Checking Channel Preemption Status; Channel Linking - NXP Semiconductors freescale KV4 Series Reference Manual

Initialization/application information
DADDR, and NBYTES, which decrement 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.

23.5.5.3 Checking channel preemption status

Preemption is available only when fixed arbitration is selected as the channel arbitration
mode. A preemptive situation is one in which a preempt-enabled channel runs and a
higher priority request becomes active. When the eDMA engine is not operating in fixed
channel arbitration mode, the determination of the actively running relative priority
outstanding requests become undefined. Channel priorities are treated as equal, that is,
constantly rotating, when Round-Robin Arbitration mode is selected.
The TCDn_CSR[ACTIVE] bit for the preempted channel remains asserted throughout
the preemption. The preempted channel is temporarily suspended while the preempting
channel executes one major loop iteration. If two TCDn_CSR[ACTIVE] bits are set
simultaneously in the global TCD map, a higher priority channel is actively preempting a
lower priority channel.

23.5.6 Channel Linking

Channel linking (or chaining) is a mechanism where one channel sets the
TCDn_CSR[START] bit of another channel (or itself), therefore initiating a service
request for that channel. When properly enabled, the EDMA engine automatically
performs this operation at the major or minor loop completion.
The minor loop channel linking occurs at the completion of the minor loop (or one
iteration of the major loop). The TCDn_CITER[E_LINK] field determines whether a
minor loop link is requested. When enabled, the channel link is made after each iteration
of the major 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, the initial fields of:
TCDn_CITER[E_LINK] = 1
TCDn_CITER[LINKCH] = 0xC
TCDn_CITER[CITER] value = 0x4
TCDn_CSR[MAJOR_E_LINK] = 1
TCDn_CSR[MAJOR_LINKCH] = 0x7
executes as:
1. Minor loop done → set TCD12_CSR[START] bit
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KV4x Reference Manual, Rev. 2, 02/2015
Preliminary
Freescale Semiconductor, Inc.