCD, and
specifies which lower address bits are incremented from their original value after the a offset
calculation. All upper address bits remain the same as in the original value. Clearing this field to 0 disables
the modulo feature.
shows how the transfer addresses are specified based on the setting of the MOD field. Here a
circular buffer is created where the address wraps to the original value while the 28 upper address bits
(0x1234567x) retain their original value. In this example the source address is set to 0x12345670, the
offset is set to 4 bytes and the mod field is set to 4, allowing for a 2
4
byte (16-byte) size queue.
9.4.6
TCD Status
9.4.6.1
Minor Loop Complete
There are two methods to test for minor loop completion when using software initiated service requests.
The first method is to read the TCD.CITER field and test for a change. Another method can be extracted
from the following sequence. The second method is to test the TCD.START bit AND the TCD.ACTIVE
bit. The minor loop complete condition is indicated by both bits reading zero after the TCD.START was
written to a one. Polling the TCD.ACTIVE bit can be inconclusive because the active status can be missed
if the channel execution is short in duration.
The TCD status bits execute the following sequence for a software activated channel:
1. TCD.START = 1, TCD.ACTIVE = 0, TCD.DONE = 0 (issued service request via software)
2. TCD.START = 0, TCD.ACTIVE = 1, TCD.DONE = 0 (executing)
3. TCD.START = 0, TCD.ACTIVE = 0, TCD.DONE = 0 (completed minor loop and is idle) or
4. TCD.START = 0, TCD.ACTIVE = 0, TCD.DONE = 1 (completed major loop and is idle)
The best method to test for minor loop completion when using hardware initiated service requests is to
read the TCD.CITER field and test for a change. The hardware request and acknowledge handshakes
signals are not visible in the programmer’s model.
Table 9-24. Modulo Feature Example
Transfer
Number
Address
1
0x1234_5670
2
0x1234_5674
3
0x1234_5678
4
0x1234_567C
5
0x1234_5670
6
0x1234_5674
Содержание MPC5566
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