The following table 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.
Table 22-298. Modulo example
Transfer Number
Address
1
0x12345670
2
0x12345674
3
0x12345678
4
0x1234567C
5
0x12345670
6
0x12345674
22.5.5 Monitoring transfer descriptor status
This section discusses how to monitor eDMA status.
22.5.5.1 Testing for minor loop completion
There are two methods to test for minor loop completion when using software initiated
service requests. The first is to read the TCDn_CITER field and test for a change.
Another method may be extracted from the sequence shown below. The second method is
to test the TCDn_CSR[START] bit and the TCDn_CSR[ACTIVE] bit. The minor-loop-
complete condition is indicated by both bits reading zero after the TCDn_CSR[START]
was set. Polling the TCDn_CSR[ACTIVE] bit may be inconclusive, because the active
status may be missed if the channel execution is short in duration.
The TCD status bits execute the following sequence for a software activated channel:
Stage
TCDn_CSR bits
State
START
ACTIVE
DONE
1
1
0
0
Channel service request via software
2
0
1
0
Channel is executing
3a
0
0
0
Channel has completed the minor loop and is idle
3b
0
0
1
Channel has completed the major loop and is idle
Chapter 22 Enhanced Direct Memory Access (eDMA)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
493
Содержание MK22FN256VDC12
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