NXP Semiconductors PXN2020, Reference Manual, Page: 564 / 1376

Enhanced Direct Memory Access Controller (eDMA)
PXN20 Microcontroller Reference Manual, Rev. 1
24-44 Freescale Semiconductor
The TCD.START bit is cleared automatically when the channel begins execution, regardless of how the
channel was activated.
24.5.6.2 Active Channel TCD Reads
The eDMA will read back the true TCD.SADDR, TCD.DADDR, and TCD.NBYTES values if read while
a channel is executing. The true values of the SADDR, DADDR, and NBYTES are the values the eDMA
engine is currently using in its internal register file and not the values in the TCD local memory for that
channel. The addresses (SADDR and DADDR) and NBYTES (decrements to zero as the transfer
progresses) can give an indication of the progress of the transfer. All other values are read back from the
TCD local memory.
24.5.6.3 Preemption Status
Preemption is available only when fixed arbitration is selected for both group- and channel-arbitration
modes. A preempt-able situation is one in which a preempt-enabled channel is running and a higher
priority request becomes active. When the eDMA engine is not operating in fixed group, fixed-channel
arbitration mode, the determination of the relative priority of the actively running and the outstanding
requests become undefined. Channel and group priorities are treated as equal (or more exactly, constantly
rotating) when round-robin arbitration mode is selected.
The TCD.ACTIVE bit for the preempted channel remains asserted throughout the preemption. The
preempted channel is temporarily suspended while the preempting channel executes one iteration of the
major loop. Two TCD.ACTIVE bits set at the same time in the overall TCD map indicates a higher priority
channel is actively preempting a lower priority channel.
24.5.7 Channel Linking
Channel linking (or chaining) is a mechanism in which one channel sets the TCD.START bit of another
channel (or itself), thus initiating a service request for that channel. This operation is automatically
performed by the eDMA engine at the conclusion of the major or minor loop when properly enabled.
The minor loop channel linking occurs at the completion of the minor loop (or one iteration of the major
loop). The TCD.CITER.E_LINK field are used to determine whether a minor loop link is requested. When
enabled, the channel link is made after each iteration of the minor loop except for the last. When the major
loop is exhausted, only the major loop channel link fields are used to determine if a channel link should be
made. For example, with the initial fields of:
TCD.CITER.E_LINK = 1
TCD.CITER.LINKCH = 0xC
TCD.CITER value = 0x4
TCD.MAJOR.E_LINK = 1
TCD.MAJOR.LINKCH = 0x7
will execute as:
1. Minor loop done

set channel 12 TCD.START bit
2. Minor loop done

set channel 12 TCD.START bit