Information

Stage
TCDn_CSR bits
State
START ACTIVE DONE
1 0 0 0
Channel service request via hardware (peripheral
request asserted)
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
For both activation types, the major-loop-complete status is explicitly indicated via the
TCDn_CSR[DONE] bit.
The TCDn_CSR[START] bit is cleared automatically when the channel begins execution
regardless of how the channel activates.
21.5.5.2 Reading the transfer descriptors of active channels
The eDMA reads back the true TCDn_SADDR, TCDn_DADDR, and TCDn_NBYTES
values if read while a channel executes. The true values of the SADDR, DADDR, and
NBYTES are the values the eDMA engine currently uses in its internal register file and
not the values in the TCD local memory for that channel. The addresses, SADDR and
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.
21.5.5.3 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.
Initialization/application information
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
464
Preliminary
Freescale Semiconductor, Inc.
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