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.
22.5.6 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
2. Minor loop done
→
set TCD12_CSR[START] bit
3. Minor loop done
→
set TCD12_CSR[START] bit
4. Minor loop done, major loop done
→
set TCD7_CSR[START] bit
When minor loop linking is enabled (TCDn_CITER[E_LINK] = 1), the
TCDn_CITER[CITER] field uses a nine bit vector to form the current iteration count.
When minor loop linking is disabled (TCDn_CITER[E_LINK] = 0), the
TCDn_CITER[CITER] field uses a 15-bit vector to form the current iteration count. The
bits associated with the TCDn_CITER[LINKCH] field are concatenated onto the CITER
value to increase the range of the CITER.
Chapter 22 Enhanced Direct Memory Access (eDMA)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
495
